Some tips: listen for bubbling. If a battery makes bubbling sound, that means it produces hydrogen. That’s not supposed to happen outside a short period at the very end of charging.

If it happens early, there could be one or several dead cells, causing other cells to experience overcharging. If it doesn’t hold a voltage of 2V per cell (6 cells = 12 volts), discontinue using, as a dead cell is then very likely present in the battery.