Would like to get an inkling of what could possibly be wrong in my phone based on behavior described

[Sanjit Keskar]

Of late, when my fully charged phone drops to around 80% a call or video viewing will cause it to trip and reboot without notice. When I turn the phone back on the charge has dropped straightway to 55%. This will happen again at circa 40% and te charge dead drops to 28% or so
What may be causing these sudden dead drops?
I have tried calibrating the battery twice (no success) and have scanned for possible rootkits and virus too (none are there)
From 10% to 0% charge this does not occur al all
It also does not happen when the phone is pluggin in and used at any charging level

 

[rgd1101]

sound like the battery is failing