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Understanding battery capacity: Ah is not A
- 24 October 2019Brad,
You're kind of all over the place with the units, mixing Ah and A, which is the main point of this blog post. However, for your case, most of that isn't that relevant, anyway, since you have a much more specific application, and the main challenge for you is how to charge batteries. In general, you can't just "plug in the solar panel between the batteries and the motor". Lithium-based batteries are especially touchy and dangerous, so I would stay away from them unless you have a product that is a complete solution--and then you wouldn't need to ask how to set that up.
If this were my project and I were trying to do it cheaply/minimally, I would see if a 12V lead-acid battery would give the blinds enough juice. They might run a little slower, but probably it will be ok. Lead-acid batteries are a lot more forgiving, and a 12V battery could probably be topped off by just connecting two of your 8.4V solar panels in series and through a diode.
- Jan
Understanding battery capacity: Ah is not A
- 23 August 2019Hi.
It's difficult if you cannot do any math. You could try that pack you're talking about, and if it doesn't last long enough, get a higher capacity one. If it ends up lasting way longer than you need, you'll know next time that you could get a smaller one.
- Jan
Servo control interface in detail
- 12 July 2019Since you seem interested in the details, the speed is not "12.5 μs"; rather, it's ``(12.5 μs)/(10 ms)``. To see how much the position changes from one time to anther, multiply speed by time: in 20 ms, the position will change by ``20 ms * (12.5 μs)/(10 ms) = 25μs``. Kind of like if you have a car that is going 65 MPH, and you check its position two hours after an initial check, it should be 130 miles farther. That you happen to check 2 hours apart doesn't make 65 MPH stop being a useful measure.
Your seeing that some descriptions say D and others D/2 gives us good indication that the term is not particularly well defined, or that it gets used sloppily, so there's not much point in having a "correct" definition since you still need to determine what the source using it means. I think deadband of D should be the whole region of no response, which therefore means some target plus or minus D/2.
Servos are usually not going to just have proportional control. You can take a look at our Jrk motor controllers if you want to look more into what might go into a servo:
https://www.pololu.com/category/95/pololu-jrk-motor-controllers-with-feedback
- Jan
Servo control interface in detail
- 9 July 2019Hello.
I think you can gradually increase the pulse rate to the servo while moving it around, and if it behaves normally, it will probably be fine. Be ready to cut power if it tries to go past its physical limits (end stops).
The Micro Maestro does its internal position update math every 10 ms independent of the pulse rate. That keeps the positions sent to the servos over time the same even if you change the pulse rate. You seem to be focusing on the servo side (and how it can only get an update once per period T); the point is that the servo controller only tells the servo what to do once per T, and the rest of the time, it's getting ready to tell the servo the right thing at the right time. Think of a clock with a second hand that ticks every second: it's updating the time it's telling you once per second, even though internally there could be gears and pendulums and other things moving between the times when the second hand is moving. The details of what it's doing internally to give you the correct updates are going to vary from clock to clock.
- Jan
Understanding battery capacity: Ah is not A
- 2 July 2019Jim,
Battery voltages always fluctuate as they discharge, so you definitely don't need "the exact same numbers". Also, an AC power adapter does not have to connect to exactly the same point in the circuit as the batteries, so you shouldn't put too much stock in the power adapter specs for the purposes of battery selection. But in this case, the numbers match up since four AA alkaline batteries in series will give you 6V.
I don't know what you mean by "they worked once" and what the relevance of that is. Does it mean that they don't work anymore, even after charging? In any case, alkaline batteries have a little higher voltage than NiMH batteries, 1.5V nominal vs 1.2V nominal, so four of those in series gets you to 6.0V vs. 4.8 V, so some devices don't work very well or very long if they really need the higher voltage. A charged NiMH cell might actually get to about 1.4V, so it looks a little like a somewhat discharged alkaline right off the bat.
- Jan
Understanding battery capacity: Ah is not A
- 28 June 2019Hi, Jimmy.
I think that depends too much on the details of the particular batteries and what exactly they've been through. Age-wise, those 4 months don't seem like too much of a difference, and 65 cycles also doesn't seem like that much, so my guess is you'll get more life out of the 2.0 Ah battery.
- Jan
Understanding battery capacity: Ah is not A
- 26 June 2019Hi, Maxwell.
Since we are talking about batteries and DC power, a transformer (which is for AC power) is not quite the right term; something like DC-to-DC converter is more appropriate. In your case, we are talking about just reducing the input voltage, so you could use a switching step-down regulator that can be around 90% efficient. Having double the voltage with the same amp-hour capacity means double your energy, so as long as your conversion to the lower voltage is more than 50% efficient, you will get more run time with the higher voltage. However, your battery will be twice as large, so you could probably just get a 11.1V battery with 12000mAh in the same size as the 22.2V 6000mAh one, and then you don't have to bother with the voltage conversion at all and get double your run time.
- Jan
Understanding battery capacity: Ah is not A
- 29 April 2019Mark,
I expect the maximum allowed current to remain the same whether you have multiple batteries in series or not. One battery cannot tell if there is another battery somewhere else in the circuit; all it has is the voltage across it, which would still be 6V, and the current flowing out of it (or in, if you're charging it).
- Jan
Understanding battery capacity: Ah is not A
- 14 March 2019Hello.
The "225 AMPS" might be sloppy labeling, but it might also be the maximum recommended current the battery can supply, or even an outright scam, so I would not go with that option if that is really what it says. C/100 vs C/20 is probably about discharging in 100 hours vs 20 hours, so using the 20 hour value is probably a better estimate for you. Given all of your confusion, I would be worried about your 588 Ah requirement calculation, but if it's correct, three of the 190 Ah batteries would almost cover you, so 4 might be enough. Going to 6 because of your 50% discharge guideline would give you more room for error, though that 50% rule might also vary with battery type. This is again assuming you need 588 Ah at 6V, in which case you'd put all the 6V batteries in parallel.
I don't know your time and cost constraints, but you could start with a smaller quantity and just test it. And if you need more run time, add some more batteries.
- Jan
Understanding battery capacity: Ah is not A
- 1 March 2019Hi, Keiran.
No, you cannot trade off the voltage for current in your jack. That 1A rating is going to come from things like how much the contacts or some other parts the current goes through heats up, so if you put 3x more current through, it's going to heat up possibly 9x faster (for a fixed resistance, power goes with current squared). 1A sounds low for a power jack, by the way, so it's possible it's a really cheap unit or they're being really conservative with the rating.
Separately, you should also not connect the AC adapter straight to your battery and expect it to act as a charger. Best case, nothing will happen (i.e. you also won't charge your battery); quite possibly, you'll also break something.
- Jan