A trailer for a vehicle with a high voltage traction-battery. The trailer may be a self-propelled trailer to substantially match vehicle dynamics and minimize impact on the vehicle. Alternatively, the trailer may push the vehicle to simulate hill descent, or drag on the vehicle to simulate hill ascent. The trailer may provide power for the vehicle. The trailer may provide current to a vehicle electric machine. The trailer may also provide current to recharge a vehicle traction-battery. The trailer may also recharge its own traction-battery.