The CTEK Smartpass 120S is a fully automatic 120A power management system that distributes power between the starter battery, the service battery, and onboard equipment, without the need for traditional diodes or VSR relays. The Smartpass features smart alternator compatibility and continues to deliver power to the service battery even when there is low alternator output. It also supplies power to critical equipment directly from the alternator when the engine is running. This means critical functions like starters, radios, emergency lights, and communication systems always have the power they need and take priority over non-critical functions. Additionally, Smartpass uses power from the service battery to help get the engine started if the starter battery is flat.