I’ve been able to revive a dead power tool battery before, but Ryobi has some extra protection so you can’t trickle charge them from the contact points on the outside of their batteries. I was able to use a method I found on YouTube to trickle charge from the inside and the battery is working again.
When I say “dead battery” I mean the charger status shows the battery is defective. It can do this if the battery completely loses all of its power or the cells become imbalanced. The charger has built-in safety measures that prevent it from charging batteries in this state.
Be aware that opening the battery like this will void the warranty. Due to the special screws (T10 with a hole) I bought a security bit set from Menards for $5.
Update: A second battery went dead the day this was published. This method resurrected that one as well.