An off-board battery cycler is used to condition an on-board battery system used in electric or hybrid electric vehicles. The battery cycler periodically discharges the vehicle's high voltage traction battery to eliminate battery “memory” that can prevent the battery from being fully charged. The cycler also recharges both the high voltage battery and an on-board low voltage battery used to power the vehicle's electrical system. High voltage AC power used in recharging the high voltage battery is switched using contactors physically isolated within the battery cycler, in order to protect operating personnel from coming onto contact with high voltage. Faults and malfunctions are recorded by the vehicle's on-board controller