The Offshore Voyaging Reference Site

Choosing & Installing Battery Switches

There are three usable battery switch options and one that, though very common, no one should go near:

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More Articles From Online Book: Electrical Systems For Cruising Boats:

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  4. 5 Safety Tips For Working on Boat DC Electrical Systems
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  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
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  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
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  30. Lithium Batteries Buyer’s Guide—BMS Requirements
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Raj Laud

Thanks for the helpful article, particularly the suggestion to have an alternator disconnect for option 3. Our alternator is connected to the common post of the house battery switch, which has a couple of drawbacks: 1) you could make the mistake you describe in the article, and energize the alternator with the output disconnected from the batteries (although you’d have to ignore the prominent warning placard about this), and 2) you can’t muck around near the alternator with the house batteries on. We haven’t found these to be a huge drawback in practice but it does seem like a place with room for improvement.

Instead of a relay, we have a 1-2-both switch for the engine. Position 1 is the start battery Position 2 is wired to the common post of the house selector. This allows you to start from the house bank if the start bank fails (or even combine them, but if the start battery is really dead, leaving it out of the mix altogether seems preferable to me).

Edit: one thought occurs to me. Might it be easier to mistakenly leave off an alternator disconnect than run into trouble with the alternator wired to the load side of the house bank? I’d probably notice no instruments, chartplotter, etc. before starting the engine, but the only feedback for the position of the alternator disconnect is looking at the switch itself.

Raj Laud

Thanks John. You’re right, I think we’d notice if the alternator disconnect were left off. I always check for proper charging, if nothing else out of greed for those watts while the engine is running.

I see from RC’s article that as long as the alternator disconnect also disconnects the power to the regulator, there is no risk to the alternator if it is left off by mistake. It’s important to wire that right, otherwise an alternator disconnect left off could fry the alternator.

Raj Laud

“Weird Shit” is making my head spin. My takeaway is that if the alternator disconnect both removes power from the regulator and disconnects the field wires, it should be ok if the engine is mistakenly run with the alternator disconnect off. Does that track with your thinking?

Raj Laud

That’s right, currently, the field wire is not run through the auxiliary contacts on any switch. In planning for the alternator disconnect, I’ll also plan to run them through the auxiliary contacts on the alternator disconnect switch.

Ernest E Vogelsinger

The question is if there might be some kind of alternator that keeps some residual magnetism for some time – in such a case even disconnecting field will not keep the alternator from producing current when rotating.
I have no idea how to cope with such a situation but maybe shorting D+ to ground (and disconnecting the hot wire before) might help?

Kit Laughlin


Re. “complete block diagram of the recommended offshore voyagers’ solution, including the variant,” I most certainly would be interested. Cheers, KL

Ernest E Vogelsinger

Actually I don’t believe a “general offshore wiring block diagram” would be of mainstream interest – my take would be to think through all what you wrote, plus comments, plus my own requirements, and come up with the “personal block diagram”. Might be more to the point, and should then be verified by a pro.

P D Squire

I imagine that the complete block diagram of the recommended offshore voyagers system will eventually appear in the A40 manual. Doing it now will save Maxime et. al. some effort. On the other hand, it is likely that the diagram is coming soon anyway from the A40 team.

William Elliott

I am doing an electrical refit my second boat a baba 35 and it’s a complete mess. I would appreciate the diagram.

Patrick Sell

Hi John, me too. I’m also working on a complete electric refit for my Oyster 406, and could really use the diagram if you can spare the time.

Niels Rasmussen

Me too. Middle of rebuild the electrical system on my 40 years plus Swan. Most electrical is still the original.
I have developed my own diagram, but would love to see one on this site.
A drawing is equal to 1000 words..

Niels Rasmussen

Hi John,
Thank you for the response, which makes me realise that my comment was unclear. I did not ment to ask for a complete diagram. Only the rough schematic of the components and how they should be connected according to your idea as presented in you article and the following discussion here in the forum. We all have different equipment and setup. It would be wrong to “dictate” “the” solution 🙂

Arne Mogstad

Hi, excellent article. In fact this whole series is timely for me, as I will be doing some extensive electrical (and mostly electronics) rebuild soon. Anyway, the complete block diagram etc would be appreciated, but I can live without it, since I know it will take time off other things.

But a question about the engine main switch and usage: On my OVNI they are originally placed in the port side aft bedroom, close to the batteries. Electrically, that’s a good place, but operationally, not so much. If I need to start the engine quickly (MOB, collision/grounding avoidance, etc), I need to run downstairs first, which is unfortunate. Therefore, I keep my engine main switch on when inshore, and only turn it off when well offshore. I am now considering one of those remote main switches. They seem to solve most issues. Any experience or thoughts on that? How do you use the main switches?

Raj Laud

Hi Arne,

My start battery switch is in the engine room. I’m curious if others do differently, but I leave my engine connected to the start battery when sailing. There is no drain on the start battery except when the ignition circuit is energized, so I don’t see any harm in this. Perhaps I am missing something?

