What size fuse for automatic charging relay?

Tom J · Dec 31, 2021

kjdoyle1 said:
So we are adding the Blue Sea ACR (7610) to our Hunter with 2 batteries. I understand it needs fuses between each + battery leg and the ACR. But I am struggling to find guidance on the specific fuse I should use. We have a Yanmar 2GM20F, the alternator of which provides about 55-60Amps charging current. Any advice on a specific fuse?

-Kyle

The battery fuses are sized according to the size of your cables, and should have already been in place, preferably within 7" of the battery. My C310 had the fuses installed, and I just ran new cables from them to the ACR. Charts for cable sizes and fuses are available on this forum, or the internet.

dlochner · Dec 31, 2021

Always fuse to protect the wire and the fuse should be within 7 inches of the battery posts. Wire size will be determined by the largest charging source, alternator or charger. Blue Seas systems has a wire size calculator.

Stu Jackson · Dec 31, 2021

kjdoyle1 said:
But I am struggling to find guidance on the specific fuse I should use.

Are you asking about the fuse size, or the fuse type?

If size, the manual includes that information.

Scott T-Bird · Jan 4, 2022

Blue Sea provides a wide selection of high quality fuse products. The easiest and possibly the most applicable selection for you would be MRBF fuses and fuse holders. You could also select an AMI or ANL type fuse. Read their material and it will become clear. A likely application for you could be a 60 Amp MRBF fuse with 6 AWG cable. You could get a double fuse holder so that you are separately fusing your battery-size cables and the ACR cables on appropriate fuses.

dlochner · Jan 4, 2022

Scott T-Bird said:
Blue Sea provides a wide selection of high quality fuse products. The easiest and possibly the most applicable selection for you would be MRBF fuses and fuse holders. You could also select an AMI or ANL type fuse. Read their material and it will become clear. A likely application for you could be a 60 Amp MRBF fuse with 6 AWG cable. You could get a double fuse holder so that you are separately fusing your battery-size cables and the ACR cables on appropriate fuses.

For a good discussion on fuses, see this article by Nigel Calder.

https://boathowto.com/electrics/fuses-circuit-breakers-on-boats/

Maine Sail · Jan 4, 2022

If installing a Bluesea Systems ACR you will want to read this article.. It covers fusing . Making sense of Automatic Charging Relays

Maine Sail · Jan 4, 2022

Scott T-Bird said:
Blue Sea provides a wide selection of high quality fuse products. The easiest and possibly the most applicable selection for you would be MRBF fuses and fuse holders. You could also select an AMI or ANL type fuse. Read their material and it will become clear. A likely application for you could be a 60 Amp MRBF fuse with 6 AWG cable. You could get a double fuse holder so that you are separately fusing your battery-size cables and the ACR cables on appropriate fuses.

You need to be extremely careful sizing fuses for an ACR especially if you have a small solar array. The reason for this is because the solar may have the ACR in combined mode when you go to start the engine. If this is the case when you hit the starter button you will be pulling a ton of current across that 60 amp fuseand it can go pop.... Other then relay-cycling issues undersized fusing combined with solar arrays is the number one reason we would get called out on service calls for a ACR's..Always remember the fuse is there to protect the wire not the ACR!

dlochner · Jan 4, 2022

Maine Sail said:
You need to be extremely careful sizing fuses for an ACR especially if you have a small solar array. The reason for this is because the solar may have the ACR in combined mode when you go to start the engine. If this is the case when you hit the starter button you will be pulling a ton of current across that 60 amp fuseand it can go pop.... Other then relay-cycling issues undersized fusing combined with solar arrays is the number one reason we would get called out on service calls for a ACR's..Always remember the fuse is there to protect the wire not the ACR!

Would having the Start Interrupt circuit connected prevent this?

Could the same happen when starting the engine with the battery charger on?

Maine Sail · Jan 4, 2022

dlochner said:
Would having the Start Interrupt circuit connected prevent this?

Could the same happen when starting the engine with the battery charger on?

Not if you have on1/2/B switch which the vast majority of boats still have.Same thing can happen with charger on..

Scott T-Bird · Jan 4, 2022

Change the switch to DCP and use the Start Interrupt Circuit!

You'll be glad you did! (Be sure to cover all your isolation bases)
@kjdoyle1 , I think Maine Sail might suggest that you use cables the same gauge as your fuse-protected battery cables so you don't have to fuse the ACR cables individually. They would be protected if they are in line with a positive buss - assuming your battery cables are fuse-protected. There is a lot of reading to do on these topics of wiring diagrams / fuse protection / battery selector switches. Get busy!

dlochner · Jan 4, 2022

Maine Sail said:
Not if you have on1/2/B switch which the vast majority of boats still have.Same thing can happen with charger on..

