Battery Powered Tool Brands

[MotorSeven]

There should be an automatic cutoff to prevent any of the series cells from inadvertently being charged in reverse by it going dead and the pack still being used. Some tool - battery pack combos may not support this sensing function. ... Try interrogating the charge level during use occasionally. If it gets down to one light set it aside for charging.

I agree on the automatic cut off, but clearly these Ryobi batteries are defective and out of warranty. From now on, I'm sticking to Ridgid.

 

[TxSam]

It is possible to "jump start" those dead batteries by wiring a charged one in parallel. This can provide enough charge so the charger will "recognize" the dead one.

If you try it, MAKE SURE you observe the correct polarity!

Another hack I've used for dead batteries, specifically Ryobi - is to insert the battery into the charger multiple times. Each time the battery is inserted it trickle charges slightly before the charger 'rejects' it. After doing this enough times (5-10 times), the charge increases enough in the battery for the charger to accept the battery and then begins to fully charge.