Battery Chargers formed part of Goodminton's siege group during their march on Goodfinger.
Battery charger is likely to be a siege item - similar to Siege Tower.
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.
It is possible that Battery Chargers are mount units with siege capabilities similar to Wiener-Rammers. Flesh burns.
Given that no side other than Goodminton is known to have used them, it is very likely that they are units, rather than items, as equipment such as towers appear to be universal.
Real World References
Battery charger may refer to battering ram in addition to an obvious reference to battery charger.
A charger is a type of medieval warhorse