In some examples, a control unit is configured to adjust charge termination voltage of a rechargeable energy storage device. The control unit is adapted to charge the rechargeable energy storage device to a charge termination voltage where the rechargeable energy storage device has capacity to support peak load but comes close to a system shutdown voltage after supporting peak load. The control unit is also adapted to increase the charge termination voltage if a voltage of the rechargeable energy storage device is near a system shutdown voltage after supporting peak load.