Ryobi 18V battery won't charge - any ideas?

Hey folks, my Ryobi 18V battery pack (the old blue one) won't charge anymore. Plug it into the charger, the light flashes red for a second then nothing. Tried it with my other battery and that works fine. Any tips before I toss it?

Sounds like the battery might have a dead cell or the BMS is fried. Have you tried jumpstarting it? I've seen some people do that by tapping the terminals with a 9V battery or using a multimeter. But be careful, batteries can be dangerous.

I had the same issue with a Dewalt battery. Ended up being the charger contacts were dirty. Cleaned them with a pencil eraser and it worked again. Might be worth a shot before you crack it open.

Thanks guys. I'll try cleaning the contacts first, see if that helps. If not, I might attempt the jumpstart trick. Will report back.