Arne Mogstad

Thank you both. I am just more and more agreeing with the need for simplicity! And yes, I also figured the draw was minuscule, hence I left it on most of the time, but most people I meet say that it should be off (well… just because a lot of people do something, does not make it the right thing…). Anyway, reassuring to hear!

Dave Pyle


Interesting article. As for your second example, the one you chose for the J109, it appears the Blue Sea makes breakers that would replace the interrupt switches that come standard:

Daniel Barthel

Actually you can order it that way, so you don’t have to pay twice.

Daniel Barthel

5511e dual circuit battery switch
7056 100A circuit breaker
2x 7208 Black toggle circuit breaker
2x 4111 360 panel adapter
instead of 8609 complete assembly.
I have no idea if this is economical compared to ordering the complete assembly and the replacing the two push button breakers.

Ben Garvey

All good discussion on a source of issues on most boats I’ve been on.

My system is a variation on the above shown ‘variation on the two on-off switches (using the 1-2-both)’. I have engine start batt completely isolated from the house system, and use a charging relay to connect the two (actually 3 – two house and 1 engine) banks when voltage is over 13 (and disconnects when below). It seems to work well in practice.

As for the drawback of not being able to parallel with the engine start battery… I’ve got a beefy set of nice alligator cables (jumper cables); and easy access to all batteries.

my sytem is old school – all flooded lead acid 6v GC cells; except the engine start which is a group 48 starter batt. I know it’s frowned upon to connect deep cycle and starter batts, but in this particular case, they’ve been playing well together for nearly 10 years, no visible issues…

Vesa Ikonen

Hi John.
I have what I think is a simplified combination of #2 & 4# in your article:

a dual circuit Off-On-Both switch, where 1 connects to house bank, 2 to engine battery, but the output wire going to starter is run through a simple single circuit Off-On switch.

This way you can:
-keep the circuits separate
-connect them if start battery is empty
-make sure the engine & starter is disconnected if needed while house loads remain on (the Off-On switch)
-forgo the third switch (alternator disconnect). There is no way you can start the engine with the Off-On-Both switch being off, and the alternator cannot get energized with either switch off.

Charging is distributed to both batteries with a Mastervol Batterymate (a clever device with one input and two separeted outputs with near-zero voltage drop. A model with three outputs is available also).

Alex Borodin

Hi John,
If I understood the idea correctly, it assumes a single wire for starter and alternator and the absence of a charge bus. Consequently, it wouldn’t work exactly as described on systems where alternator does not share the wire with the starter and/or is connected not to the output of 1/2/both switch, but to a charge bus. In those cases, an alternator service disconnect switch would still be necessary.

Petter Mather Simonsen

Here are my 5 cents worth of input, if is could be useful.

On my now previous alu vessel, I had two completely separate systems; 1 alternator (12v) and battery for engine and starter and 1 alternator (24v) for house bank. All AGM batteries.

The engine system had its own on/off switch and an internally regulated alternator.
In case of an empty starter battery, I was relying on the house bank inverter and a small separate battery charger.

The house alternator was externally regulated via a Mastervolt Alpha.
This possible problem
(….But the problem here is that this leaves the alternator energized when the battery switches are off. Definitely not a good idea when we are going to work on the engine.) ….

was circumvented by having the energizing wire connected to a relay that only closed when the engine ignition switch was on. So when engine was off or the engine on/off switch was off, there is not way to energize the 24v alternator.

The possible problem I was left with was in case 24v main switch was off when starting the engine. Since I was the only operator on the vessel and as all loads were connected to the 24v system (.it was natural to switch 24v to ON before starting the engine). Hence the issue did not surface during the last 11 years.

Alexander Kornman


Thanks for the great article and the links to other related posts. Thinking about the ASD and Ron’s comment about a battery switch with a built in AFD (eg, BlueSea 9004e), is there any reason not to simply use your approach of two separate Off-On switches but ensuring that the start bank switch has a built in AFD (putting to one side the start and house banks cross connecting topic)?


Robert Andrew

John, I’d like to make some diagrams of my setup so that someone else (and occasionally me!) could figure it out. Any readily available software and/or icon libraries (e.g. for depicting batteries, switches, etc.) that you could recommend?

