I have been toying with the idea of making a power bank, as I have a few old laptop batteries lying around which are still working well. I plan to use these components:

  • Constant-current lithium charger with under-voltage and over-voltage protection
  • Battery Management System
  • CC/CV 5v Converter

Would it be advisable to use these components? I realised that most guides and power bank circuits lack a lot of these protection features. I'm not sure if it's necessary, as it will bring the efficiency down. Any advice is appreciated.

  • \$\begingroup\$ Maybe i am just lucky but most very cheap powerbank circuits you can buy have phenomenal undervolt, overvolt, charge/discharge and the important one, short circuit protection \$\endgroup\$
    – asim
    Feb 27, 2021 at 11:00

1 Answer 1


If you just have parallel cells i.e the batteries will just be 3.7V, then you can combine different capacity cells without a problem.

When you series cells, they should have the same capacity - and random mix of old batteries may not. Your BMS likely has a limited ability to balance different cells on charge. On discharge the low capacity bank gets over discharged.

But you can test this out by monitoring the charge and discharge for a test and see if they are balanced. If they all come from the same packs they might be. However, some laptop batteries fail because a cell has a lower capacity.


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