inside light problem

[handyman]

I went to take the clipper back to the storage lot and something  crazy happened all the lights worked on shore power or with the gen but not with the battery, I    n fact everthing was dead even the battery gage so I plugged the shore power back everything worked but not on battery untill I started the gen for a second then it started to work fine with the battery what gives a sticking relay or what has anyone ever had this.