Please measure the voltage on the battery terminals. s/b 13.2 to 13.65 volts. Post the results.
The solution is to replace the defective charging device.
If your battery is going bad with a shorted cell it will have the symptoms of overcharging.
Only use distilled water for topping up the cells. Fill to 1/4" to 1/8" below the split tube that extends down into the cell. Just touching the bottom of the tube is absolute max. Overfilling can also push electrolyte out the caps.
Post the model number of your converter or inverter/charger for best options.