First I think the battery no longer holds a charge and that's why the voltage doesn't increase enough to turn off the red light. I don't think the charger is the problem unless you set it for the wrong voltage. It can be set to 120V or 220V. Replacement battery is available but is quite expensive, around $70 or so. You can get a battery that fit and work for less but doesn't have the raised ridge to prevent you from installing the battery backward.
I guess that the ready light on the flash is not on when the flash won't fire? If it's on and the flash won't fire then there is more problem than just the battery.
According to Metz instruction, you would charge the battery until the red light goes out and then another additional 6 hrs after that.