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Broad Reachin'
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I've finally got some time to go through our boat's system more closely and am trying to figure how our batteries are wired.

All three batteries are linked via the black/neg wires, but all red/positive wires are separate.

For some reason, the boat has two battery selector switches, each with 1, 2, all and off. The "1" post on the first selector has two of the red/positive wires. The "2" post on the first selector is wired to the "1" post on the second selector. That's all I can remember about the wiring at this point, but will examine more closely the next time I'm back at the boat.

All 12v systems function when the first selector is on "1" or "all", but nothing works on "2". However, if the first selector is on "2" and the second selector is on "1" or "all", all 12v systems function. Anyone have an idea why it's wired this way?

I can't seem to understand why there are 2 separate selector switches with only 3 batteries. Ideally, I'd like one battery on position "1" of a single selector (engine), and batteries 2 & 3 in parallel on position "2" of the same selector (house).
 

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POs can drive you nuts with this kind of thing, especially if there were multiples. One of the owners down the line may have had more batteries or different equipment. Then another owner comes along and removes some of it but doesn't bother (or get time) to clean up the switches. You don't mention if the switches are close together - could have been a convenience thing if they are in different locations.

Anyway - as the previous poster suggested - take your time at some point to clean this up. While you're at it, think about what you could improve. For example, I might evaluate using more 6V batteries over 12V singles. If you do go with 6V (or already have them), you need to wire them in series. On my boat I have two banks (starboard & port), which then also need to be wired parallel. The is lots of on-line documentation on that. Whatever you end up doing, do yourself and the next owner a favor and document it (even a simple drawing will help down the line).
 

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I've finally got some time to go through our boat's system more closely and am trying to figure how our batteries are wired.

All three batteries are linked via the black/neg wires, but all red/positive wires are separate.

For some reason, the boat has two battery selector switches, each with 1, 2, all and off. The "1" post on the first selector has two of the red/positive wires. The "2" post on the first selector is wired to the "1" post on the second selector. That's all I can remember about the wiring at this point, but will examine more closely the next time I'm back at the boat.

All 12v systems function when the first selector is on "1" or "all", but nothing works on "2". However, if the first selector is on "2" and the second selector is on "1" or "all", all 12v systems function. Anyone have an idea why it's wired this way?

I can't seem to understand why there are 2 separate selector switches with only 3 batteries. Ideally, I'd like one battery on position "1" of a single selector (engine), and batteries 2 & 3 in parallel on position "2" of the same selector (house).
Sounds like they were trying to achieve this only in a rather kludgy fashion...

Normal Use:

1/2/BOTH to #1
ON/OFF to ON (this gives dedicated start battery)



The key with this wiring is it allows you to use the 1/2/BOTH you already have yet by adding a single ON/OFF you get a dedicated start battery yet retain all the isolation and abilities to use either bank for house or start or both..

If starting from scratch, and you really want a dedicated start battery, this is a much easier solution using three ON/OFF switches.:
 

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If there are two 1/2/both switches they could also be wired so that one switch controls house loads and one switch controls starting loads.

Both switches would be wired to the two battery banks. If the house switch is left turned to (1) and the starting switch is left turned to (2) then you have isolated banks. You can combine by turning either switch to (Both) or run both loads off of a single bank by turning both switches to either (1) or (2).

I've thought about doing this so that I can switch off my starter from below (more secure than the key in the engine control panel) without affecting house loads. It also gives me more flexibility than the 3 on/off switch system shown by Maine Sail.
 

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I've thought about doing this so that I can switch off my starter from below (more secure than the key in the engine control panel) without affecting house loads. It also gives me more flexibility than the 3 on/off switch system shown by Maine Sail.
Care to expand on the more flexibility?

For most boaters two 1/2/BOTH switches lead to confusion and incorrect use. Most I come across are so botched wiring wise they make no electrical sense.

People also screw up the very easy triple ON/OFF wiring and most boaters still don't fully understand the simple 1/2/BOTH
 

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Care to expand on the more flexibility?
Your 3 switch setup doesn't allow me to swap "house" and "starter" batteries, so that the "house" battery is being used to start the boat, and the "starter" battery is being used for electronics. Two 1/2/BOTH switches do allow me to make that swap.
 

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From Maine Sail's diagram: I'm pretty sure that if you engage the Emergency switch and disengage the Start switch, then the house battery will be used for the starter....assuming the House switch is engaged. It would be a temporary swap...not sure why else you would want to swap assuming the Starter battery would be of a dedicated type.
 

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From Maine Sail's diagram: I'm pretty sure that if you engage the Emergency switch and disengage the Start switch, then the house battery will be used for the starter....assuming the House switch is engaged. It would be a temporary swap...not sure why else you would want to swap assuming the Starter battery would be of a dedicated type.
That allows you to reroute both off of one battery, but not swap them.

My boat has a borderline small emergency battery (Lifeline U1) and two group 24s as a combined house/starting battery. There is not good space available for more batteries without upsetting the boat's balance.

The downside of a combined/house starting battery is that the amperage draw while starting the engine can temporarily lower the battery voltage enough to turn off electronics. It also means that the electronics are seeing the voltage spikes from the starter motor.

For day sailing the power in my small emergency battery is enough to run my electronics. So I get the best use of my batteries by using the big bank battery (what would be the "house" batteries) for starting the engine and isolating the electronics to run off of the smaller battery.

For multiday cruising it makes sense to either run everything off of the large battery bank or to start on the small battery and run electronics off of the house battery.

I haven't wired things this way yet, but since I have a spare 1/2/Both switch that isn't in use I have considered it.
 

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I prefer a hidden switch in the ignition circuit, in the cockpit so I don't have to go below to start the engine (ever).
I would only need to go below when getting to the boat after being away for a few days. When sailing everything would be done from the cockpit. Since the "key" for the engine is also stored below I can't think of the last time that I started the boat's engine without opening the boat.
 
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