If you have access to a volt meter you can power down and check the voltage and see if it is as high as the battery says. For example the side of the battery says 12 volts..you read 10 volts or less let the battery charge for 24 hours and test again if it is still 10 volts (while not pluged in) one cell in the battery is bad and the battery needs replaceing. If you watch the connections you could change the battery yourself for less than a third of a new unit. The units rearly go bad but the batteries have a shelf life whether used or not.