Why do I have TWO 3-position Battery Switches?

[raahell]

I recently bought a sailboat and for the first time in my life I'm dealing with a switching system that is composed of 2 selector switches in "1-2-OFF" configuration. The PO did not know how it worked and advised to always leave both in the "2" position. Obviously, I'd like to better understand how this works as there have to be reasons for this added complexity!

The battery system itself seems pretty straightforward though I haven't tried to sort out the cabling yet as it's well buried: The house bank comprises four 12V gell batteries. There is a single 12V gell start battery. The boat's electrical system is 12V.

I also have a switch wiring diagram (pic). Unfortunately it's in French but much of it can be translated easily enough. Even if I do that, however, I still don't understand the reasoning for having two selector switches. It looks like the house bank is split into 2 mini-banks though I've yet to confirm that physically. Is that to provide redundancy in case a battery in one of the mini-banks fails?

So my questions are:

• Based on the diagram (and your previous experience), could you explain the basic functions of this switching arrangement?
• How is such a system to be properly employed? In other words what selector combinations should be used under what circumstances?
• This system is likely original (30 years old). Given advances in battery monitoring and protection devices and advances in thinking about boat electrical systems, are there more effective/safer/simpler arrangements than this one?

This is the part of the boat that bothers me the most so I'd really appreciate some guidance here. I readily admit that Electrical diagrams and troubleshooting are not my strongest skillset.

[Spot]

It looks like a combining setup, so any combination of all battery banks can be used to start the motor or power the house, so yes to redundancy. I don't have anything this complicated in my boat. Does the boat have stuff like solar, generator, upgraded engine alternator?

[rslifkin]

If it's wired as 1 switch for engine, 1 for house, to me, that's a far better setup than the typical single switch.

On my own boat, I've got 2 engines, a generator, and house power, so 4 switches. 3 battery banks (house, start 1, and start 2). Each engine and the generator gets a 1/2/both/off switch to select which start battery they use and the house has an on/off switch.


In my opinion, if the system is set up well, the switches are only for turning off power or re-selecting a power source in abnormal conditions. In normal use, a well configured system will just have a "normal" position for every switch and they just stay there.

[Spot]

Schematics can be beautiful or unnerving or a little of both...

Here is my interpretation of the hand-drawn one and text in the first post. The trick is that house common is hooked to engine switch terminal 2. So if 1+2 or 2 is selected at the engine switch, house power goes to engine and vise-versa. There is still an 'off' position which removes both 1 and 2 from connecting to the common terminal.

In my RV I keep house and engine start and generator start batteries all separate and only 'jump' them in emergencies. Some days I wish I had the switches...

[raahell]

Thanks Spot and RSLifkin,

Your comments and my further study of the diagram leads me to think the following:

• The house bank of four 12V batteries must be divided into two pairs (I'll confirm that at next visit).
• The left switch (Service) chooses from #1 or #2 pairs of house batteries, or from BOTH. Normally this switch would be kept in the BOTH position to best utilize the total bank Ah capacity.
• If there is a loss or serious degradation of electrical power to the boat, it may be due to a total loss of one of the 4 house batteries. In this case switch between #1 or #2 and see which selection restores power. Then you know that the other pair needs to be investigated at the next opportunity. In the meantime the good pair will work fine, though with only 1/2 of the bank's usual capacity.
• The right switch (Motor) functions as the normally configured single selector switch found on most sailboats. i.e. you use it to determine from which energy source you will use to start the engine. Choosing "1" will take from the engine start battery. Choosing "2" will choose from the house bank (which is determine by the "Service" Selector Switch) and choosing BOTH will provide power from both Engine Start and House bank.
• The PO said to leave both selector switches on BOTH, but if my understanding is correct, I'd be tempted to leave the MOTOR switch in the #1 position so as not to draw down on the house bank unnecessarily. Maybe on a cold start after a long period of non-use I would switch to BOTH to utilize every battery. Is there a down side to leaving the MOTOR switch to "1" position?

Spot, in answer to your question, the boat does have 3 solar panels (about 300W altogether, on two circuits) and a wind generator. I'm not sure about the output from the alternator yet.

[hlev00]

Your intuition to check where the alternator connects in is a good one. It could be that keeping the engine switch in both is so that whatever is charging, charges all the batteries. Likely the alternator and solar are connected along the connection between the two switches. If this is the case and you run the engine switch only on the engine battery it won't get any charging. The downside of running everything in Both is that all the batteries can become fully discharged at the same time if something is left on.

I have a setup that results in a similar arrangement. I just leave my engine switch to pull from the house bank and start from that. Once a month or so, I start with only with the engine battery to be sure it is still alive and working and let it get a little charge. Then I move back to run everything from the house bank again. You do have to be careful that switching back to house doesn't leave the alternator without a connection to some battery or you will blow out the diodes in the alternator.

