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The traditional boat battery switch for two batteries was a single-pole four-position switch with positions OFF-1-BOTH-2. The Blue Sea Systems company has a new line of battery switches which are two-pole three-position switches with positions OFF-ON-COMBINE.



The dual-circuit or two-pole switches are intended for boat electrical systems in which there is an engine cranking battery and an isolated and separate house load battery, and in normal operation the two systems are never tied together. The dual circuit battery switch controls both with a single switch while keeping them isolated.

The isolation of the two circuits means there has to be some method of charging the house battery that is distinct from charging the cranking battery, such as a second alternator charging output from the outboard engine. Several outboard engine brands offer the option of adding a second charging output to their engine, which is then connected to the house battery.

If an outboard engine is used that does not have a second charging output, then the house battery is typically charged by the connecting it in parallel with the cranking battery, but only after the cranking battery terminal voltage has risen high enough to indicate the cranking battery is near to or at full-charge level. The COMBINE position is not intended for normal operation of the system, as it parallels the two batteries and mixes the cranking and house loads together, although it could be used that way to share charging current.

Instead of manually setting the switch to COMBINE, a more convenient and automatic method of accomplishing the charging of the isolated second battery or house battery is to use a voltage sensitive relay (VSR) which will close and parallel the two batteries when the terminal voltage of the cranking battery exceeds a certain threshold. These relays are also called an automatic combiner relay or automatic charging relay or ACR. In well-designed ACR devices the closing of the relay is delayed until certain criteria in addition to just voltage are met, such as a minimum amount of time at a certain voltage before closing, or a minimum about of time being open before closing again, or the absence of an external signal that is applied to prevent closing, as, for example, when the ignition key is in the START position. But these considerations are all in the ACR, not in the battery switch itself.

The Blue Sea Systems newer two-pole or dual-circuit switches come in two sizes, regular and mini. If you are replacing a traditional battery switch, the Blue Sea Systems 5511e-series will very likely have the same layout of mounting holes as the original switch being replaced. This can be convenient, particularly if the original switch was through-bolted to a bulkhead. The 6011-series smaller switch will need new mounting hole locations due to its compact size. The ratings are slightly different for electrical capacity:

Blue Sea Systems Battery Switch
Model 5511e
Positions = 3
Battery Combine = Yes
Continuous Rating = 350-Ampere
Cranking Rating = 700-Ampere per circuit
https://www.bluesea.com/products/5511e/ ... ery_Switch

Blue Sea Systems Battery Switch
Model 6011
Positions = 3
Battery Combine = Yes
Continuous Rating = 300-Ampere
Cranking Rating = 675-Ampere per circuit
https://www.bluesea.com/products/6011/m ... itch_-_Red]Model 6011

As can be seen, the smaller 6011-series switch does not really give up much electrical capacity, so it can typically be used with outboard engine starting circuits without much concern about being under-sized electrically.

Blue Sea Systems began as an independent company but in 2014 became part of the POWER PRODUCTS LLC group of marine electrical manufacturing companies.

Another option for a dual-circuit battery switch system is to use three individual one-pole switches, two functioning as simple ON-OFF switches for each battery, and a third functioning as a tie that combines the two batteries. Such a switch cluster is made by BEP, another good supplier of marine electrical fittings. Their basic switch is a Model 701, and it has these specifications:

BEP 701
Continuous rating = 275-Amperes DC
Intermittent rating = 455-Amperes DC
Cranking rating = 1,250-Amperes DC
Voltage rating = 48-Volts DC
Operation: On / Off

When three switches are combined into a cluster, it available as the Model 715 Battery Distribution Cluster and looks like this:


BEP began in New Zealand, and in 2006 became part of the POWER PRODUCTS LLC conglomerate of marine electrical component companies.

Comparing the two types of dual-circuit battery switches, the three-switch cluster from BEP offers the user more flexibility than the single switch from Blue Sea Systems. On the other hand, operation of the BEP cluster is a bit more complicated.

With the Blue Sea System dual-circuit switch, both the cranking circuit and the house circuit are energized in the ON position, and the two are combined in the COMBINE position. With the BEP three-switch cluster, you can separately energize the START circuit and the HOUSE circuit, if you desire.

In the standard configuration of the BEP cluster, the EMERG PARALLEL switch is on the battery side of the circuit. That means if the EMERG PARALLEL switch is set to ON and, for example, on the START circuit is set to ON, then the two batteries are combined in parallel and energize only the engine START circuit; the HOUSE circuit remains OFF.

The BEP cluster could be modified so the EMERG PARALLEL switch is on the load side of the circuit. That means you could use one battery to power both the START and the HOUSE circuits, and leave the other battery disconnected. That arrangement might be useful in a situation where one of the batteries failed with a shorted cell.

Keeping track of the position of three switches and what circuits are energized will be more difficult than keeping track of the simpler ON-OFF action of a single switch. Sometimes more options (more complexity) is useful, but sometimes it can get in the way.

Before selecting a BEP cluster for your boat it may be useful to do a google search for "BEP cluster problems" or some similar search term. I believe that some reliability problems were associated with the clusters.

I followed the advice given above to search for the term "BEP cluster problems", and I found three or four threads on a website in which boaters described various problems with their batteries losing charge when a voltage sensitive relay (VSR) charging system was used. I found nothing about failure of the dual circuit battery switches made by BEP. In the discussions about problems with loss of charge with VSR components, there was little evidence presented that these problems were caused by the BEP components; miswiring or misuse of the system are more likely causes. As I mentioned, operation of the dual circuit battery switches with the three-switch designed used by BEP can be more complicated and may not be properly understood by casual recreational boat owners.

If there is some sort of common and repeated mention in discussions on other forums about failure of BEP dual circuit battery switches, perhaps someone can give a direct link to the information about those failures. Absent such pointers, I do not find any compelling evidence that one ought to be suspect of the quality of BEP dual circuit battery switches. Note that BEP and Blue Sea Systems are both part of the same corporate umbrella.

To move to dual batteries, I'm planning to use a [Blue Sea Systems] "Add a Battery Kit" product. The "Add a Battery Kit" packaging has a battery switch and an automatic combiner relay.

https://www.bluesea.com/products/765000 ... 5BBoxed%5D

Thanks for your notice that you plan in the future to install a second battery and to purchase a Blue Sea Systems battery switch and automatic combiner relay sold at retail in a single package. After you have installed the Blue Sea Systems battery switch, let us know how the installation went and how the switch is working for you with a follow up to this thread.

Although the automatic combiner relay is not the topic of this thread, I want to offer some advice regarding installing an automatic combiner relay: before you invest in an automatic combiner relay, you should realize that the relay module is only a small part of the total cost of such a system. I looked at using a Blue Sea Systems automatic combiner relay, or as they are now calling it, an automatic charging relay. I found that the installation instructions recommended so many additional components--not included with the relay module--that the total cost was going to be more than twice the cost of the relay itself.

For a thread that discusses the Blue Sea Systems automatic combiner relay or automatic charging relay, please see:

BLUE SEA SYSTEMS Automatic Combiner Relay
viewtopic.php?f=9&t=912

Please follow up in that thread if you have further comments about the automatic combiner relay.