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I'm running a simple robot involving a Raspberry Pi 3 and 2 JGA25-370 motors.
The system is powered from a single Powerbank (EasyAcc 10000MAh) which has 2 2A ports. DIY USB cable has data pins shortened so the powerbank would give it more power.

When I'm running my robot on smaller speeds everything works fine, but as soon as I increase the speed Raspberry PI restarts when I press "GO" :)

My suspicion is that current draw from the motors when they start causes a power drop.
I can see the red LED going dark on RPi even on lower speeds when I start the movement, but it doesn't really cause the restart, probably because it's so brief.

Could this be solved by putting a capacitor in front of motor controller (TB6612FNG)? If so, what the value of it should be?

Any other ideas? I really don't want to have 2 separate batteries, especially since it's only a problem when the robot starts from a still position.

Cheers!

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The initial motor current required to pull away from standstill is likely the cause of the pi being reset, you could try to accelerate slowly rather than hitting the gas too quickly.

A capacitor is used to stop the voltage spikes caused by the motor starting and stopping and reduce RF interference. The spikes could also be a cause of the pi reset. I use a 100uF capacitor across the power lines to minimise spikes and noise.

Depending on the motors size I don't think your power-bank will be suitable to provide enough current for the pi and the motors together, check the stall current on the motors and give this overhead at least, if you have usb devices plugged into the pi, desktop ui, wifi and blue-tooth enabled the pi could be using nearly 1 amp.

I use a bigger battery and have 2 dc-dc buck converters to bring the voltage down - one for the pi and one for the motors, separately fused. the battery lasts longer and is under less stress, a decent rc car battery can be inexpensive compared to a usb power bank.

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