Probably there is a short on the motherboard. Open it up. Look for any cracks, blown or inflated components. If that's not the case start looking for heat. You can try some heat sensors too. From there you just might get the broken piece. Please make sure the charging port is actually in good contitions and the replacement battery is as well good before going onto replacing SMDs. Doing so with a multimeter is not enough every time. An Oscilloscope may help you understand what kind of voltage are you getting on your motherboard.