Understanding battery capacity: Ah is not A

Posted by Jan on 12 November 2010

I used battery holders for eight “C” alkaline cells on my robot after not finding a 12V, 1A battery.

My earliest electronics projects and my first robot were powered by regular alkaline batteries, and I didn’t think about current or the capacity of those batteries. The batteries were prominently labeled “1.5V”, and I was happy in my understanding that putting four in a battery holder got me to 6 volts; when the motors slowed down, it was time for new batteries. When I began designing my second robot, I found some 12V, 1A motors (what a “1-amp motor” might mean is a topic for another post) and promptly wasted many hours dragging parents and teachers to Radio Shack and car parts stores looking for a 12V, 1A battery. No one understood that the batteries were labeled with capacity, not current, and since the smallest 12V motorcycle and alarm system batteries in town were 3Ah or 4Ah, I went home empty handed. I ended up using alkalines. Apparently, once the battery capacity wasn’t in my face, I forgot about my concern that they would force too much current into my motors.

I made many common mistakes in going about my battery selection:

  • Not understanding that my circuit would draw whatever current it wanted from the battery, as opposed to the battery forcing a given amount of current into the circuit.
  • Thinking that my motors would draw a fixed amount of current.
  • Confusing current and capacity.
  • Ignoring the “h” in “Ah”
  • Forgetting about a property, such as capacity, as soon as it wasn’t in my face.

The first two points are complex enough that further elaboration would merit their own posts; today I want to focus on some technical details of battery capacity and current and touch on the sloppy attitude that leads to the last two mistakes.

A battery stores energy; the “capacity” is how much energy it can store. Energy is measured in joules, abbreviated J, but it can also be expressed in different units such as watt-hours, abbreviated Wh (for larger quantities, such as residential electricity use, kilowatt-hours (kWh) are used; a kWh is a thousand Wh). This is similar to the way area can be measured in acres or in square miles: there are units specifically for area, such as acres, but you can also arrive at a measure of area by multiplying length by length, to get mile-miles, or the less awkward square miles. (The hyphenation imposed by English grammar does not help matters since the hyphen looks like a minus sign when we are actually multiplying the units together.) Watts and watt-hours are generally good units for electronics since they are easily related to voltage and current and since typical batteries that you can hold in your hand will have a capacity of a few dozen watt-hours.

In the case of a typical battery, where we can assume a constant voltage, we can replace watts with volts multiplied by amps. A 12-volt, 1 amp-hour (abbreviated Ah) battery and a 6-volt, 2Ah battery each store 12Wh, but the voltage is usually a critical parameter for a battery, and once a voltage is selected, the capacity can be specified by the amp-hour rating. The value in using the amp-hour is that it makes explicit our multiplication of rate, the amp, and time, the hour: a battery rated for one amp-hour can provide a current of one amp for about one hour, two amps for about half an hour, or 0.1 amps for about ten hours. I say “about” because the exact capacity will depend on the current.

The current and capacity for a battery are like the speed and range of a car. If your car has a range of about 300 miles, you can go 30 miles an hour for ten hours, or 60 miles an hour for five hours. Your efficiency will get worse with speed, so by the time you go 60 miles per hour, you might run out of gas after only four hours, for a range of 240 miles. Going back to my battery search, looking for a 1-amp battery was like looking for a car with a speed of 60 miles: 60 miles isn’t even a speed, and even if I revised my search to a car that could go 60 miles per hour, it still wouldn’t be a useful specification to look for. Most batteries on the scale I was looking at can deliver one amp, just like most cars can go sixty miles per hour. The maximum available current, like the maximum speed of the car, might be a more reasonable specification to search for, though providing those kinds of specifications might make the respective manufacturers nervous.

It is reasonable, though, to consider the maximum current a battery can safely deliver. That value will depend on all kinds of things, including the chemistry of the battery, but the maximum discharge rate is almost always tied to the capacity. That means that given a particular technology, a battery with double the capacity can deliver double the maximum current. Batteries are often specified with a discharge rate in terms of C, where C is the capacity of the battery divided by hours. For example, for a 2Ah battery, C is 2A. If the battery has a maximum discharge rate of 10C, the maximum current is 20 amps. It’s good to keep in mind that a 10C discharge rate means a battery life of less than 1/10th of an hour, and with the loss of capacity that a high discharge rate generally causes, the battery life would be less than five minutes.

As I tried earlier to recall what happened with my failed battery search, I was struck by the extent to which I ignored the “h” in the “Ah” specification and the ease with which I forgot about my critical “1-amp battery” requirement when I returned to the alkaline batteries. Unfortunately, this kind of carelessness or sloppiness is common, especially for beginners who might already be overwhelmed by all the information they need to sort through and who have not yet had the experience of losing time and destroying hardware because of inattention to details. I do not have any particular solution to this problem beyond reminding you to pay attention and think about how things should work before just hooking things up. Be on the lookout for contradictions; seeing “Ah” where you expect “A” should definitely make you very uneasy and lead you to reevaluate your expectations.

I will wrap up this article with some example battery capacities.

AA batteries.

  • A typical alkaline or NiMH battery in the standard “AA” size has about 2000 to 3000 mAh (or 2 to 3 Ah). With a cell voltage of 1.2 V to 1.5V, this corresponds to 2 to 4 Wh per cell. When multiple cells are used in series, as with the use of a battery holder or most pre-made battery packs, the voltage goes up but the capacity in amp-hours stays the same: an 8-cell NiMH pack made of AA cells will have a 9.6 V nominal voltage and a 2500 mAh capacity. There can be quite a range in capacities depending on the quality of the batteries. For larger cells, such as C and D size, the capacity should go up approximately proportionally to volume, but some cheap units (they’re usually light) can have the same capacity as the smaller cells. Alkaline cells have a more pronounced drop in capacity as the current drawn out of them goes up, so for applications requiring several hundred mA or more current, NiMH cells of the same size could last significantly longer. For low-current applications that need to run for months, alkaline batteries can last much longer because NiMH cells can self-discharge in a few months.


9V battery.

  • 9V alkaline batteries can be convenient for their high voltage in a small size, but the energy density (watt-hours per given volume or weight) is the same as other batteries with the same chemistry, which means the capacity in amp-hours is low. In approximately the same size as an AA cell, you get six times the voltage, so you also get about six times less in the Ah rating, or about 500 mAh. Given the high losses incurred from discharging in anything under a few hours, 9V batteries are impractical for most motors and therefore for most robots.


Coin or button cell batteries.

  • Coin or button cell batteries vary in size and chemistry, but you can generally expect 1.5 to 3 volts with a few dozen to a few hundred mAh.


12V, 8Ah sealed lead-acid battery.

  • Lead-acid batteries are popular for larger projects since they are usually the lowest-cost option and are widely available. Sealed lead-acid or gel-cell batteries are available in 6 V and 12 V versions (other multiples of 2 can be found), with the 12 V versions weighing about a pound per amp-hour. 12 V car batteries store a few dozen amp hours, and they weigh a few dozen pounds.


11.1V, 1800mAh Li-Po battery.

  • Lithium-based rechargeable batteries have around double the energy density of alkaline and NiMH batteries by volume and even better improvements by weight. These newer batteries are far less standardized in terms of battery size and shape, but since they are usually intended for applications where capacity or maximum battery life are important, these batteries usually have their voltages and capacities prominently labeled.

