PCM or PCB ( protection circuit module or board ) is "heart" of Li-Ion battery pack. PCM or PCB ( protection circuit module or board ) is "heart" of Li-Ion battery pack. It will protect Li-Ion battery pack from overcharging, over-discharging and over-drain, therefore it is must have to avoid Li-Ion battery pack from exploding, firing and damage. For > 5 cells or 18.5V Li-Ion pack, you shall ...