My approach would be: disconnect the battery and connect multimeter leads to the battery connector on the motherboard. Then, plug-in a charging cord and look for voltage (might not be continuous, could be pulses). If the motherboard is putting voltage to the battery connector, then you can assume that the issue is with the battery itself. If the motherboard is not putting voltage to the battery connector, then you may have a damaged PMIC, or a short circuit on the charging system.