Question: Recently I was checking out the engine room space on my boat and noticed the device in the picture here on the starboard side bulkhead. After checking my owner’s manual I discovered that the device is a diode-type battery isolator. I’ve been told that these may not be the best choice of equipment to use for the purpose of preventing one of my batteries from discharging into one of the other batteries in my electrical system. What are your thoughts on this, and what things would I have to consider if I were going to upgrade this isolator for a better design?

Diode-type battery isolator

Diode-type isolators like this one have drawbacks compared to more modern solid-state units.

Answer: You’re quite right, the device in the photo is indeed what is known as a diode-type battery isolator. This device has been a mainstay in battery systems on boats for the last 40 years or so. But to your question, they are not necessarily the best choice for modern systems.

Diode-type battery isolators definitely have some disadvantages compared to the more modern electronic battery combiner devices available to us today. First of all, the finned case as shown is important because these isolators get quite hot when they are in operation. The diodes used to facilitate electrical isolation of one battery from another generate a significant amount of heat that must be radiated away by those cooling fins to prevent the diodes from burning out, causing component failure. That said, one of the most common causes of failure for diode type isolators is that they get mounted in a location on the boat (like engine rooms) with inherently high temperatures and/or poor air circulation.

Another cause for failure with these diode types is amperage rating mismatch.   These devices all have to be rated to handle the maximum amount of amperage the system alternator is rated for. Often boat owners install higher output alternators and forget that if a diode type isolator is installed (quite often), then it may also have to be upgraded to match the higher amperage output of the new alternator.

Yet another drawback of the diode-type alternators is that diodes have a troubling little electrical nuance that can create some interesting problems with modern charging systems for batteries. All diodes used in electronic systems have an inherent loss of electrical voltage as current passes through them, typically about 0.6-0.7 volts, described by techies as “voltage drop.” This can adversely impact the efficiency of a boat’s battery-charging system depending upon how the installer connects the isolator into the circuit.

The bottom line here is that today, a modern solid-state battery combiner is the best way to go for this application. They have minuscule amounts of voltage drop and do not generate the heat levels that the diode-type isolators do. Companies like Balmar, Blue Sea Systems, Charles Industries, Newmar, West Marine and Xantrex all produce suitable battery combiners today. Diode-type battery isolators are a lower cost alternative that is fraught with compromises, but still used in OEM installations to save a few dollars.