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For my robotics project I would like to utilise readily available mobile phone 'power banks' to simplify the power system for my robot. However, such power banks output 5V, great for the logic systems but not for the motors.

I was wondering if I could wire the outputs of two power banks in series and get 10V or is this a very bad idea? Should I wire them in parallel and use a boost converter? Is a custom solution using 'ordinary' Li-Po batteries and associated charging circuit the best answer?

Additional Information:

  • This will be a two wheeled robot.
  • 5V Logic
  • 7+V Motor driver
  • Power Banks: 5V 2.1Amp 2100mAh
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Go ahead and use the phone battery for your logic supply. Then, series wire individual, high current L-ion cells to make your motor battery. It's good practice to keep the noisy, high-current stuff on a different buss, away from your processor.

Invest in a smart (processor driven) charger that can monitor the voltage of each cell and charge them evenly. It can also detect a cell that is weak. Weak cells charge and discharge very quickly, and they must be removed from the battery.

DC/DC converters are not very efficient. They generate their own noise on the source and sink sides. And, they get expensive for the current levels required by a pair of motors.

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If 5V blocks were your only option, then I would say "yes, this is fairly safe" with the caveats:

  • If one cell discharges faster than the other, it will start to deteriorate much faster than the other -- it will last for fewer recharge cycles
  • You should re-charge them separately, not in series

However, if you're looking for an off-the-shelf 10V lithium-ion battery pack, you should consider the 10.8V batteries that are available in many models of cordless drills.

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