Monster Power Bank

This is the biggest power bank I've made yet using 16 recycled 18650 cells from defunct laptop batteries. I now have a decent system for identifying the useable cells and preparing them for use in a power bank.

The first think to be clear about is that after messing about with soldering recovered cells together I've given up on that as unneccessary when you can buy power banks with spring connections ready for the cells to be slotted in, just like any battery-operated device. The trick is to use a Dremel or Minicraft drill fitted with a cone grinder to clean both the positive and negative connections on the cells. A squirt of contact cleaner helps as well.

Fit all the cells and charge to 100%, which with a 16 cell unit can take a while, but the USB-C 3 amp charging speeds things up. The next bit takes patience because you need to leave the thing for at least 2 weeks to see if any cells are dud and not holding a charge. As I've said before, most cells will be serviceable, usually 5 out of a 6 cell laptop battery.

After a couple of weeks take all the cells back out again and test with a voltmeter. They should all be at least 3.7 volts and any that aren't should be discarded (safely recycled, lithium is a crtical mineral).

If you're smart, you can charge a few spare cells using your power bank, taking the main set out and then putting them back in. You can use a power bank to charge a single cell if you need to. This gives a few spares to swap in for any dud ones. Obviously the spares need to be holding 3.7 volts after 2 weeks as well and any that aren't get discarded.

In my case the overall charge level had dropped to 91% after 2 weeks, a good indication that cross-charging was taking place and at least one cell was dud. Testing revealed one dud cell (holding only 0.6 volts) and 15 useable ones. I swapped in a known good spare cell and recharged.

Now 2 weeks on, I'm getting a solid 100% charge level. This thing has at least 40000mAh (or 40Ah) capacity, which means it could run a small device with a 100 mA draw for 3 weeks.