How can I tell if I have a bad battery?

The battery’s voltage should always be higher than it’s standard voltage (12, 24, 48VDC) in order for it to run equipment.  For example: 12VDC battery at float charge should be 13.5-13.8VDC.  If the battery is fully charge but the voltage fails to reach 10.5V, chances are it has a dead cell or two.