Battery Switch Guide

We recently received an interesting question from one our clients regarding how to operate the battery switches on their boat. For background, the boat is equipped with one house bank, one engine battery and one generator starter battery. With the engine running, the boat owner had assumed that the alternator was charging the dedicated starter battery, the generator battery and the house bank.

The boat is equipped with a Dual Circuit Plus battery switch (OFF-ON-COMBINE). The purpose of this switch is to isolate an engine battery from the house bank. When the switch is turned to ON, the engine battery is connected to the engine and the house bank is connected to the house loads. The engine battery and house bank are completely separated so that all loads are connected to their respective batteries only.

The Dual Circuit Plus switch differs from the more common OFF-1-2-BOTH switch. With the OFF-1-2-BOTH switch, there is no separation between engine and house loads. All loads are always powered by whatever battery is selected, either 1, 2 or both. As a result, the operator cannot assign battery 1 to the engine only or battery 2 to the house only, as with the Dual Circuit Plus battery switch.

Using a Dual Circuit Plus battery switch prevents the possibility of draining the engine batteries with house loads. In effect, the engine battery is left with only one purpose, to start and run the engine. When the switch is in the ON position, the engine battery is connected to the starter and the house loads to the house battery bank. The most common method for connecting the alternator is to have it wired directly to the starter post. The alternator can be wired in a few other scenarios, but for now let’s review the typical setup. Effectively, whatever battery runs the engine will also receive the charge from the alternator.

When the switch is turned to COMBINE, the alternators will charge both the engine and the house batteries. While underway, turn the switch to COMBINE. The moment the engine stops, make sure that the switch is turned to ON. If left on COMBINE while not running the engine your house loads will drain both the house and the engine battery. Consequently if the batteries are drained too much, it’s possible that neither engine nor house will be able to start the engine.

As a boat operator, you should be curious as to how your alternator is wired aboard your boat. Above, we discussed the common practice of wiring the alternator directly to the starter. Although this is the easiest setup for the engine manufacturer it does provide two drawbacks.

The first drawback is the possibility of disconnecting your alternator from the batteries while the engine is running. This is a mistake we see when a boat owner attempts to switch battery banks and passes the battery switch through the OFF position while the engine is running and the batteries are charging from the alternator. With the alternator suddenly disconnected from the battery, the output of the alternator has nowhere to go and the alternator suffers from a catastrophic failure.

The second drawback is that the battery that requires the most charge on a boat is the house battery and yet the alternator is connected to the starter, which in turn is connected to the engine battery. The engine uses lot of power to start, but once running only needs a modest amount of power to recharge and operate the engine. While underway, the engine battery is quickly topped-off and the alternator limits its output to match the draw. Consequently, you have an alternator that is throttled and only able to output a modest amount of power.

To offset those drawbacks, boat electrical designers have introduced two different alternator set-ups.

Alternative Scenario One  In the past, the second most popular setup was to have an alternator wired directly to a diode combiner and have the output of the combiner go to multiple battery banks, for instance, the house and engine battery. The advantage of this setup eliminated the two drawbacks above, but unfortunately seriously reduced the alternator output, because the diode combiner introduced a 0.6 to 0.7 VDC voltage drop.

Alternative Scenario Two  Over the last decade, Automatic Charging Relays (ACRs) or Voltage Sense Relays (VSRs) gained popularity. These devices allowed two batteries to be connected in parallel, automatically, if there was a charging voltage and disconnect once a pre-determined voltage was meet. This allowed for the alternator to be directly connected to the house bank, which needs the alternator charge the most. Once the charging voltage was reached, the house and engine batteries would be automatically combined so that engine battery would also receive a charge. Once the charge was gone, either battery became separate again. In our opinion, and for most cases, this is the best way to wire an alternator.

Another important reason to isolate the engine batteries from the house batteries while being discharged, is the difference in the flooded battery types. Engine, or starter, batteries are designed to deliver a high amount of power for a short time and if slowly discharged will provide only a fraction of their expected battery life. House, or deep cycle, batteries provide a low amount of power for longer periods of time to power refrigeration, appliances, and so on. These batteries often look identical and the only way to tell them apart is to read the labels. There is a dual-purpose flooded battery, but the end result is that it does neither starting or deep-cycle applications very well.

Special Note: 

Even when your battery switch is in the OFF position, a bilge pump, propane sniffer, VHF or stereo memory should be directly wired to the battery. Take the time to confirm this behaviour, by turning your switches to OFF and trying to run essential loads above. Unfortunately, we sometimes encounter some boats that do NOT provide this functionality.

Shorepower and Generators.  It is a different story while connected to shorepower or running your generator. The shorepower or generator powers the AC onboard and the AC, in turn, powers the converter (battery charger). In a typical setup, most battery chargers have three leads, one charger lead to the house battery bank, one to an engine battery, and one to another engine battery. While connected to shorepower or running the generator, there is no need to turn the switch to COMBINE or BOTH. You can confirm this by watching the voltage rise for all three battery banks while the charger is on and energized by an AC power source, without the switch turned to COMBINE or BOTH.

Switch Use Guide. Assuming you have an alternator connected directly to your starter, a Dual Circuit Battery or Off-1-2-Both switch and do not have an ACR or VSR, refer to the table below to determine what position the switch should be in and when. If you have a two-bank system, as described above, let’s assume bank one is the engine battery and bank two is the house battery:

As elaborated above, the charger aboard your boat should have multiple leads directly connected to each of the batteries. That way it doesn’t matter the position of the switch as each battery will get a charge even if the battery switch is off.

If you have switches installed on your boat, take a moment to familiarize yourself with their operation and wiring. It is not uncommon for a boat owner to think the system works one way when in fact, it works completely differently.

About the author: Jeff Cote is the owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: