Battery to battery chargers are charging devices that are put between typically a starter battery and a service / domestic battery (output). They are designed to charge the output battery at a user defined 4 stage charging profile and a charging current limit.
They have become particulary popular as a result of the Euro 6 engines that came in from ~2016 onwards that gave unpredictable alternator voltages.
They are also perfect for charging lithium batteries due to their ability to current limit to protect both batteries and alternator from damage.
