Interesting question. There are still a number of derelict battery pits along the former CNJ mainline, now NJT Raritan Valley Line. At one time the Jersey Central employed some 40,000 (!) from what I've read so there certainly was the possibility of manpower intensive changeouts, in the days before solar chargers. Before solid state electronics, even where utility power was available, chargers would be somewhat problematic, I suppose. Maybe drycells were used.
~Ken :: Fairmont ex-UP/MP C436 MT-14M1 ::
Black River Railroad Historical Trust