I have recently sent a trackpower stick pack back to flightech. My charger also said low voltage. When I put a multi meter across the pack, it had 7.34 volts in, basicly, the fault was with the balance port, the wire had "snapped" inside apparently. Flightech replaced the pack no quibbles. it was only around 3 weeks old though.
I would try what Garry says, and you might be pleasantly suprised.... or check the voltage with a multi meter...
