Reviving tool batteries that won’t charge

You can revive tool batteries that won’t charge by jumpstarting them.

Most of mine are 12V batteries, so I like to use a 12V car battery charger set at 10 amps. I stick a couple small jumper wires down into the battery connectors (for small flat connectors I had to flatten a copper wire with a hammer), then connect the positive to positive on the charger and negative to negative on the charger. I left a bit of wire coating on one wire so I can visually tell them apart.

It only takes 20-30 seconds on the 12V charger at 10A to give them enough juice to start working again on their regular charger.