116 comments

Thanks for the article Jan!

It's probably worth noting, particularly for lead-acid batteries, that the capacity is usually listed assuming that the battery will be discharged over a 20 hour period. If you're using it in relatively high current applications (i.e. robotics or motors of any kind) then you can expect almost half of that. This is something that caught me out when I was looking at battery options.
Great comment; I'm sad that I neglected to mention that. Another variable to consider is how far the battery is allowed to discharge before being considered fully drained.

- Jan
Very helpful!!
This is very, very helpful thank you.
great help, thanks...
Immensely helpful: 1 AH = 1 amp for 1 hour, or 1/2 amp for 2 hours... I always wondered about this! Thanks very much!
It is a good writting for the new robot builders
Es un buen escrito para los nuevos constructores de robots.
Great article. Really helpful for battery selection
Helpful post.

You wrote
"Most batteries on the scale I was looking at can deliver one amp..."

How many amps can a single AA battery deliver?

I want to run a 1 amp (12v, 15 watt max) motor at low rpm (as part of a solar tracker prototype) using rechargeable AA NiMH.

How many 12 volt arrays in parallel needed to draw current adequate to move a panel (other than trial and error)? Let's say a panel weighs around 10 pounds (or 5kg).

10 AA's @ 1.2v in series gives me 12v, but I don't think I can run a 15 watt motor with it. Do I need 10 more batteries in parallel? 20 more?

And what is/are the unit/s for maximum (or optimal/safe) discharge rate? I guess it doesn't come up for AA batteries much. Though some of the electric car companies are making their batteries similarly, and laptop batteries are similarly configured (10+ cells computed to get to the right balance of watt-hours and current-draw).
Hi.

Most NiMH chemistries will probably tolerate at least a few C discharge rate, so an AA should be able to deliver at least 5A, which is plenty for the motor you're describing.

I have no idea what you're asking with this solar stuff, probably because you have little idea. Among your likely confusions: weight is not at all enough to tell us how much power you need, and getting more current available in your batteries is not going to force more current into your motor.

You should not be putting batteries in parallel unless you really know what you are doing, which in this case you do not.

Maximum, optimal, and safe discharge rates are completely separate. Any maximum rating better still be safe, and there will be a wide range of safe discharge rates. Optimal in terms of energy delivered will probably be around a few mA over a month, maximum could be 20A for a few minutes.

- Jan
I need to replace an electric scooter battery pack. Yhis one cosistes of 4 x 6DZM-10 batteries which are marked 12v 14Ah.
I can only find 12v 12Ah or 12v 15Ah. Would using batteries with different Ah than the originals be a problem?
The discharge rate could be an issue, so that's one reason going with 15Ah is probably better. However, the higher capacity battery could be an issue for the charger, depending on how that works. But 14 is pretty close to 15, so I would just go with the 15Ah. I would be very surprised if the 12Ah didn't work, though, so if that somehow fits better or is much cheaper, it should be fine (don't forget the 15 will give you 25% more run time than the 12).

- Jan
Hi there,
Ok, since I'm newbie please be patient...
I'm going to buy a 24 channel RC servo controller and for sure I'd want to add to the combo, the most suitable battery pack option for the better and worst case (which is 1 Servo or 24 at the same time). Let's say that the servo I'm interested in is the famous Hitec HS-311.
Hitec specs that this servo will drain 7.7mA iddle / 160mA no load (normal working?) at 4.8v and 7.7 mA iddle / 180 mA no load (normal working?) at 6.0v.
Please, correct me if I'm wrong with the following assumption:
if I plug 24 servos, I'll need a battery pack capable of deliver 6v 4320 mAh in order to have all servos working for just one hour...
If I plug just 1 servo, since I'd need just 180mA, my new hypothetical battery will power it for 24 hs.
Which is the most suitable battery pack in sale here that would you suggest for this example?

Many thanks in advance!

Nico.
Hello.

Your basic math is right, but keep in mind that capacity depends on discharge rate, so depending on the battery and how it's rated, it might not necessarily give you the 4320 mAh if you discharge it at 4.3 A. We do not carry any packs with that high of a capacity, but that's in the range of what you would get with "C" sized NiMH cells. You should be able to find receiver or servo packs with that kind of capacity at a hobby store (a quick search at Tower Hobbies led to http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVTG1&P=7 ; at first glance, it might be what you're looking for). You could also consider lead-acid batteries; they will be bulkier, but they will probably be cheaper and easier to find in capacities exceeding 4Ah.

- Jan
how much time it will required to complete discharge of 2000mAh cell phone battery.
Did you read the post? Anyway, 2000 mAh should last about 1 hour at a 2 A discharge rate, 2 hours at a 1 A discharge rate, 10 hours at a 200 mA discharge rate, and so on. Typically, the capacity is based on some particular discharge rate, and capacity will be lower if you discharge faster than that. By the way, a modern cell phone battery is going to be a lithium-based battery, which you have to be extra careful not to over-discharge, and the battery module might have some integrated electronics to protect and monitor the battery.

- Jan
Hi there,

I just happened to stumble across this post in my search to find out about maximium current draw for Ni-MH AA batteries i have two applications i am going to use. Correct me if I am wrong but I currently understand that if I have equipment that requires a voltage of 6v and a current of 1amp that (as you said) I will need 4 AA batteries (alkaline 1.5v) to get 6v and these should provide the 1amp comfortably! But if I have 4 AA Ni-MH batteries (1.2v) I am only getting 4.8v! I am under the impression that as the voltage is lower than required, it tends to draw more current to make up for the voltage drop!
I am thinking like this as I have been testing RC Toy Car run time with alkalines and then swapping out for Ni-MH (2500mah uniross) and there doesnt seem to much difference in the run time,there was only a 23mins extra with the Ni-MH!

Application 1
I am a plumber and I am constantly working in roofspaces and small dark places so I really hate it when my torch dies and i have to go to the van to get a 240v light lead!(the torch never gives enough light anyway)
i have went down this road before and i rigged up a small 12v 10watt (900mah) LED to my ryobi torch! This works quite well and give brilliant light but the rechargeable batteries for my ryobi are expensive and i need them to power the rest of my tools!Plus they will only run this LED light for around 1hr 15mins (with 45min recharge time)
I have been researching plenty of high output LEDs but anything over 3watt and it needs a heatsink so i have been reduced to smaller LEDs. So instead of 10 x 12v 3watt i am going with 10 units of 12v x 48 SMD LED that each draw less than 1watt (10watt maximium) not sure what the mah is yet still waiting on delivery so 480 SMD LEDs in total!!
I am quite confident that these will run fine on 10 AA Ni-MH batteries, but as I dont yet know the current draw for each set of leds I cant be sure of how long it will last. Am I right??

Application 2
This is where I need advice on the Maximium Current draw of a 2500mah uniross AA Ni-MH battery. I am only being specific as these are the batterys i have found to be most reliable and i have plenty to hand!
I bought a set of xenon bulbs for my old car (Rip Corsa) and i have now got a new car but the bulbs do not fit so instead of throwing them out (because no-one would buy second hand) i was thinking again of using 10 x AAs to get my 12v but i know that the ballast unit that starts the bulb needs a minimium of 6/7 amps for a split second to ignite the light once it is lit the bulb will only draw 2/2.5 amps. These bulbs run equivalent to 150watt halogen bulb or 3200-3500 lumens!!
So my real question is can an AA battery push out to 7 amp maximium and if it can, would prolonged use or this type of ignition for a light destroy the batterys?
For this application I am only looking to use for a flashlight! (ie not a very long run time maybe 15-20mins)
I Know I can ramble on but i wanted to give as much information as possible so you understand in full the purpose of my application!!