Bob Andrew

Devon Rutz-Coveney

Hi John!! OK… Switches: Off,1, 2, Both.. we have been using these for 30 + years onboard. But not for (perhaps) what they are intended. For High amperage loads they work great. For example: selecting between charge sources. Alternator or genset. I admit that ‘both’ is seldom selected but it gives one the choice to select/power up systems. The alternators are enabled by selecting ‘alternators’, the genset by selecting ‘genset’. It is a simple matter to change the ‘cable/number’ label designations on these switches. I Imagine that many have more than one alternator attached to their engine. We do. We are able to ‘select’ which alternator by having the the regulator ignition wires (Brown) interrupted by a toggle switch that enables the external regulators for each. It is a system that works really well. One can select manually which alternator, if any, they want enabled/charging. And for those using LFP batteries, a relay controlled by the BMS inline, controls overcharging if one is not already paying attention. On the subject of LFP I note that the you guys have changed your opinion. When I first commented on LFP a few years back you commented then that it was not a viable option for complexity issues. Any way… better late than never.. As RC wrote many years ago, it is a ‘no brainer’. LFP has been a game changer for us. Saves fuel for charging (less time), nearly 100% efficiency, less weight, kinder to the environment…etc, etc. We use about 150 cycles per year living onboard 365 days/year. With a lifetime of 2000+ cycles, this is substantially better than what we experienced before with our AGMs, not to mention the savings in charging costs/efficiency, fuel savings, environmental impacts.
The usage of a hybrid LFP/Lead Acid system I have also mentioned in previous discussions is still gong well onboard. Our Hybrid system was inspired/ written of by Eric Bretscher of Nordkyn Design. This too, in our install, is using one of the ‘ Off, 1, 2, Both’ switches. In parallel with LFP we have an AGM 100Ah battery. Position 1 designated/labeled to load buss, 2 designated/labeled to the charge buss.
This is all that gets connected (‘Both’) for running DC loads while we are using shore power. The battery charger keeps it topped up like a ‘power source’. The LFP bank is shut down (via it’s own battery switch) whilst on shore power (sitting at about 40-50% SOC) and the AGM runs all the loads/charges. At sea, we use the Battery select switch to parallel the LFP/AGM to mitigate against any weird/unwanted battery disconnects instigated by the BMS. If the BMS shots down the LFP for some bizarre reason, it will sound alarm 1st. When/if this happens, the AGM will take over system/boat DC loads while we figure out what happened and correct it. This does not happen very often, if at all, but is an ‘insurance policy’ should it happen. At anchor we have the AGM/ hybrid switch turned to off.
It sounds perhaps complicated but in reality it is not. It has been in use by us for over 5 years now. In the end everyone chooses the layout that works best for them. The reason I wrote all this is because I think, from our own ‘real world’/24-7 usage experience, there are good uses for the ‘Off, 1,2, Both’ select switches. Just saying.
Also I wanted to again point out the value of the ‘hybrid’ layout for LFP/Lead Acid (when implemented correctly).
All the best, Devon

Devon Rutz-Coveney

Fair enough… Thanks John for all the great info. Like I wrote, ‘people choose the layout that works best for them’.
All the best, Devon

Crane Stookey

Hi John,
I understand why, in option 3, you connect the alternator to the start battery to avoid the problem of starting the engine with the house battery switched off. But in option 4 you say to connect to the house for better charging, and you address the issue of the alternator being energized and needing a switch, but doesn’t the setup still allow the problem of leaving the house switch turned off when starting the engine? There’s something here that I’m missing I think. Thanks.

Bob Hodges

Hi John,

On our Dragonfly 32, we have the following:

Engine Battery On-Off Switch (in aft cabin)
Parallel On-Off Switch (in aft cabin)
12V House Battery Breaker Panel with 12V House Battery Main Switch (in aft cabin, house battery main switch mounted on the breaker panel)
12V Switch Panel (in main cabin, serviced by house batteries)
Battery Separator Solenoid mounted in main battery compartment (in the main cabin that houses the house and engine batteries)
Bowthruster/Windlass On-Off Switch in forward cabin
One (1) 55aH West Marine Engine Battery in main battery compartment
Three (3) Victron Super Cycle AGM 60aH House Batteries in main battery compartment
One (1) Interstate 70aH AGM Bowthruster/Windlass (shares a small compartment below the V-berth with its On-Off switch listed above)

The boat’s wiring diagram does not show the battery separator. This system seems to address separation of the house/engine batteries similar to the preferred systems you detail.

For normal operation, engine, house, and bowthruster/windlass battery switches are all on and parallel switch is off. Per Quorning, the only time the parallel switch is on is if there is insufficient power in the engine battery to start the engine.

Any thoughts or comments on how our Danish friends set this system up?




Bob Hodges

The system is all factory stock. We replaced the two house batteries that came with the boat when we bought it with the three Victron Super Cycle AGM’s which seem like an interesting alternative to lithium batteries and we replaced the forward windlass/bowthruster battery as the existing was just over 6 years old.

The battery separator solenoid listed in our system looks very similar to the Blue Seas L-Series solenoid above but it is not shown in the power wiring diagram of the boat so I am going to address that with Quorning’s electrical engineer to understand how it interfaces and functions. I’ve done my own process flow diagram for the NMEA network for instruments on the boat for better reference and I anticipate I’ll probably draw up my own power distribution schematics to supplement the factory schematics provided with the boat.

Thanks for the feedback.