Check the source that the charger is plugged into for proper polarity and a good ground. A $7 tester from Home Depot will do fine for this.
Use a VOM to check continuity between the power cord gound and a negative battery post. AC safety ground and the battery negatives should be tied together. If you have batteries wired for 24 or 36 V, then that doesn't apply, but the starting battery neg and the AC safety ground should show continuity.
Plug the charger back in and measure between a battery ned and the outside case of the motor. If you read any AC voltage, you likely have a defective charger. DC will not give youa shock, only AC, so if there isa problem, it has to be charger or AC wiring related. A bad AC neutral can cause the problem you are describing..
Hope this helps.
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