It wasn't any of my chargers (Triton, Hitec, Hobbico). Have used Panasonic, Sanyo and Venom, just unreliable. If I have the time, I let them rest an hour or two then retry. Only use them in 2nd or 3rd plane of the day. Lots of my buds have up and switched to other chemistries. These are hard-core RC hobbyists who welcome a challenge.
The car guys do seem to have good results, tho. I use 2 7-cell Venom car packs on my starter, and I made up the 10-cell 3A pack from matched Reedy cells. That pack is over 10 years old, still works but false peaks if charged per instructions. Hence, I 'massage' it.