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 choose PCM with equilibrium function to keep each cell in better balance and good service life. For high voltage Li-Ion pack ( > 20 cell),  you shall consider advanced BMS ( battery manage system) to monitor each cell's performance to ensure battery safer operation.

A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and / or balancing it. A BMS may monitor the state of the battery as represented by various items, such as:

• Voltage: total voltage, or voltages of individual cells
• Temperature: average temperature, or temperatures of individual cells
• State of charge (SOC) or depth of discharge (DOD): to indicate the charge level of the battery
• State of health (SOH), a variously-defined measurement of the overall condition of the battery
• Current: current in or out of the battery

                                                                                                                                                                                          The above from Batteryspace.com.