Be-four the starting repair process; you have to check the battery condition, its current or volt. Normally all mobile battery are output volt 3.7to 4.2 volt or its current about 800ma to 3500ma.its also mention over the battery.
How to check a battery
You can easily check a battery with a DC POWER supply, select the power supply rotary in 4.2 volt condition than connect battery through power supply output pin. when connect battery to power supply, the power supply both meter one amp &volt meter reflect parameter, if the batter dry then amp will be show 1400 ma after some time it will be go down to 1200-100 when it will came into 800 or 700 ma then its ok & ready to On a mobile phone. otherwise the battery faulty , if you have no power supply you can also check through a multi meter select meter band in DC volt (10v), then connect battery tips through meter probe by polarity (+ or -) it will be show the volt .
Next check the hand set current consumption through DC power supply , after connect handset with power supply you can identify what is the main causes behind the dead phone if current consumption is normal near about 30 ma then try to upgrade latest software of flashing ,
Or if current consumption is very hi then check for short component in mother board
Set the meter probe set in 50ohm value, then connect mobile battery connector through multimeter by polarity +,- or vies versa if its show value near about 10ohms that men set ok or dead due to software or component failure .if its show value 0 ohms that’s means its dead for component short in motherboard.