It might be that your power supply is weak...
If that is the case, as the battery approaches full charge, it takes more and more VOLTS from the charger/power supply combination to push the juice into the pack... Especially if the battery has some age on it, the IR will have gone up and the voltage it peaks at can climb -- if the power supply is weak, it will lose voltage when it is working hard...
"If you cannot win, make the one ahead of you break the record."
Biff Racing Team #420 (Ah... The "good old days"...)
The local indoor Offroad Track: The RC Race Barn
TLR22-3.0, TLR22T-2.0, TLR22SCT-1.0