Battery combiners contain voltage detection circuitry. When the voltage on one battery (often the engine start battery) rises above a preset level, a second battery or battery bank is paralleled with the first battery. The rise in voltage shows that a charge source is available and so both batteries get charged at the same time. When the voltage falls the link between the batteries is disconnected and the batteries are discharged individually. The advantage over battery isolators is that there is no voltage drop associated with the diodes in an isolator.