It is possible that the batteries are unable to hold a charge and will need to be load tested to check for damaged cells or bad connections between cells within the sealed battery. A battery load tester will apply a load to the battery while still reading the battery voltage. To load test each battery, first recharge the batteries. Apply a standard automotive style load tester (12 volt), applying the load for about 10 seconds. The batteries may pass a load test but still have problems maintaining the charge over a period of time. If the battery load tester shows a reduction of more than one volt under a load, it is a good indication that it is time to replace the batteries. A deep cycle battery tester is the most accurate way to test deep cycle batteries. This tester will run a battery through a complete cycle and determine if the output of the battery is within tolerance for its rated capacity.