Thanks for taking the time to read this post!!
Hi, John.

You're kind of all over the place there, so I will just give you bullet point comments as I go through your post:

* I don't know where you're coming up with this "(as you said)" note.
* All kinds of devices (including motors and light bulbs) draw less current when you give them less voltage. You have to have some somewhat special electronics to draw more current when the voltage goes down. Some LED flashlights could certainly be in the "somewhat special electronics" category.
* I'm used to RC toy cars running for maybe 10 minutes, so 23 extra minutes is huge. Obviously, it's not in your case, but you should see it's useless for you to just mention 23 minutes without some baseline.
* Your Application 1 text is difficult to follow and seems to have a lot of extra unnecessary details.
* If you are asking for confirmation that you cannot calculate the battery life without knowing the current you are drawing, then yeah. But in that case, I don't know where you are getting your confidence about running off a particular battery configuration.
* If 1W is your input power at 12 V, you can figure 1/12 A (or 83 mA) for your current.
* I don't think a lot of little LEDs is an efficient way to get a lot of light. That's based on the good flashlights not seeming to do that.
* I expect a decent AA NiMH cell to be able to put out 7 amps for a few seconds, but if you really care, you should try to get the specs for your particular batteries.

- Jan
nice blog,very easy to grasp for non technical readers! :D

is it true batteries are like memory cards or hard drives?
e.g. when you buy a 5gb memory card the usable space will be far less than the 5gb capacity.

if that is true,how can you compute the usable power vs the mah rating of lithium ions?

thank you :D
Hi.

I don't think your statement about memory devices is correct. The battery part is in the post: as with range on a car, you will get more or less out of a battery depending on how quickly you try to get it out.

- Jan
hello Jan (et al)
let me just rephrase the memory card/hard drive analogy: :)
I bought a portable terabyte drive disk,its advertised as 1000gb but when you are about to check the fresh unused drive, it only has 930gb usable free space even without putting any files on it.

reference (see comments) http://news.cnet.com/2100-1015_3-6105515.html

I was wondering if this also applies to batteries,ie: if you buy a 200mah battery will you also get a full usable 200mah?

cheers! :)
With the hard drive, that's mostly a matter of how your end device is using and reporting the available memory; I'm pretty sure the hard drive itself will have at least 1000 billion bytes on it. Even if you want to feel ripped off about the 10^3 vs. 2^10 distinctions, I think 7% is not really worth calling "far less".

Anyway, there's no equivalent issue with mAh as far as the unit goes. The point is, as with the MPG on a car, you can't capture the capacity with a single number. I think that with most reputable battery manufacturers, you will get the advertised capacity if you suck the power out at the right rate. If you discharge the battery quickly, over something like an hour, you will likely get less than the nominal capacity; if you draw it out over something like a week, you will probably get more. If your applications always discharge the battery quickly, it's reasonable to expect that you'll always get less than the claimed capacity.

You might think of something like ketchup in an 8 oz. bottle: there's 8 oz. there, and you can get it all out if you wait long enough, but you'll get a lot less if you only have 5 seconds per bottle to get it out.

Of course, keep in mind there are many more variables with batteries, like how much that capacity changes over the number of times you recharge, how the temperature affects the capacity, and on and on.

- Jan
hi Jan, :)

this is getting interesting..

so it all depends on the sucking or pulling of the reciever.

sample items:
a portable external battery charger w/ 100mah rating
a gadget w/ 20mah rating
(figures above are simplified,not real ratings)

the gadget can get 5x from the charger at a right rate.BTW the gadget is on 'off' mode while charging.Is my approximation correct?

do you have a formula for the speed of charger drain using the pairing sample above?

other non battery info that might help.
charger's charger rating:
input: AC 100-240v 50/60hz 150mA MAX
output: DC 5.3V 1000mA

gadget's charger:
input: 100-240V 50/60hz 0.5A
output: 5.0v 2.0A

all the best :)
Nice article JAN!

Please forgive my noob assumptions..

Using your ketchup analogy & lithium's hard drive analogy

When you buy an 8 0z ketchup (or 8mah battery) you will get a bottle filled w/ 8 oz ketchup (or 8mah battery energy).

So the Hard drive industry is different from the battery industry.

Hard drive industry: you get LESS on what you pay for.
ie: buy 10gb,but 9gb is only usable

Battery industry : you get EXACTLY what you pay for
ie: buy 10mah battery,you get usable 10mah power.(either slow or fast suck).

???
I think you're getting sloppy with your units, and I don't follow the charger stuff or what you are asking.

With the hard drive, you make a mistake when you switch from what you get to what you use. You buy 10gb, and you get 10gb; how you or your device use it is a separate thing. When you buy an 8.5 x 11 inch piece of paper, that's what you get, even if your printer can't print on the whole area.

- Jan
Hello Jan.

I have a simple question about batteries:

Discharge Rate: Fixed 5V, 1,000mAh

Charge Rate: 5-5.5V 450mAh

the unit mAh is a unit of electric charge, like Coulomb. I don't understand what a fixed 5V discharge rate means. Also what does the 1000 mAh mean?

Why does the charge rate have a smaller mAh?


Thanks,
Kavan
Kavan,

I can confirm that your two specs do not make sense. Without any context, it's difficult to guess the intended meaning.

- Jan
Dear Jan Thanks for the great and continuous efforts up there! I wanted to use a battery with 4 v and 2A supply such that the 2A is burst supply during transmission of signals in GSM module. I want this battery to be used without charging it for 2.5 years. Everyday once during this period, the GSM module will transmit an SMS of about 50 characters and then go to autosleep mode. IN this scenario, do i have a chance to get one such battery!? If not, what is the best alternative?
Anand,

You are not providing the right kind of information to answer your questions, and I do not know if this is the appropriate venue for going into the details of your project. But, here are some general points that might be helpful to you and others:

* I think 2.5 years is unlikely to be practical for a rechargeable battery. (Maybe that was a typo since "charging" and "changing" are pretty similar.) I think there are various non-rechargeable battery types, like lithium batteries, that are supposed to last a decade or more.

* It is not helpful to go into details like how many characters are sent without talking about the time it takes. And, for all I know, the wakeup and autosleep stuff might take longer than sending the message. Anyway, let's say your power requirements are 2A for 5 seconds every day. You would need 2 A x 5 seconds = 2.8 mAh per day. Multiply that by 365 and then 2.5, and you get to a little over 2.5 Ah for your 2.5 years. That's not some particularly huge number, though you might need to pad it quite a bit since the capacities might be based on very low discharge rates, not the 2 A pulsed discharge.

* How do you expect anyone to know the "best alternative" for your application? We cannot know if your system will be in the sunlight for some solar option or how practical it is to power your system from a wall outlet.

