Choosing a Battery Isolator: Diode or Relay?

Most of our vehicles are equipped with a single starter battery at the factory, but some of us may want to add a secondary battery or two to power a winch, a compressor, extra work lights, an audio system, or to augment our camper batteries – or some combination of these – all without idling the engine.

But to do so, we usually have to take steps to isolate the starter battery from the secondary battery so that our vehicle will start when it’s time to head home.

A larger alternator is sometimes required as well.

One solution is to plug an extension cord into a vehicle’s seven-way trailer connector and use the battery charging circuit to charge the secondary battery. Modern trailer battery charging circuits are deactivated when the vehicle ignition is switched off, so the starter battery is isolated from the secondary battery and can’t be discharged by outside loads.

A Y- adapter would still allow a trailer to be connected to the tow vehicle. A possible drawback, however, is that trailer battery charging circuits are limited to 30 A. This may not be enough to charge several batteries and run other major DC loads, such as a three-way refrigerator.

More common solutions employ a diode or a relay isolator, each with its own pros and cons. A diode isolator installs between the alternator and the batteries; the built-in diodes allow the vehicle alternator to charge both batteries but otherwise keep them electrically isolated.

This configuration allows for loads to be connected to the secondary battery without discharging the starter battery. Medium- and high-current diode isolators for one alternator and two or three batteries are readily available.

The diode isolator pros include being easy to install, all passive, no moving parts, and therefore very reliable. Its cons are that its heat sink is relatively large (especially for high-current models), and the voltage drop across the diodes can reach ~0.7V, which could result in undercharging the batteries.

However, the alternator field sense line can be relocated to an isolator output, and the alternator voltage regulator will adjust charge voltage accordingly.

An isolator relay also installs between the alternator and batteries. Like a vehicle’s trailer battery charge circuit relay, a relay isolator is closed when the vehicle ignition is switched on, allowing both batteries to charge. When the ignition is switched off, the relay opens, and the secondary battery is isolated from the alternator and starter battery.

The relay isolator pros are that it’s relatively small, which makes it easy to locate, and there’s no voltage drop across the contacts. The downside is that it must be wired into the vehicle’s ignition circuit, it contains moving parts, and the relay contacts are prone to pitting and wear from the high DC arcing when the contacts are open and closed.

One of these approaches might be just the solution you need. Give us a call at 800-621-1271, and we’ll be happy to further discuss how the right isolator can help meet your power requirements without discharging your vehicle’s starter battery. Or visit Inverter Service Center for additional battery management needs.


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.