DIY: How to Revive a dead Li-ion power tool battery

Fix your dead battery by using a DC power source to raise its voltage. Note: do NOT use a DC power source with more voltage than the maximum rated voltage of the dead battery. If you have a regular 18V or 20V battery pack, that's 21V max (4.2V x5). Do not use a DC power source more than 21V to revive the dead 21V battery.
About the wires with alligator clips: make sure you use heavy gauge wires size AWG 14 or bigger. The alligator clips with attached wires from Harbor Freight are not going to make it as the wires are too thin and they will heat up and just melt or burn the insulation right away. And make sure to monitor closely what you're doing cause it might heat up things quickly. As soon as you get the desired voltage (about 16V or more for this kind of batteries), just remove the alligator clips and put the battery in the charger. In other words, just do the parallel charging as minimal as possible and just enough for your charger to recognize the dead battery.

Видео DIY: How to Revive a dead Li-ion power tool battery канала vuaeco