- Jan
Dear Jan
Thanks a lot for your response! Sorry for not providing proper information. Firstly, the battery cannot be re-chargeable. Secondly, I wanted a very small circuit, hence no scope for recharging. Also, the size of the end-product should be as small as possible.
Thanks for those inputs like wakeup and autosleep taking longer than sending the message. Yet, would a 2.5Ah non-rechargeable lithium battery suffice my purpose of lasting 2 years at least.
I am sorry if this is not the forum for this discussion. You may please let me know the same and i shall post this info there. If you need more inputs, i am ready to share them.
Best Regards
Anand
I showed you the exact assumptions and calculation that led to the 2.5 Ah result.

- Jan
Thanks Jan.
Too slow in catching up the logic!
Best Regards
-Anand
Great post.

The fact that the capacity is never mentioned on Alkaline batteries is driving me nuts… It's arguably the most important criteria of a battery… Instead they push marketing slogans like "last twice as long as other batteries"… Why not simply give the capacity so everybody can see for himself ?

I think it should be mandatory. Imagine if you bought a bottle of water and they wouldn't tell how much water is inside…
Hi, Joan.

I'm not sure what you mean by "mandatory", but in the typical, government-backed sense, that is a very bad thing. Imagine if you bought a bottle of water without knowing how much water was in there and someone put you or the seller in prison.

If you're just talking about making a capacity specification a personal prerequisite for you to consider buying a battery, keep in mind that there are all these other variables, including discharge rate, temperature, how old the battery is, how low the voltage can be before you consider it discharged, and on and on.

- Jan
Thanks for the article! How does voltage decrease with time? I see batteries labeled with voltages, but surely this does not stay constant while discharging? Does it just decrease linearly?
It typically drops quickly, then flattens out around the nominal voltage, then drops quickly again. You should search for something like "battery discharge curve" for more details.

- Jan
very nice . tnx
Hi Jan, thanks for the informative article. It's obviously a topic of much interest as these Q&As have been going on for two and a half years!

I have an old but nice Bosch cordless drill/driver (PSR 7,2 VES). It's one of the Swiss made ones! The battery pack is a Ni Cd 7.2V and rated 1.4Ah. The replacement cost is £57.

So if I make an assumption that were I to run the drill constantly (which you can't because it would overheat) it should run for maybe 20 minutes? That gives me a rough guess figure of 4.2 amps constant current?

I am thinking that if I were to fabricate a simple AA size battery holder, in place of the original battery, I could use 6 in series Ni-MH 1.2V - 1900 mAh AA batteries (Panasonic Evolta)? A value for C of 2.2 using my guess.

Is the above reasonable? I think it worthwhile to save a good drill from the landfill. Note to gearheads etc.: I know a 7.2V drill is probably considered a toy but I find it useful and it's what I have. I'm not in the market for something like a DeWalt at many £100s!

--
Stephen
Stephen,

That generally sounds reasonable. The current at peak loads might be higher than what you calculated, so the performance you see in the max torque sense could be lower that what you saw with the original NiCd pack (when it was new). I have no experience with the Panasonic Evolta, but if the performance ends up being limited, you might look around for NiMH cells that are better about high discharge rates.

Also, you probably should not use the original charger for the new NiMH pack.

- Jan
Jan, thanks for your response.

Your point about not using the original charger is taken. I intend to assemble the new pack in such a way as to be able to remove the batteries individually and use a brand AA charger.

Just to share the information: the Panasonic Rechargeable Evolta advertise as purposed for long term storage quoting 80% capacity after 1 year. I wonder if this is some 'new' variant of Ni-MH as the chemistry much be different to enable that long term storage of charge. Anyhow, perhaps a debate for another venue.

I will look out for further information about discharge rates. Best wishes for the success of Pololu and your team.

--
Stephen
Hi, I dont have any questions. I just wanted to say thanks for the article, I found it highly informative. Feel free to add me on google plus, I'd like to network with electronic professionals! Thanks!

vinnythestick@gmail.com
Dear Jan,

Please guide me I am a student and currently working on a Self Balancing Two Wheel Robot System, using a NI board which works on supply of 9-30Vdc. Power requirement is 24.7W. Kindly let me know what all things I have to look for while selecting battery and what about current and capacity? (I calculated current to be 2.74Amp)
Dear Jan,

Other thing I want to know is how to convert my Ampere value to Ah. In my case 2.74Amp=?? Ah.
Hi-

The following is 'exempt' from the need to obtain a building permit in Ventura County, Ca. :
"8. Electrical wiring, devices, appliance or equipment operating at less than 25 volts and not capable of supplying more than 50 watts of energy." ((this is also found in building codes across the country)

I drew a 12v solar power system with a 12v battery and fuses that are 4amps, therefore limiting the output in any circuit to 48 watts.

I met with the head of the department of Building and Safety, and he says that NO part of the system may have the capacity to supply power greater than 24v, or 50W. Therefore, the solar panel must be no larger than 50 watts.(easy enough) But when it comes to the BATTERY, (for nighttime use) I can't figure out what to use. If the 12v battery discharges more than 4.1 amps, it puts out more than 50 watts. So- I can't figure out how to construct an exempt, 12v system. (I am going to power 5Watt L.E.D. bulbs, so I could power one for 10 hours, or 2 for 5, in theory, with 4 Ah))

Do you have a suggestion as to how to satisfy the requirements for exemption? (what battery to use, or how to satisfy this beaurocrat some other way)

Thanks- David dfeigin@yahoo.com
Ashish,

Did you read the post? Or the comments? Your question is answered many times.

- Jan
David,

I'm not sure what you are asking. Is your safety guy saying the fuse approach is not acceptable? If you're thinking about using a 12 V lead-acid battery, those are commonly used in alarm systems, and I doubt they all need to get building permits. If they do accept the fuse or other current/power-limiting device right at the battery, the capacity shouldn't really matter, and you could use a 40 Ah battery (it would be the size of a car battery) to give you 4 A for 10 hours.

- Jan
Hi Jan,

I read the article and comments and I think I now understand the difference between A and AH.

I have a little question, I want to power a tripath amp. Input is 20V and current is 3A. I'm thinking of connecting 2x12V SLA batteries inline and they each have a capacity of 12AH. The batteries have "Initial current: less than 3.6A" written on the side. If I've got this right I should expect about 4hrs usage out this setup, where I'm a bit confused is that the battery says "12V 12AH/20HR " on the side; this is touched on in the comment by J. Lumsden but I'm still a bit confused by how that now works.

Andy
Andy,

You are correct about the calculation for the four hour estimate, and I think the "12AH/20HR" means that the 12 Ah capacity is based on a 20-hour discharge (i.e. discharged at 12/20 A, or 600 mA). In general, you will get less than the 12 Ah out of the battery if you discharge it faster, but you will probably still get close to the four hours.

- Jan
Hi,
Here's a real life battery problem for you that probably would be a good test question:

I want to power a bilge pump in a motorless vessel in VERY cramped quarters. It draws 1.9 amps @12 volts or 2.6 amps @ 13.6 volts. Due to space limitations, I have the choice of powering it using 8 D-cell alkalines in series or a 12 volt 3.3 amp-hour LSA battery. It also uses 0.2 amp-hours per day to monitor the water level. The battery would be needed for no more than a month at a time and would likely have to actually pump water in short bursts of 10 minutes or so a few times over that month. Which battery do you think would power the bilge pump longer and more effectively?

Thank you if you can help with figuring this out. A functioning bilge pump can be a literal life saver.
Robert,

Your question boils down to: do D alkaline cells store more than 3.3 Ah? http://www.allaboutbatteries.com/Energy-tables.html lists 17 Ah, so if that's correct, it's no contest.

