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When should you use multiple separate batteries vs a single battery with multiple UBECs?

I'm trying to design the power system for a small 2-wheeled robot. Aside from the 2 main drive motors, it also has to power an Arduino, a Raspberry Pi and a couple small servos to actuate sensors.

  • the motors are each rated for 6V with a peak stall current of 2.2A
  • the Arduino uses about 5V@100mA
  • the Raspberry Pi uses about 5V@700mA
  • the servos each use 6V and have a peak stall current of 1.2A.

So the theoretical max current draw would be 2.2*2+.1+.7+1.2*2 = 7.6A.

Originally I was planning to use three separate Lipo batteries:

  • one 12V using a step-down converter to power the main drive motors for 6V@4.4A peak
  • two 3.7V lipos each with step-up converter (rated for 5v@3A) to handle the servos and logic separately

Then I discovered UBECs, which sound too good to be true, and they seem to be both cheap (<$10) and efficient (>90%) and able to handle my exact volt/current requirements.

Should I instead use a single high-current 12V lipo with three UBECs to independently power my drive motors, sensor motors and logic? Or will this still suffer from brown-out and power irregularities if a motor draws too much current?

What am I missing?

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A single battery powering everything is more convenient and reliable (only one battery to be charged and monitored etc.). It is generally better to use a higher voltage stepped down rather than a lower voltage stepped up. The battery current will be lower and voltage headroom can be higher, so there is less chance of a brownout during current surges.

A 6V UBEC should operate down to 7-8V input voltage, while a good 11.1V Lipo will hold its voltage above 9V (3V per cell) provided its rating is not exceeded.

Use a separate 6V 3A UBEC for each drive controller, another one for the servos, and a 5V 3A UBEC for the Pi and Arduino. With separate regulators you should not have any problem with interaction between power supplies.

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