If you have a second person present who can operate the truck,while you take voltage reading of all the cells individually. Do readings of cells before you operate truck. The only function needed to operate is lift while you do voltage check on each cell. If you have access to a specific gravity reader you can check the state of cells. If a charger is not switching off ,then the charger is faulty. Check charger is correct for battery pack. you will need the large number on battery plate eg 500Ah this is battery capacity. divide it by charger Amp output. too high will cook cells