By the way, your 0.2 Ah/day will use up that 3.3 Ah in about two weeks even without any pumping, so that is not a viable option, anyway.

- Jan
Jan,

First of all, thank you for providing this service to those of us who are not electrical engineers!

I was concerned with whether the drain when pumping would be excessive for alkaline batteries, and whether the power curve would drop faster. I also noticed one of the previous comments said SLA battery ratings were based on a slow draw-down and high demand would cut the available Ahs in half. I don't know if any such rule applies to the alkalines.

I should have mentioned, too, that the circuit would only be activated when under way, which would average about 4-6 hrs/day. I could eliminate the monitoring circuit, so the batteries would see demand only if actually used (though I would prefer to keep it automatic).

I guess then the two-part question is, given the two scenarios, and the fact that the pump draws less amperage as the voltage drops off:

1) how long would the 8 D-cells pump water if the monitoring circuit were deactivated, and
2)would the pump be able to pump (or pump or very long) near the end of two weeks or a month of intermittent (~6 hr./day) activation of the monitoring circuit?

I realize there's no way you can know with any precision at just what voltage the pump will stop pumping, but I presume you have a lot better experience base than I do to make a decent approximation (WAG).

Nothing's ever simple, is it?

And thanks again!

Bob
Just to complicate matters, what if I used two 26 Ah alkaline 6V lantern batteries in series? It might be more difficult to waterproof package them, of course, and they'd be a bit bulkier.
How well different battery types handle high loads is generally a very relevant consideration, but your ~2 A discharge rate is not that much for these batteries, so the alkalines having 5x the capacity should far outweigh the variations in high-discharge rate characteristics.

This is not that complicated, and your latest question is whether 26 is bigger than 17. If you gave two more similar numbers, like 17 and 19, you should just try it and time it if it matters much if one does 8 hours and the other does 8.5.

- Jan
Jan,

Thanks for all the advice.

Bob
Hey there!

I only have one question. I understand amps per hour * volts = watts per hour = watt hours. That part is easy. Given any two values, you can determine the remaining value.

What I still don't understand though, is how can one find the maximum amp draw of a given power source?

For example, one AA battery might have a full capacity of 2 amps an hour, or 2 amp hours. But how many amps per hour can flow through the battery? How do you calculate the limit before resistance and heat become so great that the amp draw cannot go any higher? Could you literally draw 4, 8, 16...120 amps out of the battery? Of course understanding that the cell will fail in under a minute at such a high draw rate.

Thank you.
I did some more googling and it sounds like you just need the "C" rating. 1C means you can discharge at whatever the max capacity is. For example, a 2AH 1C battery can be discharged at 2 amps per hour. And a 2AH 10C battery can be discharged at 20 amps per hour.
Matt,

As long as you are talking about "amps per hour", you do not understand it. "Per" means you are doing division, so something like "watts per hour" is very different from "watt hours", which is watts *times* hours. Units like amps and watts are already rates (coulombs per second and joules per second, respectively).

As for your main question, I specifically went over it in the post, and you got it right in your follow up post, except for the "per hour" part: a 2Ah, 10C battery can deliver 20 amps.

- Jan
Hi Jan,
I have an electric scooter that has a 1,000 Watt motor and came with 12V/12aH batteries. These batteries are unable to take me round trip to work, so I replaced them with 12V/15aH batteries. The motor failed on my first trip. Suspecting it had burned out, I touched the housing immediately after stopping, but it was cool. My question is, did the higher aH batteries cause the motor to fail, or is this just a coincidence?
Hi.

Just changing the batteries for higher capacity ones shouldn't cause your motor to die. However, there might be other factors like the new batteries being heavier, which could put more strain on your motor and lead to the failure.

You also wrote "batteries" (plural); if you originally had something like two in parallel and changed it to two in series, you would have gone from 12 V to 24 V, and that could definitely lead to your motor wearing out quickly.

A final thought I have is that the motor is not necessarily what broke; have you verified that independently? The things like higher load and higher voltage that could damage your motor could also damage the control electronics.

- Jan
You are correct, Sir. (3) 12-volts; a 36 watt system. It was my presumption the motor had burned up; it turned out to be a pebble in my brakes. I have installed the new batteries and the old girl is now able to go 'round the horn without difficulty. Thanks for the confirmation I was only increasing capacity. Cheers!
Mark,

I am happy to hear you figured out your problem. For anyone else reading this, I should point out that the three batteries in series make it a 36 V system, not a 36 watt system (that will depend on the current).

- Jan
Doh!
Hi,
I have one question, I have a device consuming a 3V button cell every two weeks, would it be possible to substitute this button cell for two AA batteries expecting longer replacement time? Or are button cells providing lower current than standard batteries?
Since AA batteries are about the amount of mAh in my phone, could I theoretically use some sort of apparatus that plugs into my phone and runs on AA batteries to charge my phone on the go? I know lithium is more convenient because it is rechargeable, but AA is more convenient if cost-defective because I can buy it on the go and don't need a charger for the car or outlet when I'm really busy.
By the way, credit for still looking at comments and replying after 3 years.
Hello Jan!

My question is: If I have two batteries, one being a 12v 1.2 Ah and another being a 12v 60Ah
will the battery with bigger capacity push more amps through a fixed resistance circuit just
because it has more capacity?

I know it is a bad example but.. If you lick the terminals of a 9v 1.2Ah battery (plese don`t
lick batteries... REALLY... DON`T) you only get a tingly sensation on your tongue...
But if you lick the terminals of a 12v 60Ah car battery (Again, don`t ever try this, it is stupid,
you will almost certainly get badly injured... And Darwin will be laughing his a** off)... my guess is
you will probably get a very, very nasty burn on your tongue: Even though the voltage is only 4 volts higher..

Can two batteries with the same voltage push different amps trough you if they have different
capacities (Ah ratings)?

Does this have anything to do with the discharge rate: the "C" rating?

Hey! thanks for your support!
And kudos for you, for still answering this questions after two years!
Manuel: Two AA batteries will likely last a lot longer than a button cell battery, which is usually used because of space or weight constraints. So if those are not issues for you, you should be fine.

Ark A.: yes. You might start by looking at http://learn.adafruit.com/minty-boost .

M. Milani: no, batteries do not "push" current into your circuit. The higher capacity battery will just deliver the current for a longer time.

- Jan
Great!! Thank you
Hi Jan,

Big thanks to you for sharing these information on basic understanding of batteries. Many
people can clear their doubts regarding batteries.
Also, replying to queries means you really deserve a Hats off plus standing applause.

Thanks again to you for your efforts.

With Regards,
Ranjit Singh
Hello Jan,

Total newby to battery specs. I stubbled across this thread looking for an explanation to batteries on phones and how power is transfered to them by portable battery packs. Sorry this is not about running a robot, but you seem to have your head on straight about this subject of batteries.

I have an iphone 5, it has a 1440mAh battery. I am looking to buy a 13500mAh portable battery pack to recharge it.

13500/1440= 9.375.

Is it correct to assume that this battery pack will recharge the phone from 0% to 100%, 9.375 times or does some of the stored power get lost in the transfer? Or are there other factors involved here?

http://www.newtrent.com/store/iphone-external-battery-iphone-portable-battery/nt135t.html
Hello.

