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Monday, June 23, 2014

Arduino Robot - LiPo batteries update

I said (here) I did not know much about LiPo battery output and their maximum discharge rate.
I made a few assumptions and simply said that my 1200mAh 25C should have been more than fine (for this project) since if it could power an RC car/helicopter, then it should have been more than enough for my  two small DC motors and few other digital electronics.

Still, it was a good opportunity to understand some  more and I researched the meaning of that "25C", here the results :

When you have a battery pack that is declared at xxx mAh you actually know the maximum charge that can be stored in it, but that does not tell you how fast you can (safely, possibly) discharge it or in other words, how many Amperes of current you can draw from it.
1200mAh means that if you draw 1200mA continuously, a completely charged battery will be depleted in 1 hour (assuming you can pull all the charge, which is quite unlikely due to voltage drop that goes with it, plus it is advisable to pull only 80% max of the battery charge to prolong its life).
That does not leave any clue on how you can use that charge, is it going to be at 10mA at a time, at 10A or else?

That's  where the "C" rating comes into place for LiPo batteries.
From what I found the C coefficient has to be multiplied by the total charge of your battery (1200mAh in my case) and that gives the maximum (peak) Amps you can draw.
Now, a bit of math tells us that a 1200mAh @25C can output a whopping 30Amps peak.. at least in theory.
Honestly I don't really think that figure is anywhere near a realistic one, 30A is a scary amount of current even at the moderate voltage we are dealing with (average 7.4V, up to 8.something).
We are talking about 220Watts of energy (7.4 * 30)!
Granted at 30A you would drain the fully charged battery in only 2.4 minutes (1.200/30.000*60)... but I still believe that 30A * 7.4V for two continuous minutes are still a serious business.
I initially thought I had it all wrong, but searching on the web it seems these are the correct figures (!!!).
One of the sources here.

However, when I look at those small wires coming out of it, I see something that seems to have pretty good quality, but still I hardly see them carrying 30Amps without transforming suddenly in a smelly ball of smoke.
I would not feel really safe exceeding 5A, which again is about 10x more than I need.

In conclusion : The max current seems more than adequate for my purpose, all hail to the magic of LiPo batteries... yes but...
Right, is this all so cool, no flipside?
Well, it is until you happen to short circuit your battery, in that case I fear the best thing that can happen is a bit of smoke, fire if it was not your lucky  day and a nice "kaboom" if your horoscope that day was not favorable (in that case you probably deserve it, at least for believing in horoscopes!).
With big power comes big responsibility, play it safe! 

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