I have a 4000mah battery pack, need to figure out the amp draw, is my math correct?

Question

So, I would like to purchase a 10,000mah battery pack. But the pack is only able to handle 40amps continuos. So I do not waste money I'm trying to figure out what I am currently using amp wise.

So here is my math.
4200mah=4.2 amph hours.
So this 4.2 amp pack would last an hour if there was a continuos 4.2 amp load.
At 8.4 amps continous this pack would last 1/2 hour.
At 16.8 amps continuos this pack would last 15 minutes.
Since my current 4.2amp pack lasts for 12 minutes about, then I'm probably only pulling 20 amps continuos through this pack?

So it would be safe to use the 10,000mah pack since it is rated at double the continuos load.

The reason for my doubt, is this electric motor was supposed to be able to draw 80amps continuos.

Since I have room to play with Amp load, I could add a couple of cells to the pack, go from 7.2volts to 9.6v and also put a faster gear on the motor, while still being safe in the 40amps continuos range?

Watts you think?

Answer 1

Your calculations are not correct. As another poster stated, when you are discarcging the battery at very high rates then you get less total electrical energy out (the balance being waste heat) to power your motor. The way you get the rated energy capacity is at low discharge rates, the opposite of your electric motor. So you are probably actually putting significantly more than 20 amps through your motor. You can actually measure the current you are getting from your battery by putting a device called a shunt in series. This devices has a known, but small resistance across which you measure the voltage. Ohms law gives you the current. There is a new battery technology coming out of MIT with super high discharge and carge rates. Should be awesome for electric RC.

Answer 2

If you increase the amps draw (current) the amp hours will decrease because of the heat waste, and the nature of batteries, also I think that rating is like under optimum conditions, like low draw for long time. If you add more cells in series to increase the volts the motor rpms will increase and so will the stall current which can make lots of heat and torque. Dc motors are rated amps at full sustainable load at rated speed. The torque is directly proportional to amps available and speed is directly proportional to volts available
You can double the power pack in parallel and get more torque and battery life. You didn't state your application so I can't give more advice.. By the way it is OK to overload dc motors severly for short periods of time without serious consequences, but like an engine, it does shorten the life.

Answer 3

Don't change the voltage. If you increase the voltage, your motor would be able to draw more current than it was designed for. You could also burn out the control electronics.

You're correct in your amp hour calculations. Yes, you will be able to use the 10,000mAh (or 10Ah) battery with no problem. At 80A, it will last 1/8 hour. But, you won't be drawing 80A all the time. I assume this is some kind of RC car?