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Regarding Battery Selection

Hey Guys!

For controlling two 12V motors, I'll be using an 25A sabertoooth motor driver. For powering the two motors I"m choosing an 11.1V/850mAH/3S/25C Li po battery pack.  Each motor draws 9.5A(max), so totally for both the motors the current draw is 19A(max). The battery provides 21.25A. So is this battery pack wise? Or should i go for an battery pack that provides higher current?


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Basically the maximum output current of the battery should be higher than your expected peak current draw. If your robot draws 20A under stress and your batteries can deliver a max of 10A you could burn the connecting cables or electrodes and lower the life of your battery.

SLAs are probably not a good idea for a battle bot because of their weight and low discharge rate. I would instead stack up some LiPo packs.

Hmm..another reason I'm prefering SLA now is because of its price. They are pretty darn cheap compared to Lipo's and for charging Lipo's i have to get an special charger. An 4.5AH SLA weighs about 1.5KGs. That weight doesnt seem to bother me now.

The above mentioned Lipo pack provides a max current of 21.25A. How much max current will an 4.5AH SLA battery give? How should i calculate that?

And What do u mean by low discharge rate? Is that a real problem that should be looked into?

The Li-Po pack you first mentioned had these specs: 11.1V/850mAH/3S/25C

I believe the 25C at the end means it supports a peak current of 25A. So it is just within range of your peak motor load.

25C means it has a peak discharge or max current of 25*Capacity = 21.25A. On the original pack the only problem with it was that it was too small to give a run time of more than 4-5 minutes.

Your math seems to check out fine. But this applies to magical batteries that live in the theoretical world only.

The problem with bats is: the actual current drawn (on average or at any given moment) will greatly influence the capacity the battery will be able to give out. More current means less capacity. Or at least this time around. The data sheet for a battery will hopefully be able to tell you more about this effect.

The inverse is true as well: when you use your battery conservatively (low current), it will be able to "squeeze out" more Ah, over the course of this one charge.

Different battery chemistries behave differently in this regard. I am not sure how Lithium batteries are better than Lead or Cadmium or Manganese.

Check out this walk through for the nitty gritty.


thnx buddy:) Nice article!