The problem could be the battery pack however batteries tend to wear out a little more slowly than that so we recommend to test the output Voltage of the battery charger to make sure that it is working, and to test the Voltage of the charger port to make sure that it is working and transferring the battery charger's current to the battery pack.
The battery charger should output around 29 Volts, and the charger port should have the same Voltage as the battery pack.
When testing the charger port be careful to not short circuit the pins inside of the port.
Please let us know how it goes.
Danny