Just in case( I got this from a website in geocities):
How does a Battery Isolator work?
It's an electronic sealed unit that contains two diodes.
Diodes permit current to flow in one direction only. Power supplied to the middle stud of the Battery Isolator flows to each of the other studs. Current cannot flow back from one of the batteries to the other battery. Charging current flows preferentially into the battery with the lowest charge (because of the higher potential difference).
Because they're solid state, isolators have a long life. They're not susceptible to contaminants, vibration, or extremes in temperature.