What size fuse for automatic charging relay?

Stu Jackson · Jan 5, 2022

I used to spend a lot of time helping people out with their wiring diagrams. I eventually gave up, because as time went by there became two camps: 1) those that "got it" and 2) those that kept trying to reinvent the wheel.

So what I did instead was to gather a lot of different wiring diagrams in one place for people to use as guides. Why? Because regardless of what some electridiots would say, there really are only a few different ways to safely wire recreational boats in the range of the ones we talk about on this forum. No need to reinvent the wheel.

You have a good start and appear to be organized. Now you need to drill down on what is going on and how connections are made and WHY.
***************************
Here are some links: (these all come from my Electrical Systems 101 Electrical Systems 101 )

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

1-2-B Considerations (New 2020 - Rod (Maine Sail) finally got around to diagramming what I had done in the above link in 2009

)
1/2/BOTH Switch Considerations

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.sailboatowners.com/showthread.php?t=137615

This is a newer primer for boat system wiring design with a thorough diagram: Building a Good Foundation (October 2016)
Building a DC Electrical Foundation

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

What are ACRs, Combiners & Echo Chargers? (by Maine Sail) [scroll to the top]
http://forums.catalina.sailboatowners.com/showthread.php?p=742417 and Battery isolator / voltage regulator / batteries

Making Sense of Automatic Charging Relays (2019)
Making Sense of Automatic Charging Relays - Marine How To
*****************************
******************************

kj, in your case it's a nice diagram but I don't understand it.

1. What does "engine" mean? As far as electrical systems are concerned, there are only two parts of the engine that are involved: the starter - which gets power FROM the batteries and usually goes through a switch or switches(s); and the alternator - which produces power and goes TO the battery bank, and most all knowledgeable boat electrical folks recommend to the HOUSE bank and the VSR will charge the "almost always full" start bank.

2. The BS DCP switch has some serious limitations for sailboats. Here's why:
AGM Battery Issues and the Blue Seas Dual Circuit Switch (from Maine Sail) "DARN AGM Batteries"
Darn AGM Batteries

3. Fuses are not required on (-) wires

4. Repeated mantras: the battery is the power source, consider it a big bomb; fuses for equipment; fuse for the wire size. BlueSeas has a good writeup for how to do it on their website.

5. There is no such thing as an ABCombo switch. Please don't make up stuff. It's called either a 1-2-B switch (usually) or an A-B-Off (rarely). You need to identify the posts at the back of the switch in your diagram. With the DCP there already, I'm not sure what you are trying to do, and don't intend to spend much more time on it. Rather, I suggest you review the material provided here for YOU to better understand what you are trying to do. The reality is that I've helped over a hundred skippers both in forums and offline individually, over the past 25 years, so instead of continuing to do it one-by-one, I developed the links.

Listen to Maine Sail, he's the best. He is recuperating from a stroke and just got back to participating in forums. You should consider yourself blessed by his attention to your issues.

Happy New Year

Tally Ho · Jan 5, 2022

Looks pretty clean...what is the purpose of the fuse on the ground circuit? Is that normal or called for in the ACR specs?

Your battery wiring looks better…you do want the fuse where you have it labeled with a ? off the positive battery terminal (maybe you are asking what size fuse).

i plan to add one of these to my 2) 6-volt GC battery bank (215 AH).

But I guess I don’t know how to size the fuse…100/ 200 / or 300 amps?

On the DC panel, I don’t think you need a fuse between the switch and panel.

Looking good.

Greg

Greg

jssailem · Jan 5, 2022

Battery wiring positive wires and negative wire are consistent with the information being provided by @Stu Jackson, a very smart guy. It demonstrates that you under stand how the batteries should be wired to optimize their charging. If you look down at the batteries and see a crew member miss wiring them in the future you will know it is wrong and know the solution.

I would encourage you to continue reviewing the areas that Stu identifies. I do not understand putting the "Yanmar Charging" positive wire to the "Switch Blue Sea" nor the purpose of the "A/B/Combo".

The intent of the wiring structure identified in Stu's supporting web links (and as I understand MaineSails articles) is to make the system less complex, function more automatically, and create less opportunity for the boat operators/crew to screw things up by turning the wrong switch.

