Our system has a solid state battery monitoring function. In stand-by mode while the truck is parked up, the truck battery voltage is constantly monitored. Should it drop below 12 volts the engine will start ( a buzzers sounds to alter you should be near the engine) and automatically begin to recharge the truck for a period of 30 minutes. After this time the engine will stop, re-sample the battery voltage and should they be above 12.5 the units goes back to "monitoring" status. This means you can leave your truck parked up indefinitely with the fridge on and not end up with the hassle of flat batteries.