In general, you should go off of the watt-hours specification, not amp-hours, because the voltages might be different. In your case, the batteries are about the same voltage, but going by the Wh specs of 49.9Wh for the power bank and 5.45Wh for the phone still gives you a slightly different ratio of 9.16.

You definitely are not going to get perfect energy transfer. I don't know how efficient the power bank is at boosting its 3.7V battery to the 5V for the USB port output or how efficient the phone is at charging, but if we guess 85% efficiency for each of those, we get a net efficiency of 0.85*0.85=72%. Multiply that back by that 9.16 ideal case and you might expect 6 or 7 full charges.

- Jan
Hi Jan,

I have read and reread this article and spent the last 3 hours researching the internet to find an answer but I am still not 100% clear on this so I apologize but without any background in engineering I am excusing my ignorance at this stage :)

I am trying to power a 24V DC fan (I started with a 12V version but it did not have enough air throughput for me) but I would like it to be battery powered and I am trying to calculate the most efficient way to do this.

The technical specs for the fan says the following:
Power input 13W; therefore I have calculated the current required (the draw?) to be 0.54A using P = V x I.

Making this simple if I round the current required to 0.5A and I obtain a battery rated at 24V 1Ah (which are not very common and are very heavy), should I assume that the fan with run for 2 hours? (at an assumed 100% efficiency).

Apologies if this is answered in some other way in the previous posts but I have read them all several times and I want to make sure that I am understanding this correctly.

Thanks,

Jack
Jack,

You're basically right, but you should keep track of which direction all of your rounding is going: you're drawing *more* than 0.5 A, your efficiency will be *worse* than 100%, and your capacity at that discharge rate will be *less* than the rated 1 Ah. So, I would expect more like one hour of run time.

By the way, it should be pretty easy to find 12 V batteries, so you can put two of them in series to get to 24 V. "Very heavy" is relative; I expect that at 12 V lead acid battery weighs about a pound per Ah, so you should be able to get a solution that weighs about two pounds. 1 Ah might be a little hard to find, but a quick Digi-Key search yieleded this 1.2 Ah unit that weighs 1.3 pounds:

http://www.digikey.com/product-detail/en/BP1.2-12-T1/522-1007-ND/

That would weigh 2.6 pounds for 24 V and they claim 1 hour discharge will give you 0.787 A, so you should be able to count on more than an hour but less than two of operation with that.

- Jan
Hi Jan,
Its great post about batteries.

I'm new robotic stuff, apparently I have to build a line following robot with Obstacle avoidance. I'm using a Arduino and SeedStudio motor driver to build a robot. I'm using four 1.5 Alkaline batteries to run this robot. It is working perfectly when I used it only as line following robot. But I added a ping sensor to avoid the obstacles along with line following, it is not moving at all. Interesting thing is when I connected the USB cable to Arduino along with batteries it is moving forward. So I could draw some conclusion that current required for circuit with ping sensor is enough when I connected USB and batteries. But I wann know I can increase the current in my circuit.

You reply is much appreciated. If let me know if you need more details.


Thanks
Kiran,

I'd like to keep the discussion here focused on batteries and their capacity. I suspect the answer to your question has more to do with the rest of your system than with the batteries, so please ask your question in a more appropriate place such as our forum. You could try NiMH batteries instead of alkalines to get a quick idea of whether it's a battery current limitation.

- Jan
Great post. I knew the information, but this makes it very succinct for a beginner.
Hello, im also new in the forum and not so good with electics.
I have a bosch hammer batery driller, im using it for making rock climbing routes, over 1 year period the bateries are almost dead, and they are quite expensive to buy.
I was thinking if i can use an motorcicle batery 12v 6ah for my driller.
The original bateries are rated 24v 3ah.

Thanks in front

Kire S.
Kire,

12 V x 6 Ah = 24 V x 3 Ah = 72 Wh, so the energy in the two is about the same. However, you're probably not going to get much performance with just the 12 V battery when the motor expects 24 V, so you would need to connect two of your motorcycle batteries in series. You should also make sure the batteries are sealed if you will be using your contraption in all kinds of angles.

- Jan
Thank you Jan, i will try it first with only one battery, if the power is not suficcient i will conect two of them.

Thank you and i will write after the work is done.
Hi Jan,

Great information here. Very educational and explained in a very basic manner.

Just one correction though about M. Milani's question:
"My question is: If I have two batteries, one being a 12v 1.2 Ah and another being a 12v 60Ah
will the battery with bigger capacity push more amps through a fixed resistance circuit just
because it has more capacity?

I know it is a bad example but.. If you lick the terminals of a 9v 1.2Ah battery (plese don`t
lick batteries... REALLY... DON`T) you only get a tingly sensation on your tongue...
But if you lick the terminals of a 12v 60Ah car battery (Again, don`t ever try this, it is stupid,
you will almost certainly get badly injured... And Darwin will be laughing his a** off)... my guess is
you will probably get a very, very nasty burn on your tongue: Even though the voltage is only 4 volts higher..

Can two batteries with the same voltage push different amps trough you if they have different
capacities (Ah ratings)? "

Your response was that, "no, batteries do not "push" current into your circuit. The higher capacity battery will just deliver the current for a longer time."

I have a diferent opinion, that since he is "licking" the batteries, he is shorting the terminals and causing a current surge. In this case the battery with 9V can only give him a tingle on the tounge, whereas the bigger battery (size, rating and voltage wise) will have a higher charge stored and also a higher voltage, and so will give him some burn on the toungue too as it will deliver more Amps when shorted.

Please correct me if I am wrong here.

Thank you!
Faisal,

The question was about capacity in Ah, not about voltage. Higher voltage will cause higher current to flow; M. Milani specifically asked for the same voltage.

By the way, talking about a battery "storing charge" is probably not helpful since it's not a capacitor. Also, be careful about mixing various characteristics like voltage and maximum current with attributes like size: physically larger batteries are not necessarily going to have higher voltages than smaller ones.

- Jan
Good stuff. I'm an inside wireman for IBEW and I'm going to take the "Skilled Craft Battery Test" for Union Pacific railroad for a position called "Electrician Diesel Engine". I've worked with alternating current primarily so this was very helpful.
I'm building a tricycle with a front hub motor and a small audio amplifier on it. I've checked the tech specs and it does 300watts rms at 14v and draws 22a rms I was looking into using new lifepo4 batteries but idk how big of a battery I need. I use two kinetic hc600 there 25ah 12v. I want to be able to play the music for about 6hr like I do now. I want to cut weight but keep a big reserve for pedal assist but all these lifepo4 specs and discharging things are confusing me.
Daniel,

I think you should be able to figure this out if you read the post and other comments carefully (or your question is something more complicated that I am not following). I'm not clear on this audio amplifier vs. motor you're talking about, but on the most basic level, if you need to supply 22 A, a 25 Ah battery will give you about an hour tops, so you'd need at least six of those batteries for around 6 hours. If your 22 A spec is not continuous but something like just when you're accelerating or going uphill, you might get a lot longer battery life based on the actual load you put on the motor over the course of the 6 hours. In that case, it seems like you already have these parts so you could just see how long the batteries last in your typical scenario. If your two batteries last 4 hours, you'd need one more to last 6 hours, and so on.

- Jan
Hi Jan

Glad i came across this blog, you really seem to know your stuff!

