Hi. We have a battery backup zoeller 507 for our very active sump pump. the battery backup has been unplugged from the wall for a while. we plugged it in and the red light went on indicating power, but the yellow light, which would indicate that the battery is charging) is not on, nor obviously the green, which would indicate that the battery was fully charged. what does this mean? do we need a new battery? unit? please help we are very technologically unsavvy and hurricane sandy is approaching!