When design a lithium battery pack for your device, you must have two level of protection. The primary protection should be from the device for charge and discharge the battery. The secondary protection is from battery's protection circuit board which protects batteries from over-charge, over-discharge and short circuit. You must not rely on battery's PCB/PCM protection only. Let's use Lithium-ion cells as an example:
Primary protection: When designing protection on device, the charge circuit should cut off at 4.2V/cell and discharge circuit should cut off at 2.75V/cell.
Secondary protection: When designing protection circuit board (PCB/PCM), the over-charge threshold voltage should not exceed 4.35V/cell and the over-discharge threshold voltage should not below 2.3V/cell.
Please also consider using thermistor to monitor the temperature, thermostat to cut off at high temperature and polyswitch to limit the current when necessary.
Please always check cell's specification to determine the cut off voltage and charge/discharge current.
Design of enclosure: The battery should be attached to enclosure firmly. Due to the nature of polymer cell, it might swollen over time. Please leave at least 10% more space to allow the cell to expand.
For medical device, we strongly recommend use brand name cylindrical cell, like LG,Panasonic and Samsung, 18650 cells.
Suggest to replace the battery pack once every two years, used or not used.