# 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?

Thanks!

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You are confusing battery capacity with battery output. 850mAh is the battery capacity, 21.25A is the max output. If you assume your motors will draw an average current of about 6A over the course of the match then 12A out of a 0.85Ah battery means your battery will be dead in a minute.

You can`t power a 30kg fighting robot with a 200g battery.

This is better. 2200mAh means you might get about 5 minutes run time.

Now I'M confused. Check my math, Ezekiel.

0.85 AH x 60 minutes/hour = 51 Amp-minutes.

With your assumption of 12A average current consumption, I get:

51 Amp-minutes / 12 Amps = 4.25 minutes.

A 2.2 AH battery x 60 minutes = 132 Amp-minutes. With the same 12A average current assumption, that's:

132 / 12 Amps = 11minutes.

Am I missing something?

You`re right, I hate doing math so I try to avoid it if possible! I did edit the final time from 10 minutes to 5 just after posting it because I think 12A is pretty darn conservative for a battle bot - pushing adversaries, jumping from forward to reverse, running weapons.

With a current draw that is 6 times the battery capacity you will get 1/6th of the original running time(1hr).

Hmm..So I should opt for an high AH rated battery rather than a battery that provides High output. For example if i use an 15AH rated SLA battery,

15AH*60=900 Amp Minutes

With an average current draw about say 20A, I would get..

900/20= 45 minutes of run time..Am I correct?

But the main problem with SLA's is that they are heavy. The reason i was opting for Lipo pack is because of their light weight. I could add up the weight in Building an sturdy chassis.

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!

You have the calculations correct. How long does a match last? You will want to make sure your bot can last through a whole match, probably with a 2-3 times factor of safety. Are the matches timed or to they just go on until one bot wins or the other cannont continue?

Design is all about balancing your requirements. In this case, weight vs battery life.

The match lasts for a maximum of 5 mins. So i think the best option is to opt for an 4.5AH SLA battery as it weighs lesser than 15AH battery. 4.5AH battery would last for around 13.5 mins approximately. Considering the losses, it should last atleast for around 10 mins rite?

Another doubt regarding Li-Po battery packs guys!

Consider the pack I mentioned in my first post. The maximum current it can give is, 0.85*25=21.25A. While calculating, for how long the battery will work for an given amount of current draw, we are using the Ah rating of the battery. So when must the maximum current output of the battery must be considered? In this case for what purpose 21.25A must be considered?

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.