First we must check if this device would benefit from battery revival / recharging. Plug in a charger and make sure that the ear piece red lights illuminate. This will indicate that the onboard charger, regulator, and electronics are functioning.

However, if the unit is incapable of holding a charge, you should see that it will pull no current from the wal-wart charger. Not required, but you can see charging current from a USB power monitor such as the Klein Tool ET920.

If the red lights do not turn on, then this repair guide will not able to assist you; a deeper level repair is required.