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I've bought this motor driver, which features a 5v output pin. I connected this and the ground pins to a micro-USB adapter, so that I could use a 2-cell LiPo battery to power the driver and the driver to power the RPi3, without needing extra power sources.

However, after trying it out, the RPi loses wi-fi connection and seems to have random CPU throttles. We believe it is because the driver just doesn't output a stable 5v and 2.5A.

Are we doing something wrong or is this the expected behavior of such (cheap) motor drivers?

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The 5V pin should only be used as an input to the motor driver board. Its purpose is to provide the 5V needed to power the motor driver board logic circuitry. It is not meant to be used to power external devices.

You can supply 5V logic circuitry power to the motor driver board in one of two ways.

  1. from an external power supply such as the Pi's 5V pin.
  2. from the motor driver boards motor power supply (if in the range 5V to 16V). There is a jumper on the motor driver board which if fitted means the 5V is derived from the motor power supply.
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  • You are incorrect, ive powered the RPi already with it. "Note that the 5V regulated power on pin 5 above is an output when the 5V_EN jumper is in place." – BlueMoon93 Oct 7 '18 at 22:07
  • @BlueMoon93 No, you are incorrect. It is not designed to be an output. I'd have thought the fact that you are having problems would have made that crystal clear. – joan Oct 7 '18 at 22:10
  • Maybe Im missing something then. What does the phare I quoted mean then? – BlueMoon93 Oct 7 '18 at 22:15
  • Also, tutorials like this use it to power Arduinos, if it's relevant – BlueMoon93 Oct 7 '18 at 22:18
  • A hammer is not designed to drive screws. Some use hammers to drive screws. The 5V pin is designed to provide 5V logic power to the motor driver board circuitry. Alternatively this power can be derived from the motor supply power if it is in a suitable range. Someone noticed that the on-board regulator can supply more than the motor driver board needs and noticed that it was sufficient to power a low power device such as a microcontroller (e.g. an Arduino). That is not what the supply was designed for. – joan Oct 7 '18 at 22:42

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