cpu (Quallcomm cpu) heating burning battery dying in few minutes

hello friends

i need your help to diagnose fault with my iphone 5.

when i got it from an friend it was with no power with no past history.

i tried to charge it ,charging symbol appeared.I checked it after 30 min.still it was displaying red line charging symbol not powering on.even after 1 hour same thing i.e. red line charging symbol not power on.

then i opened and replaced battery with new battery .device get powered on with symbol connect to itune. i tried to restore and update but failed with error code 50. i tried 3-4 time same results i.e, error 50.

when i touched its cpu (Quallcomm cpu) it was heated burning. and full battery got died/empty within 12-15 minutes.

i tried to charge device but battery was not charged.

so whats all i found with this device is "its cpu (Quallcomm cpu) got burning heating within 1-2 minutes whenever charged battery is connected to it .It emptying/discharging fully charged battery within 10-12 minutes . display redline charging symbol but not charge battery.Cpu got heated even when device is off and battery also getting discharged to empty in few minutes(however burning heating of cpu is only when device is powered on) "

can you tell me where i should look for fault on logic board.

thank you