From ErfWiki

Speculation

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.

It is possible that Battery Chargers are mount units with siege capabilities similar to Wiener-Rammers. Flesh burns.

Real World References

Battery charger may refer to battering ram in addition to an obvious reference to battery charger.