After flying for a couple years with full charge batteries 4.20 charge and experiencing lots of short life and extremely unbalanced cells batteries, Less than 50 charges On Hobby King cheap batteries .
I have cut down the max charge to 4.14 volts and went to a over capacity battery . With changing to a thunder-power battery , the battery voltage is 3.8 after flight , right at storage charge.
The model needed the nose weight , so extra battery capacity and lower voltage full charge was not a issue .
No battery failures with over 100 cycles on battery 1
Might be something to not charging to 100 % and not discharging too deep.
Lesson learned , do not buy chaap batteries, they are not really cheaper and follow Igor's advice