I can't say this for sure but check what kind of battery charger you have. Years ago I had a full Craftsman 19.2 volt set and after several years the Nicad batteries started to die. I went to Sears and they now had Lithium-Ion 19.2 batteries. I bought 3 of them but was told I would need a new battery charger as the one I had would only charge Nicad batteries. I bought the new charger and never had a problem. Could it be you are trying to charge Lithium-Ion batteries on a Nicad charger? I'm throwing this out as a possibility but I could be off base.
FYI--I use Ryobi 18V One-plus stuff now (The 20 year old Craftsman stuff was passed onto my son who still has and uses them) and I have never known or heard of any of my friends to have any issues with their batteries--to include the off brand ones bought off of Ebay.
You mileage may vary...................
