My Dewalt drill battery won't charge - any ideas?

Hey all, my 20V max Dewalt battery (the compact 1.5Ah) stopped taking a charge. It's about 2 years old, used lightly. Plug it in, charger shows solid red light (fault). Tried another battery on same charger and it works fine, so not the charger. Anyone else had this? Is it worth trying to revive or just toss it?

Classic issue. Sometimes the cells just go out of balance. Try a 'jump start' with a known good battery: connect positive to positive, negative to negative with wires for like 10 seconds. It can rebalance the voltages. But be careful not to short anything. If that doesn't work, it might be a dead cell. Not much you can do then. Dewalt has a 3 year warranty btw, you might still be covered if you have proof of purchase.

I've taken those packs apart before. If you're comfortable soldering, you can replace individual 18650 cells. But honestly for a compact 1.5Ah, it's often cheaper to just buy a new one on sale. Also check the terminals on the battery - sometimes they get gunk on them. Clean with a pencil eraser or contact cleaner. And make sure the charger contacts are clean too.

If you decide to replace, go for a higher capacity like the 5Ah. Better runtime and you can get them as part of a combo kit. But for diagnosis, try this: measure voltage across the battery terminals with a multimeter. If it's below like 15V, the charger might see it as defective and not charge. If it's around 20V, then it's a communication issue. Dewalt has a thermal and a data pin. Could be a bad connection on the data pin.

Thanks guys! I'll try the jump start first since I have another battery. I don't have a soldering setup. If that fails, I'll check the voltage and clean contacts. If it's dead, I'll just get a new one - maybe a 5Ah. Appreciate the help!