This could be due to any number of things, as the battery, load, and charger must all work together as a system. Check the expected runtime of the load against the size/capacity of the battery. Then check the voltage settings on the charger and make sure they are correct. If the voltage settings are ok, leave the battery charging until it is fully charged. Also check the cables from the charger to the battery and make sure they are connected properly and aren’t damaged.