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[on:]

I am using a 2S 8.4V 2000mAh 2C Li-ion battery for my small electronic projects and I would like to buy a bigger one - a 3S 12.6V Li-ion battery - for my remote controlled robot that would consume about 2A. But I am a bit confused about what mAh rating should I select.

From what I've read from various websites, I understood that a 2000mAh battery will get completely discharged if 2A is drawn from it continously for 1hr. But what does this term 'completely discharge' mean? Is it that the battery's voltage will come down to 0V from its full value?

Also, the manufacturer warns that the voltage of individual cells must not be less than 3.5V otherwise standard chargers won't be able to charge the battery pack. Taking this into consideration, for a 12.6V battery, its voltage should not be less than 10.5V. So how do I approximate the time required for the voltage to reduce from 12.6V to 10.5V based on the mAh rating if the robot draws about 2A?

Thanks in advance.

[Reply #1 on:]

You're overthinking this a bit. Simply put you can accept that your 2000mAH battery will supply 2000mAH as it discharges from its maximum voltage (typically 4.2V per cell for LiPo) to its "discharged" voltage (3.5V).

So how do I approximate the time required for the voltage to reduce from 12.6V to 10.5V based on the mAh rating if the robot draws about 2A?

1 hour. The actual length of time varies based on the rate of discharge, age of the battery, temperature of the battery, whether or not the manufacturer is lying, etc. so it's typically not worth the effort to calculate the capacity beyond the basic definition.

[Reply #2 on:]

Hi,
   You do need to think about the 3.5v, I have ruined more batteries than I would like to admit by taking them below their minimum charge.

   If 8.4 volts is enough for your application I would stick with 2S LIPO batteries and get something around 3,600 mah. These have been around for a long time - so are cheap and reliable. The higher capacity batteries up around 5000mah are pushing the technology, are way more expensive and much less reliable, personally I wouldn't touch them.

   You could also look at having two batteries - not connected in parallel, but one for the left side and another for the right side (assuming its some type or symmetric vehicle), this will also extend your run time as each battery is only having to do half the work.

To protect your investment in batteries, get yourself a LIPO Alarm or build one into your Arduino using a voltage divider and an analog input, sound a buzzer and stop the vehicle if either batter gets below 3.6v.

Duane B

rcarduino.blogspot.com

[Reply #3 on:]

Thank you Chagrin and DuaneB for your advice.

My robot requires a 12V supply and it should work continuously for an hour(with short gaps in between) while drawing less than 2A and what I'm concerned about is the battery's discharge rate. If the voltage drops to 10.5V quickly, I will have to go for a 4000mAh pack.

You could also look at having two batteries - not connected in parallel

Is it not recommended for Li batteries?

And one more question: Is it necessary to take extra precaution against high current draw? Will the battery be damaged if current more than the rated value is drawn from it?

[Reply #4 on:]

Hi,

The ability to deliver a sustained current is given by the 'C' rating of the battery, my 3600mah batteries are 20C, this means that are happy to deliver 3600/1000 = 3.6 Amps * 20 C = 72 Amps.

I could not give a confident answer so I suggest doing your own research regarding connecting various battery technologies in parallel or not.

Duane B

rcarduino.blogspot.com