I'd like to change to something like an Automatic Charging Relay to simplify some of this. Maybe this winter yet.

Harry

[sinnerman]

Three battery setup. TWO House batteries and ONE Start battery.

Service battery switch allows you to select from which house battery you are powering your electrical devices. (House -1, House-2, or 1+2 [BOTH]).

Motor switch connects the STARTING battery to the starter motor/solenoid. (Position-1 NORMAL).

If the starting battery is dead, you can use the house battery / batteries to start the motor (Position 2- EMERGENCY CROSS CONNECT TO HOUSE BATTERY BANK).

I cannot see WHY you would need to have 1+2 on this switch, as if the start battery is DEAD, I wouldn't want to connect good house batteries to a dead start battery...

An important question is how the engine alternator is connected back to the batteries to charge them. (Battery combiner, isolation diodes etc.)

[Kamen]

What purpose does the service switch serve? Why not simply combine the two house batteries?

[Uncle Bob]

Hi, I have two three position switches on my boat, one powers all the house loads, the other powers the engine loads. Both switches are connected to both battery banks, house bank is six six volt golf cart batteries, engine is a pair of group 31 batteries. The engine bank is also wired to the windlass, the philosophy being that when leaving an anchorage I have the engine running which pumps amps to where it is immediately needed.
This allows the connection of either load to either bank.
Alternator runs to engine bank, with a VSR to the house bank, only energised when the engine is running.
This works well and is easy to manage.

[Hudson Force]

I had this same configuration on my '73 Morgan as shown in my avatar. I would isolate a battery bank for engine starting only when cruising with little engine use,- particularly long terms of sailing while using my 12VDV autopilot; however, most often I kept both switches at "both" with frequent use of my diesel generator and active wind generator and solar panels.

[Mithril Bham]

I have seven. Trick is to not run down starter battery. Turn that off when not charging and only drawing house. Don't want to leave that on at anchor. Don't want to wake up not being able to start. If starting battery is weak then combine.

[mitiempo]

My preference is a single on/off for start and a single on/off for house loads. A separate on/off can be wired for starting with the house bank but need not be exposed as it will only be used if the start battery is dead.

House batteries should not be divided but all in one bank for more efficiency and longer battery life.

[Squanderbucks]

like my setup for two banks and twin engines. Two switches. Each 1-2 -Both-Off.
Very simple to me. Start batteries hooked to 1. House batteries to 2. Normally one switch set to 1 and the other set to 2. One alternator goes to Start battery and the other to House batteries.
Generator starts off the Hose set. If I ever would need to use house for starting as a boost just set both switches to Both.

If one set of batteries are faulty that bank can be turned off by simply setting both switches to the desired bank.

Want everything off - Turn Both to Off.

I find it easiest to think of the switches by their function Start and House. Where do I want the Start to get its power from. Batteries 1 or Batteries 2. Then just turn the switch to that option.

[Clivevon]

From a non-electrician boatowner with 30 years ownership experience.
Useful rules (in no particular order):
1) You NEVER want your engine battery to do anything else except start your engine. That is a full stop. You call them periods?
2) The bigger your domestic battery bank is, the longer it will last, so do NOT sub-divide your domestic battery bank. Just have ONE domestic battery bank for ALL needs except starting the engine. That applies to all boats. (Catamarans might have to accept less than ideal battery arrangements due to weight distribution issues.)
3) Always run your engine when using your electric anchor windlass.
4) Best way to alternator charge your engine is to connect to engine battery plus connect to domestic battery bank using an electrical relay which means the engine battery is always disconnected from the domestic battery unless the engine is actually running. No switches at all. its automatic.
5) You NEVER want to use your engine battery for domestic loads but it is conceivable (but unlikely) that you might want to use your domestic battery to start your engine. So fit a solenoid to join them on demand AND carry a set of heavy duty jump leads to connect them together because the solenoid is likely to have frozen by the time (if ever) that you need to use it. Or just carry the jump leads & be quick..
6) Connect your batteries so the load is balanced between them. See: SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

So - you have two 1-2-both switches. My boat also had 2 of those rotary switches when I bought it. The (36hp) engine battery bank was 2 x 100 amp, connected to one rotary switch. The domestic bank was 2 x 100amp plus 1 x 100amp connected to the windlass, again using a rotary switch.
I ditched both switches & changed that to 1 x 110 amp engine battery (more than big enough) & 4 x 110 amp domestic batteries. Engine charging is via a relay as above. Same amount of batteries but much more efficient. I use standard leisure 'truck' batteries (not real deep cycling) which cost maybe £80 each & last me about 5 years. I replace them all at once.

Maybe your 2 switches relate back to a previous set up - since changed. My advice is ditch them & rewire as I have done- it will save you money in the end.

[Clivevon]

Sorry - should have said i also have master on/off switches for both the engine & the domestic battery banks.