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:
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.
40 Comments New Comment
24 Nov 2010
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.
24 Nov 2010
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.
21 Mar 2011
4 Jun 2011
This is very, very helpful thank you.
10 Aug 2011
great help, thanks...
9 Sep 2011
Immensely helpful: 1 AH = 1 amp for 1 hour, or 1/2 amp for 2 hours... I always wondered about this! Thanks very much!
28 Sep 2011
It is a good writting for the new robot builders
Es un buen escrito para los nuevos constructores de robots.
19 Feb 2012
Great article. Really helpful for battery selection
26 Mar 2012
"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).
27 Mar 2012
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.
27 May 2012
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?
29 May 2012
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).
17 Jun 2012
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!
18 Jun 2012
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.
28 Aug 2012
how much time it will required to complete discharge of 2000mAh cell phone battery.
31 Aug 2012
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.
16 Nov 2012
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!
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??
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!!
19 Nov 2012
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.
9 Jan 2013
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
9 Jan 2013
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.
9 Jan 2013
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?
10 Jan 2013
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.
10 Jan 2013
hi Jan, :)
this is getting interesting..
so it all depends on the sucking or pulling of the reciever.
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
input: 100-240V 50/60hz 0.5A
output: 5.0v 2.0A
all the best :)
10 Jan 2013
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).
11 Jan 2013
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.
6 Mar 2013
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?
7 Mar 2013
I can confirm that your two specs do not make sense. Without any context, it's difficult to guess the intended meaning.
30 Mar 2013
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?
1 Apr 2013
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.
3 Apr 2013
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.
4 Apr 2013
I showed you the exact assumptions and calculation that led to the 2.5 Ah result.
4 Apr 2013
Too slow in catching up the logic!
1 May 2013
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…
2 May 2013
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.
12 May 2013
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?
13 May 2013
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.
19 May 2013
very nice . tnx
20 May 2013
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!
20 May 2013
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.
21 May 2013
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.