Looking at the diagram in that area of the switches, and tracing the possible connections I see where if I got on your boat I could flick the switches wrong and make you very upset.

Learning this stuff and then applying this stuff is empowering to you. You are certainly on the correct path.

dlochner · Jan 5, 2022

Tally Ho said:
Looks pretty clean...what is the purpose of the fuse on the ground circuit? Is that normal or called for in the ACR specs?

In order to power the ACR it needs a return to DC -. The ACR draws its power from the + connections to the battery which are delivered with a large cable. The ACR does not need a lot of power to operate the relay. It is impractical to add a fuse within the ACR to protect the ACR brains, thus, the small wire that provides the DC- connection is fused. If there is a short in the ACR brains the fuse in the - wire will blow and current will cease to flow through the ACR brains. This is one of the few times a fuse on the negative side is allowed and makes sense.

dlochner · Jan 5, 2022

kjdoyle1 said:
Hey guys, I can't thank you enough for your advice. The sailing community is amazing!

Here is where I'm at after your input. I'm still a little fuzzy on the Blue Sea schematics vs the ones on the Marine HowTo site that @jssailem posted. I added a A/B/Combo switch to be able to start on house bank. I think I have that right. If I wire in the start isolation circuit to the ACR I guess I just need to figure out where I find that. I guess it can't be too difficult to find behind the key start panel.

How's this?
View attachment 201509

DC wiring on a boat that is hard to comprehend and doesn't make sense until it does and then you wonder what the big deal is about. You're in the middle of that process.

@Stu Jackson has provided valuable links to the information you need. Read, reread, and study them.

Sometimes when dealing with complex systems it is best to understand each component first and then to put the subsystems together. It is the answer to the question about how does a mouse eat an elephant? Easy, one bite at a time.

The boat's DC system has 3 main subsystems, charging or generation, storage, and use or demand. Understand each of those systems first and then begin to integrate them. The pieces will begin to fall into place.
*****
A fuse is needed between the battery and the positive bus. It should be within 7 inches of the battery.

As drawn, the battery parallel wiring appears to be correct.

The ACR negative does need a fuse. Fusing the negative is a rare event and unless you are Beneteau never done.

If you are using the BS Circuit Plus switch, an A/B combo switch is not necessary. What is necessary is a simple On/Off switch between the house battery and the start/reserve battery and the starter.

The alternator + should go to the house battery or better yet to the DC positive bus bar. It is likely that the alternator output on your boat goes to the starter. It is better for it to go directly to the house battery or the DC positive bus bar.

My preference is to have all DC connections, ACR, Charger, Alternator, DC panel, connections go to a busbar (with fuses) with only one cable from the house bank to the bus bar.

Ward H · Jan 5, 2022

@kjdoyle1 I think you're getting it. Here are some comments on this revision.

Sometimes you don't want to combine both battery banks if one fails. So you used what you're calling an ABCombo switch. I believe your intent is to allow starting from either the engine/start bank or the house bank without having to combine both banks. If that is the case then the switch will be a Blue Sea 3 position selector switch. Just change the name of the switch, maybe to A, B, Off switch. It saves from adding an on/off switch at each battery bank. Either way that switch should be mounted out of the way so guests, trying to be helpful, do not have easy access to it.

It's more common to use the term "Starting Bank", not "Engine Bank". Minor thing.

You show the ACR wired correctly. Dave explained the fuse on the negative ground and the SI wire does need to go to a engine panel to the start side of the engine switch.

You note that all wiring, unless noted is 4 AWG. The Blue Sea wire ampacity chart shows for the lowest temp rated wire insulation 125A outside of the engine room and 94A in the engine room. Looks like you could use 100A fuses.

4AWG sounds small and may cause voltage drop depending on the length an amp draw. However, boat builders used the smallest they could get away with. My Catalina was wired with 4AWG. ( I rewired with much larger wire). If what you have is original to the boat, that can be a future upgrade if you have low voltage issues.

If you have 4 AWG going from the battery to the positive bus bar, then to the battery switch, then to the DC panel, you do not need a fuse between the switch and DC panel.

I believe your diagram is meant to show what you have existing and with suggestions from the group, what you are adding new.

