The charger indicates they are charged but they don't power up the beltpac.

Aging of Lithium-Ion batteries is affected in several ways: age, use, charge-cycle and storage conditions.
Lithium-Ion (Li-On / Li-Poly) Battery General Care and Maintenance• Typical life span of a lithium-ion (LI) battery is 2-3 years• Typical number of discharge/charge cycles is 300-500• Li batteries lose capacity due to cell oxidation thru normal use• Elevated temperatures hasten permanent capacity loss• LI batteries should be stored in a cool location at ~59⁰F• High charge levels hasten permanent capacity loss• LI batteries prefer partial rather than full discharges• LI batteries do not need to be cycled, they have no memory• LI batteries should be stored partially charged at ~40% charge• LI batteries should never be charged at temps below 32⁰F• Internal protection will prevent an LI battery from being charged if the cell voltage falls too low ~1.5V/cell