When motors at full speed, robot turns off

I’m having a problem with the motors on the 3pi. If I run the demo program (The one already programmed onto the robot when you get it), and go to the motor test. I can run both motors fine. But when I try to use both motors at once, when they reach close to full speed, the robot just turns off. I don’t think it’s a problem with the code, because I didn’t change it, and it used to work. The battery level is fine and over 4500mV, and all the other tests in the program work. Help would be appreciated.


Battery level of 4.5V is definitely not fine, especially since that is probably at no load. Fully charged batteries will typically give you above 5.4V, and you should probably recharge by the time you’re seeing 4.5V with no load, especially since the behavior you are describing is exactly what would happen with dying batteries.

- Jan

I guess that’s what threw me off. I tried changing batteries, and the new pair I guess was also used. I’ll try again with a completely fresh pair, thank you.

EDIT: It worked, thank you. I’m also thinking of now editing the program to give a value of percentage full, what would be a good value for completely charged?


That will depend on whether you’re using NiMH cells or alkaline cells. The easiest way to find out is to put a set of new alkalines or freshly charged NiMH cells into your 3pi and see what the reported voltage is. In my experience, freshly charged NiMH batteries are usually around 5.5 V, give or take a few 100 mV, and as you’ve seen, they’re quite close to drained when you see a voltage of 4.5 V without any load.

- Ben

I’m using alkaline cells at the moment, and plan to get NiMH ones soon. By assuming that 5400 mV is full and 4500 mV is empty, my percentage change still drops very fast. What would be better values for alkaline batteries? The four I’m using aren’t fresh, so I wouldn’t know right now.

Alkalines should start at a bit over 6V (1.5V per cell), so 4.5V is even more dead for them, especially since they tend to have a higher internal resistance. You might use 6V and 4.5V as your 100% and 0% points, though as you’ve seen, 4.5V doesn’t mean you have no power. Battery voltage tends to drop quickly at first, then flatten out for most of their usable range, and then start dropping quickly again. You can search for something like “battery discharge curve” if you want to see some graphs.

By the way, because of the 3pi boost regulator arrangement, the drop off at the end is even more severe since at lower input voltages, the 3pi will draw more current, draining the batteries even quicker.

- Jan

Thank you very much, I’ve been more of a programmer for my team anyway, and this is my first time really dealing with my own hardware. This will help a lot though.