It's a steep learning curve but you are getting it.

Scott T-Bird · Jan 6, 2022

kjdoyle1 said:
Hey guys, I can't thank you enough for your advice. The sailing community is amazing!

Here is where I'm at after your input. I'm still a little fuzzy on the Blue Sea schematics vs the ones on the Marine HowTo site that @jssailem posted. I added a A/B/Combo switch to be able to start on house bank. I think I have that right. If I wire in the start isolation circuit to the ACR I guess I just need to figure out where I find that. I guess it can't be too difficult to find behind the key start panel.

How's this?
View attachment 201509

You are close ... You have followed advise to use a DCP switch. Those of us who advocate this switch do so because we want to isolate the start bank and the house bank under normal conditions. I'll speak plainly ... Stu HATES the DCP switch and will go to any length to denigrate it. Most of us have learned to live with him ...

And he has a valid reason. On its own, the DCP switch does not provide a means to isolate a good battery bank from a bad battery bank when you use the DCP switch in the Combined position. Of course, we never use the switch in the Combined position but that is beside the point. We would need to use the switch in the Combined position if we ever had a catastrophic failure of one bank or the other.

There is a work around, and Maine Sail has described it. You should install a simple On-Off switch between each battery bank and the DCP switch. That way, if you have a catastrophic failure of one bank or the other, you simply turn off the switch to the bad bank and use the DCP switch in Combined position. Yes, you have to add 2 On-Off switches. Your A/B/Combo switch isn't being used properly in your diagram and you can just throw it out. I put the 2 On-Off switches in a hidden compartment near the batteries. They are always On and I don't want anybody else seeing them or touching them. They are there for only one purpose ... turn off the one bank or the other in the event of a catastrophic failure. That way, I can use the good bank for house and start loads and isolate the bad bank. In your diagram, you can't isolate your house bank if you need to switch the DCP to Combined. You will want to correct that one simple issue.

Stu Jackson · Jan 6, 2022

Scott T-Bird said:
Those of us who advocate this switch do so because we want to isolate the start bank and the house bank under normal conditions. I'll speak plainly ... Stu HATES the DCP switch and will go to any length to denigrate it. Most of us have learned to live with him ... And he has a valid reason. On its own, the DCP switch does not provide a means to isolate a good battery bank from a bad battery bank when you use the DCP switch in the Combined position. Of course, we never use the switch in the Combined position but that is beside the point. We would need to use the switch in the Combined position if we ever had a catastrophic failure of one bank or the other.

There is a work around, and Maine Sail has described it.

Yes, Scotty, we have learned to live with you, too.

I do not hate the DCP switch.
What I do dislike is people who promote it WITHOUT inclusion of THE CRITICAL "work around" that both you & I have included here; mine in my oft-repeated link to Maine Sail, and you in this thread.
Indeed, the entire reason that Maine Sail had to write his "work around" and why you have needed to respond as you have, thankfully, is that BlueSeas themselves neglect to mention it in their own literature!!!

For unknowing boaters, I find BlueSeas failure to do so negligent.
You, Scotty, have learned to understand its limitations and the necessity of the two additional switches to avoid the potential catastrophic failure of the remaining power supply that could/would occur if a bad bank was combined with a good one. You also understand how the DCP "system" works. You also made a conscious CHOICE to employ it because of its ability to avoid startup voltage drop to electronics. This is a good thing. It is also an OPTION as to how to wire a boat.
However, in most cases and wiring quedstions that we deal with here on sbo and every other of the many boating forums we all follow, the folks who ask these questions are new to boat electrical systems.
What I have objected to, all along and will continue to do so, is when folks promote the DCP WITHOUT MENTIONING THE DRAWBACKS AND NECESSARY ADDITIONAL SWITCHING.
I thank you for doing so today. :beer:

Stu Jackson · Jan 6, 2022

Not indulgence, it's simply the way things are here with our merry band of brothers and sisters.

I like the :beer:

idea!

I believe you engaged folks properly and that we have provided you with appropriate information to enable you to see the options available to you to allow you to make informed decisions. If so, we have all been successful.

Enjoy, I "get" the snow thing, we just got a foot last night!