Thanks, that explains why I blew a fuse on the ACR before I had the SI wired in.

On the two boats I owned that came with 1-2-B switches the first electrical upgrade was isolating the house and start batteries with an ACR and ditching the 1-2-B switches.

Scott T-Bird · Jan 4, 2022

I don't know very much about charger/inverters but I suspect that the reason for the warning is to avoid using the inverter to any extent that it discharges your starting battery. You want to use the inverter with your deep cycle house battery bank that should have the greatest capacity in amp-hours. Obviously, the charging element of the equipment will be charging your start battery via the ACR, so wiring the charger to the house buss is suitable. You should also wire the alternator positive lead to the house buss if it isn't already. If you have the boat builder's set-up, the alternator is probably wired to the starter, which backfeeds the alternator current through the starter to the start battery. You will be better off if you wire the alternator output to the house bank.

It looks like you just have 2 switches to isolate your house bank and start battery rather than the ubiquitous 1+2+B. How much do you use an inverter? I've removed an inverter, finding no desire to use one. I prefer a stand alone charger rather than the expensive charger/inverter combos.

dlochner · Jan 4, 2022

kjdoyle1 said:
Guys, thanks for all the input. In the Blue Sea it mentions ANL fuses.

Here's what I have as of now. I found the inverter/charger manual (Freedom 10) and it says, "Do not use car batteries or engine starting batteries of any kind with your inverter/charger." So i guess the inverter/charger goes on the house side only?
View attachment 201470

Getting closer!

A nice easily read schematic.

A few comments.

I'm not certain that this is the best way to wire the house batteries. I think a more standard arrangement is to wire + to + and then to the bus bar and - to - then to the negative bar. Also, the - cable to the negative bar should not come from the same battery that the + cable comes from. Follow the electricity flow, it should leave the + terminal flow through the system back to the - terminal through Hose B and then House A.

I don't see a way to parallel the start and house batteries. If the start battery fails, how would you start the engine with the house battery?

All the charging sources can be led to the + bus bar, this leaves fewer connections on the battery, a good thing. The ACR can also be led from the + Bus Bar to the Start battery.

Keep plugging away!

Ward H · Jan 4, 2022

Yes, nice diagram.
A few things I noted.
You do not need a fuse between the DC Panel and switch.
You do need fuses between the house battery and switch #1 and engine battery and switch #2. Those fuses need to be within 7" of the battery terminals.
You have isolated house and starting circuits now so installing a Dual Circuit Plus switch with SI (starter isolation like @Scott T-Bird mentioned) would give you a method to start on the house or use the engine battery for the house side **.
Maybe notate the size cables you are using to help you identify the correct size fuse needed. As others have said, you are fusing for the wire size.

** You should have a way to isolate bad house or engine battery, then put the DCP+ switch in "Emergency Both" position. You could install isolation switches at each battery bank or keep a stubby offset box wrench handy so you could disconnect a bad battery quickly.

Maine Sail · Jan 4, 2022

Ward H said:
You do not need a fuse between the DC Panel and switch.
.

if the wire gauge changes /drops down a size,you absolutely do need a fuse to protect that wire

Maine Sail · Jan 4, 2022

kjdoyle1 said:
Guys, thanks for all the input. In the Blue Sea it mentions ANL fuses.

Here's what I have as of now. I found the inverter/charger manual (Freedom 10) and it says, "Do not use car batteries or engine starting batteries of any kind with your inverter/charger." So i guess the inverter/charger goes on the house side only?
View attachment 201470

Again, if you have solar or a shoreside charger & the ACR is still closed when you go to start the motor or the ACR is still in parallel mode you will pop those 80Afuses.please take the time to read the previously linked article it will save you a lot of time and headaches. that article has been proof read by the blue sea engineers and they use it for tech support calls regularly

-house bank needs a fuse
-inverter/charger needs a fuse and its own on-off battery switch
-if those are 12 V deep cycle batteries they are wired incorrectly and will unbalance.

jssailem · Jan 4, 2022

KJ
I have been looking at your battery wiring. It looks like the wiring may not maintain a balanced charge. The objective it to charge through your house batteries.

Compare your wiring to this one form MaineSail's site.

I am looking at the way the batteries are connected (I undertand you have 2 12 Volt batteries and you want to operate them in parallel.

While your negative connection is ok but you have two positive connections. This could be problematic.

In case you did not read the recommended ACR report.

Here is the image from MaineSail that shows the optimum way to wire the batteries for most cruising boats.

Ward H · Jan 4, 2022

Maine Sail said:
if the wire gauge changes /drops down a size,you absolutely do need a fuse to protect that wire

Thanks for pointing that out.

What size fuse for automatic charging relay?

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SBO Weather and Forecasting Forum Jim & John

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