I would like your advice about some power banks I have recently purchased from china as i am currently in dispute over the specification with the supplier. The power banks i purchased state that they are 50Ah capacity. I do have some degree of electronical knowledge as i studied a-level technology and briefly studied electrical engineering several years back. As you have mentioned in a previous point i was going on the basis that this would provide 1A for 50 hours and inversely would take 50hrs to charge at a rate of 1A. After some research and charge / discharge tests it seems the actually capacity would be about a 1/5th of this as it was fully charged in about 10hours. I have included two extracts (Sorry about the big post) from the supplier below and would hope you can make more sense than me of this as the english is terrible! What I would like to know is if i am misunderstanding the specification and the correct definition is actually 50Ah or is the supplier trying to con me?

"Power bank real capacity is 12000mah. The really full charge 50000mah battery is more than 5KG weight .It is very dangerous and inconvenient. So the market is almost no more than 13000mah, let alone 50000mah. Special Note: our solar power bank built-in electric current and voltage protection of 50000mah, if your area voltage instability or excessive current input, our 50000mah play a very large role in the protection, safeguard the security of each buyer, which is also the advantage of our products and new technologies, so this product is defined as solar power bank 50000mah, you understand it?

"Our electronic components and batteries imported technology from Japan, its quality is very good. 50000mAh mean explanation can carry 50000mAh energy, charge for 48 hours or even 72 hours can be achieved 50000mAh. Simultaneously protected and will not explode this is the EU's safety standards, but it is when the output current. impossible to achieve 50000mAh energy. scope of its output value to between 12000mAh 15000mAh. 12000mAh lowest output standard, you understand?"

Thanks

Gerard
Hi, Gerard.

First, a little nitpick: a battery that can do 1A discharge for 50 hours does not mean it can get charged in 50 hours at 1A. But still, if you're getting a full charge at 1A in 10 hours and not doing anything special voltage-wise, you can be sure the capacity is nowhere near 50 Ah.

Your supplier seems to acknowledge that the actual capacity is 12 Ah and gives you the basic math that a battery with 50 Ah would weigh over 10 pounds. I don't know what your actual thing weighs, but if it's a couple of pounds, then that would be another sanity check that your battery has nowhere near 50 Ah.

The last part does start sounding like BS. Maybe he's just trying to say that the electronics is good enough to support a 50 Ah battery if you had one there. Perhaps this is some modular product where the battery part can be upgraded or retrofitted.

- Jan
Hello,
New bee here. I am building an e.v. and my motor states that it is 3.2amps. 90volts. What should my battery specs be if I put 48 volts only? My load should be 200 lbs. Please help.
Follow up. Reason for under. Voltage is weight and space.how powerful a battery should I install? BTW , what happens to ah(amp hour) if I put the batteries in series?
Are you building a motorized scooter?

Load = name for an electrical circuit (in your case, a motor)
At 48 Volts, your motor will probably turn if it's still new, but it'll be performing at half power, likely at 1.7 Amps.
You should be supplying your motor with 90 volts like it says, then I assume you have some kind of circuit for modulating it's speed???

I think you missed the point of the topic. The "Ah" of the battery is it's capacity, not how much current it can discharge into a short. A car battery can discharge a lot more current a lot faster than other types of batteries. For your applications, lead acid should be sufficient. I going to pretend you find 90Volts like the specs demand. If you find a 90 Volt, 12Ah battery, your motor should run great for about 3 hours, then it'll be slowing down.

Remember, 120V AC doesn't seem very scary. But you stick to DC voltage. 90 Volts is enough to kill you. Be careful.
Oh, and the Ah don't add or anything if the batteries are in series.

Just your voltage goes up. If you put different rated batteries in series (you shouldn't do that) then you can more or less take an average of the Ah rating. But if you use different types of batteries with different ratings it'll affect the performance and life of your batteries.

So 8 X 12V batteries all rated at 10Ah each, connected in series, will make you a 96V, 10Ah battery.
Hi Jan,

This was a great article. I'm a little confused though. Are you saying that a single AAA battery will last longer than a single 9 volt battery? Why aren't there any simple charts out there that list how many mAh an AA, AAA, and 9V battery will last?

Thanks,
"How many mAh a battery will last" is not a good question formulation because it sounds like you want to know how long a battery will last just based on its capacity. It's like asking how many days until you need to refuel your car that has a 12-gallon gas tank without saying anything about how much you drive per day.

As I said in the 9V alkaline battery example, you're basically going to get about the same amount of energy per weight for any given battery chemistry, so since a 9V weighs about the same as a AA, it's going to have about the same amount of energy, and since energy is measured in Wh, which is Ah times V, a 9V battery will have about six times lower mAh rating than a 1.5V battery of the same weight. If you happen to know your application for the 9V draws the same current as a specific application for AA or AAA, then yeah, it will not last as long, but that kind of situation is rare since there aren't many products that can run from either 9V or 1.5V.

- Jan
The capacity of any particular battery also depends on several things other than its size, including its chemistry and manufacturer, but this Wikipedia article lists some typical capacities for common sizes:

http://en.wikipedia.org/wiki/List_of_battery_sizes

- Kevin
I am not sure if I did this correctly or if this will post twice, but I am looking for battery feedback on replacements for an electric scooter. I am concerned that the three factory installed ones are rated at 12V 12A/5h, yet the ones I see on line claiming to be the acceptable substitutes are 12V 12A/h. Would you expect that this means the replacements would have 1/5 the stamina of what was in the scooter at the start ? Thank you.
Dan,

I don't know what that 12A/5h notation indicates. My first guess is that it's a typo or other mistake; my second one is that it's 12Ah at a five hour discharge rate. If the batteries weigh about the same and have similar dimensions, their capacities will be similar.

- Jan
Ok, i was reading thourhg the post, but didn't get really andswer of what my question was, so i allow myself to add some new!!!

well, here it goes.
I am user of an audio device, the H4n recorder, using phantom power mic on. The h4n is using 2 AA batteries but by giving 48v to the mic, the power runs down very fast. And i need to be able to record sound for longer period than one hour!!!

so i am trying to find a solution.
the H4n as a input of 5v 1A...
i am thinking of building a little set that would give this power to the machine.

but i can't understand what to expect.

let's say i am plugging together 4 rechargeable AA, i should get 4,8 v. But when i check the battery they give more 1,25V than 1,2V, so than would come pretty close to 5V.
Now... the batteries are 2500mAh.. more or less...
But i can't understand the link.
If i put 4 AA, i am supposed to get way more than 1A... or? Is there a risk of burning the audio device?
Or this 1A just means that is what it request from the battery to operate, and they will gently give it enough....

Some electrician in a store told me i would have hard time to run the device, as i would hardly get the 1A requested...

so?
Shall i make to sets of 4 AA in parallele, getting still 5V but more power....

or this is way too much for the audio recorder...

if you can help in these dark thoughts!!!
have a good day!!!
I stumbled across this article as part of some research into project. I will be turning the back of my pickup truck into a small camper and will be powering an inverter with a deep cycle battery from a semi. My main concern is with charging. From what i understand about lead acid batteries, they require little to no circuitry for charging. As such I am planning on connecting this secondary battery up directly to the rest of the charging system (primary battery included). The only circuitry I will be adding is a relay to disconnect the secondary battery when the ignition is turned off so I do not accidentally discharge the primary whilst I am camping. My main concern is whether or not I will see any adverse effects. What do you think?
Tchopko,

I am concerned that with the level of electronics understanding you have presented, you might be risking damaging or destroying some fairly expensive equipment. It seems like you might be better off looking for some pre-made solution, like a general-purpose external/backup battery for phones or other electronics.

