yep forkingabout is right, a good charged 48v battery should read 50v+ when fully charged.
Are you sure its charging?
check the charger output while charging
as far as your load test the volt drop doesnt sound excessive so i wouldnt say you have any bad cells as far as that goes, but you can take a volt meter and check each cell and see if you have any that are particularly low, reading below 2.2v.
Also check the SG per cell and see what the state of the acid is in. A decent specific gravity reading on a charged battery should read anywhere from 1.275 to 1.300 or higher.
If they all are about even then i'd go back to the charger.
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