I have an MP3 player, and the battery no longer takes a good charge, I have 2-3 similar batteries in other MP3 players, that I haven't used in years. Any point to find the box ?
Otherwise I think I'll tape re-chargable AAA's to it, and with a regulator chip if needed.

 

A lot of the battery maintenance chip has a small current draw and when left unused for years can drain the cells down below where it should be recharged, I believe that point is around 1.8 - 1.9V, I have successfully recharged ones that have gone down to about 2.1V.

 

I've charged them from graveyard dead many times before (0.0V at the cell terminal ends, before the regulator) but at this point, you're pretty much gambling on a battery that has seen its better days. It may work fine for years to come, or it may, as you found, not be worth it. I would consider directly connecting a set of AAA's directly, or you may also do well to scan Amazon for your specific battery. If you find one, go for it. (They're usually fairly cheap nowdays. You'll be surprised.)