2.4 volts means 1.2 volys times two, there's likely very little to no charge control on teh charger just a flat constant current when it's on it's based, that's likley what kills the battery, once full it causes eleveated temepratures which degrade the battery chemistry over time. If they're NiCad batteries it's again no wonder the fail, short use and then back on the charge right away is the worst thing you can do to a Nicad Pack. Why not just replace the batteries? It's likley AA or AAA batteries in a plastic housing.