A vehicle includes an electric machine, a traction battery, a low-voltage power system, a DC to DC converter, and a controller. The electric machine is configured to accelerate the vehicle and to slow the vehicle. The traction battery is configured to deliver electrical power to the electric machine and to receive electrical power from the electric machine via a high-voltage bus. The low-voltage power system has vehicle accessories, an accessory battery, and a low-voltage bus. The DC to DC converter is configured to transfer electrical power between the high-voltage bus and the low-voltage bus. The accessory battery is configured to deliver electrical power to the vehicle accessories and to receive or deliver power from or to DC to DC converter via the low-voltage bus. The controller is configured to controller the transfer of electrical power between the high-voltage bus and the low-voltage bus.