A DC to DC battery charger is a device that converts the DC voltage from one battery to another, usually at a different voltage level. The purpose of a DC to DC battery charger is to recharge a secondary battery from a primary battery or power source, while ensuring that the charging process is efficient and safe.

DC to DC battery chargers are commonly used in applications where the two batteries have different voltage levels or chemistry types. For example, a DC to DC charger could be used to charge a 24V deep-cycle battery from a 12V starter battery in a boat or RV. It could also be used to charge a lithium-ion battery from a lead-acid battery in a solar power system.

These chargers typically use a step-up or step-down converter to match the voltage level of the charging battery with that of the charging source. They also use a charging algorithm to regulate the charging process and prevent overcharging or undercharging of the battery. Some DC to DC battery chargers also include additional features such as temperature compensation, which adjusts the charging voltage based on the temperature of the battery.