Some tips: listen for bubbling. If a lead acid battery makes bubbling sound, that means it produces and vents hydrogen. That's not supposed to happen outside a short period at the very end of charging (equalization period).

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 at least 2V per cell (6 cells = 12 volts), discontinue using, as a dead cell is then very likely present in the battery.

If the batteries have a cap on top that you can open with just your fingers, you should open it and make sure the lead is submerged in the liquid.

If not, add DI water to cover them.

A “battery tender” or similar can sometimes help batteries that aren’t too far gone. And check water levels like the other comment mentioned.