[BrianG]

I'm not familiar with that particular charger, but it sounds like the internal AC power supply isn't putting out enough voltage to actually charge the battery. For charging to occur, the power supply voltage has to be a little higher than the battery's voltage (plus a little extra for circuit losses) for current to flow into the battery. It may be that your house mains voltage is a little lower (like 110v instead of 120v) which would affect the internal power supply. If that charger accepts 12v input, I would try that before getting a new charger.