This was purchased to isolate an RV auxiliary battery used for starting the generator from the start battery bank for my engine. The original design had these two batteries banks connected together for charging purposes, but not isolated. Since the auxiliary battery was a smaller capacity battery it could drain the chassis battery during long storage periods leaving me with a dead chassis battery. Now with the battery isolator the two batteries are disconnected from one another unless the engine alternator, or other charging source, is supplying a charging voltage to the chassis battery. The battery isolator then applies the full charging voltage to the auxiliary battery, when needed, allowing it to recharge . Diode isolators have been used for this purpose and work ok, but they have a voltage drop across the diode which limits the charging that can occur. Diode isolators are solid state devices and can be susceptible to voltage spikes. This device is a much better solution. its rugged, delivers a full charging voltage to the load battery, and isolates the charging source battery when there is no charging voltage being applied. No dead battery after 6 months of winter storage.