The battery may even need to be replaced, not just charged.
I use an external battery charger with my PS-1 engine and remove the battery for charging.
For PS-2 engines I bought the MTH wall charger that plugs into the locomotive.
I feel that it is safer to charge batteries this way then to track charge the battery, as my electric flickers too much down here in south Florida and I don't want to see my MTH engines hit the wall or the floor.
Lee F.