Understanding battery usage

[Jwerkjr]

Having come from the power boat world, I never worried about battery usage. Now as a sailor concerned about battery draw I find myself confused. I have established that my engine will start on battery switch position 1 or 2. I’ve always assumed 1 is the starting battery and 2 is the house. Is it safe to assume if I leave the battery switch on position 2 while sailing that battery 1 Will always be available at full charge for starting the engine?

[LongRange]

I suggest finding a wiring diagram as a starting point. It is unusual that the engine starts with _either_ bank. Perhaps there is a combiner somewhere that is switched to "together" mode, and you probably don't want that except in an emergency.

In general, the combiner/isolator should be set up so that both banks are charged from a single source (engine usually), but the bank outputs are separate. In other words, it should be rigged so that the starter bank - and only the starter bank - can start the engine.

[Kelkara]

Originally Posted by Jwerkjr

Is it safe to assume ...

If you've bought an old boat, then the answer to that question is "No".


It sounds like you have a simple 1-2-Both switch with house and start loads both wired to the common pole on the switch. But if you want to be sure you're going to have to trace out the wires that come from each battery and see where they go. For example my boat has a bilge-pump hard-wired to battery-1, if water gets into the boat that will slowly draw down battery-1, no matter what the position of the switch is.

[Stu Jackson]

Here's why & how:


OEM 1-2-B Switch Wiring History Alternator/Batteries & "The Basic" 1-2-B Switch BEST Wiring Diagrams

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowner...d.php?t=137615

This is a newer primer for boat system wiring design with a thorough digram: Building a Good Foundation (October 2016)
http://forums.sailboatowners.com/ind.../#post-1332240

The Short Version of the 1-2-B Switch Stuff: Electrical Systems 101 This is a link to the Electrical Systems 101 Topic, reply #2


Comes from this collection:


Electrical Systems 101

[john61ct]

You can't assume anything may be for charging maybe loads.

Trace the connections and start a detailed wiring document.

Keep it updated as you fix issues and make changes.

[Jwerkjr]

Thank you all! Looks like I've got some reading to do!

The boat is a 1984 Hunter 34 which does have the simple 1,2 or both switch!

Seems to me like it should only start on one battery but I'll do the tracing and figure out what's up.

Thanks again!

[S/V Illusion]

Originally Posted by Jwerkjr

Thank you all! Looks like I've got some reading to do!

The boat is a 1984 Hunter 34 which does have the simple 1,2 or both switch!

Seems to me like it should only start on one battery but I'll do the tracing and figure outt what's up.

Thanks again!

Unless a PO did something silly, the explanation isn't that complicated. The switch most likely does what it says -selects either of the two batteries or combines them at the "both" position.

Assuming both batteries are the same size/type, the switch position determines which one runs/starts the engine and boat systems. Stated differently, selecting either 1 or 2 isolates the other.

Charging is a different picture as most boats have an isolator/combiner/relay which can charge both simultaneously.

[Stu Jackson]

Originally Posted by S/V Illusion

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Charging is a different picture as most boats have an isolator/combiner/relay which can charge both simultaneously.

Not really. I believe.


Reason is that "technology" wasn't and still isn't part of OEM boat building. To be so would imply that all PO of these boats knew about them and added them.


Every single "isolator/combiner/relay" I have ever seen has been added by a knowledgeable skipper. Well, except for the isolator crowd, which were active in the 70s until relays came about. Smart skippers removed the voltage drop isolators right quick!

[john61ct]

Originally Posted by S/V Illusion

The switch most likely does what it says -selects either of the two batteries or combines them at the "both" position.

But isolate-selects / combines what exactly?

Yes in recent years, with the use of VSRs etc for charging purposes, many have learned this switch is best used as a LOAD selector only, and perhaps it does that for engine cranking **only**.

But I've also seen them used for generalized "use which batt for House?"

Often to manually direct charging currents (why else would Blue Sea have developed their AFD feature?)

And sometimes for windlass or thrusters!

So, needs diagnostics and documentation, all the rest is guesswork unless original equipment left alone.

[Tillsbury]

It’s likely that originally the switch simply selected one or both batteries. So you switch to 1, start the engine, switch to both (to charge both), then remembered to switch to 2 when you turned the engine off (so you don’t discharge the start battery). My old 70s boats were like this.

But, of course, as stated above some previous owner might well have done something a bit more clever to avoid the obvious problem of forgetting to switch back to battery 2.

I would suggest it might be well worthwhile paying a good marine electrician an hour’s labour or so to go through the wiring and show you what’s there, and perhaps make any suggestions as necessary. From the other end of the internet much more information would be partly guesswork.