On to your specific case. First off, you should realize that with four batteries instead of the built-in two, you will get at most 2x the battery life. You should not put your batteries in parallel, and if you just put all your batteries in parallel, you would only have the 1.2V individual battery voltage anyway. You should also go through a regulator, too, to make sure you actually give your device the 5V it is expecting since the fully charged batteries might be 5.5V or more and damage the device, and they might get too low as they discharge. Something like this step-up/step-down regulator could work:

http://www.pololu.com/product/2123

With that regulator, it would be better to go with 6 AA batteries if you have the room for it. But, if you go down this path, make sure you get a better understanding of what you are doing and that you are ready for the consequences if you make a mistake.

- Jan
R. Keisler:

I don't know enough about your system to know for sure. There's got to be some limit on your alternator and whatever battery charging circuit is in your car, and while my impression is that lead acid batteries are among the more forgiving regarding charging, you still might limit their life or otherwise damage them if you just do what you are proposing.

- Jan
Hi and thank you for sharing of your experience. Clearly there is a need for information such as this.

Here is my inquiry:

I plan on running two 50mm fans inside a costume helmet for air circulation. The fans spec at 5v 0.1watts under normal load.

I wanted to power these two fans using one of those portable lithium-ion cell phone back up batteries mainly due to their compact size. (Limited space inside a helmet) Most other costumers use 4x AA batteries (6v) my question is can I expect similar battery life using a 5v 2600 mAh battery pack instead?

Other than voltage and capacity no other specs are provided for the battery pack.

Thank you in advance.

Adam
Adam,

Why don't you just try it? (And I don't mean that in the "go for it" sense. I'm wondering what motivated you to write here as opposed to trying it and timing it.)

- Jan
Thank you Jan, my motivation was hoping to get a rough idea of run time based on mathematical formula before I invested in some equipment. :)

Is there a way to estimate on paper? But you are right I could just try it...hahaha

Regards,

Adam
As a rough approximation (within a factor of two), I would expect similar battery life. The main thing I'm skeptical about is that 5V is not a natural voltage for a lithium ion battery. If there's actually a 3.7 V, 2600 mAh battery inside and the converter to 5V is 90% efficient, you'd be getting the equivalent of about 5V, 1750mAh.

- Jan
Thank you Jan for the info. I will be testing the set up shortly. I only need run time of 5 hours at a time.

Adam
Adam,
I'm going to try to answer your question.
Jan,
Please comment if I got this wrong.

Two fans each 5v and .1w
5v 2600mAh battery

To figure out the current draw of one fan.
Watts = volts x amps
.1 w = 5v x .02 amps
.02 amps x 1000 = 20 milliamps or ma

So one fan will draws use up 20 mamAh if left on continously for 1 hour (corrections by Jan)
(or 10 mamAh for 30 min, 5 mamAh for 15 min, etc.)

Two fans x 20 ma = 40 ma draw per hour
2600 mAh battery / 40 ma = 65 hours

In reality it will be less than that, but it gives you an idea.

But more than 5 hours. Stay cool. : )
Anonymous,

You start out ok, up through where you got the 20 mA, but then you got sloppy with the units in exactly the way this blog post was supposed to help people avoid. 20 mA is already a rate, and as long as that fan is running, the current is 20 mA, and it is wrong to say 20 mA per hour or 10 mA for 30 minutes. It would be correct to say the battery will deliver 10 mAh in 30 minutes.

I've crossed out the incorrect parts in your comment in the hopes that it will help others avoid this kind of mistake. The 65 hour result based on the starting assumptions is right, though.

- Jan
Hi Jan,

Thanks for the very helpful information, I've learned quite a bit. But I am still unsure of how to go about connecting my 5 watt portable amplifier. It is powered by an AC adapter or 6 "C" type batteries. I want to play my acoustic guitar hooked up to the amplifier but since I live in S. America, "C" type batteries are hard to find, and very expensive. I am thinking of buying an inverter (smallest I could find was 200 watt capacity) and a 12 volt motorcycle battery for portability but I am unsure of how to proceed with the 12 volt battery. What aH would suit me best? I would only be using the amplifier for 20-30 minutes at a time. Thank you for your time and expertise!

-Jon
Jon,

More Ah will just last longer, and since you don't need much, it seems like the primary consideration should be size or weight. Separately, though, I think this 12V battery->inverter->AC adapter route is quite awkward. You can probably get by with just using AA batteries. You could either set up your own battery holder or get some AA to C battery adapters, which are just sleeves or shells that fit around AA batteries to make the final diameter match that of C batteries.

- Jan
Thanks for your prompt response. I've thought about the AA to C battery adapter route, but was afraid I may do some type of damage to the amplifier or its components. I don't know much about these things, but do you suppose this would be safe to use? I don't much about using those adapters but I am willing to give it a try! I will have to look up what the general consensus is regarding using rechargeable batteries for amplifiers. I could either do the rechargeable C batteries or the rechargeable AA batteries into the C adapter. What do you think? Thanks so much in advance! Since I am in S. America, people are not used to anything but plugging into the wall and know nothing on the matter.

-Jon
If you can easily get rechargeable C batteries, definitely just try that. (Rechargeable C batteries are often just effectively AAs in a C sleeves, anyway.)

- Jan
Is it okay if a battery ( Ni - MH ) rated 6 volts w/ 2800mAh will be charged with an 11-13 volts( output), 850mA charger?
Is 9 volts, 1 ampere charger is much better than 11-13 volts( output), 850mA ?
Jun,

It is not appropriate to think about an "is it okay" question based on the specs you have given. You can use the current and voltage specifications of the chargers to get a rough best-case estimate of how long it will take to charge the batteries, but whether it will work at all or be safe is a completely separate issue. You should only use chargers that are specifically made for your kind of battery pack (and you should make sure you know how to configure them appropriately).

- Jan
I'm building a power pack for recreational use, what would be the most efficient battery if I were trying to run my car charger off of a battery for my phone? (1
2v 2amp draw)
Tyler,

Your post looks sloppy and lazy, making it difficult for anyone to help you or to want to help you. For instance, I suspect you are not actually trying to run a 12 V cell-phone charger off of your phone battery. But even if I assume you want to make a portable power pack into which you can plug in your phone charger, I still cannot really understand your question. What are you trying to achieve? Why do you care about efficiency? Typically, in an energy conversation, the relevant efficiency is how much energy you spend charging your power pack vs. what you get back out of it, but it seems like you might care more about being cost efficient or size efficient.

- Jan
Very well written report Jan! You've managed to explain technical stuff while keeping your language in common terms.

Just an FYI to the readers who are new to electronics, that the AMPERE is just one of many SI units. The "Système International" was invented by a bunch of French scientists and they consolidated all the units in a way that you can very easily do mathematical conversions and calculations. Some examples:

Water freezes at 0 degrees Celsius and boils at 100 degrees C.

1 Gram of water = 1 milliLitre = 1 cubic centremetre
So if i only had water and scales, i could work out Volume!

By the way, the metric system is a part of the International System of Units. Americans really need to get with the times and ditch Imperial!

http://en.m.wikipedia.org/wiki/International_System_of_Units

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