Battery charger is likely to be a siege item - similar to Siege Tower. Battery chargers were used by Goodminton.
A column of Goodminton's siege was ambushed by a group Haffaton's firemen.IPTSF Text 6 Firemen burned four siege towers and a battery charger - which implies that the battery charger is not a unit.
Real World References
Battery charger may refer to battering ram in addition to an obvious reference to battery charger.