DC gearmotor stall current and battery capacity

I'm working on a self-balancing robot project and am in the process of motor sizing. I've read that with self-balancing robots the motors need to switch direction quickly, which can cause them to briefly draw more than twice the stall current if changing from full speed in one direction to full speed in the other direction.

So I'm wondering if, when I find a motor with a suitable rated torque to drive the robot, would I need to work out if the battery can supply a current twice (or more) times the stall current of the motors/how long it would last before being drained?

e.g.

Motor Stall Current: 5000mA = 5A
12v Ni-MH Battery: 3800mAh = 3.8Ah $$I_{stall} = (5A*2) = 10A \,\,\,\,\,(2\, motors)$$

$$C_{battery} = 3.8Ah$$  $$I_{stall} = \frac {C_{battery}}{t}$$ $$t = \frac {C_{battery}}{I_{stall}}$$

$$t = \frac {3.8Ah}{10A} = 0.38h$$

So the battery could supply twice the stall current for 0.19h.

Am I understanding this correctly?

You will almost never get the motors to sink their stall currents... But yes, your robot will last 0.38 hours. Also, it's good to know that you should not reach the 80% of the battery's nominal voltage (at least not in LiPo or NiMh). So be careful!

• Could you go into a little more detail about the motor stall current and 80% nominal battery voltage? – somers Jul 9 '17 at 1:50
• @5_5 A motor's stall current is its maximum! You will never get over 5A. – Iaka Noe Jul 10 '17 at 10:34
• @5_5 Talking about batteries, from my bad experience, I can suggest you never get to 80% of the nominal voltage of the battery. For example, a 2 cell LiPo nominally has 7,4v (3,7v each cell). Although, its maximum charge is about 8,4v (it all depends of the single battery). And, for your security, never go less than 80% of 7,4v (~6v). I don't know why, but that's the lowest number that worked correctly for me. Also your robot won't work nicely at "low" charge so... ;) – Iaka Noe Jul 10 '17 at 10:35
• Thanks Iaka :) So what I read about motors briefly drawing more than twice the stall current.... is incorrect? My Ni-MH battery has 10 cells, each having a nominal voltage of 1.2V. So that's a total nominal voltage of 12v. And to fully charge each cell their voltage needs to be pushed to 1.6v-1.65V. At the end of the charge cycle the battery voltage should be around 16v-16.5v? And I should not go less than 9.6v (80% of nominal voltage) ? – somers Jul 10 '17 at 18:24
• @5_5 now you're right – Iaka Noe Jul 10 '17 at 18:27