You can monitor them by checking the digital monitor. Temporarily unplug after a couple of hours and select the battery status setting on your monitor ( probably labeled with the brand, Xantrex). When it registers something between 12 and 13 you are charged. When not plugged in, be sure to push the store/use button into the "store position. Even then you may get a drain if stored for a long period of time. If this happens often, you will kill your battery. I unhook the negative terminal on my battery when stored more than a couple of weeks.