MRBF Fuse Block question

[sailcapt340]

I have been watching, with great interest, a posting about adding an ACR Switch to a battery bank array.
On my boat, a 2000 340, (new to me) came with a very standard wiring of 2 battery banks. Bank 1 (House Bank - 2 / 110 AH batteries) and Bank 2 (1 Starting Battery). And a Guest 1/2/Off/Both Switch, with 2 fuses on the panel itself.
Over the years, I have added a 3rd battery to the House Bank, but never a Fuse on the battery banks themselves. And never had any issues with batteries.
I’m on a mooring, with no electrical need to speak of. Refrigeration very rarely each year.
So, should I add the MRBF Fuse Block or the MEGA/AMP? I guess my question is...is it a choice or a need, to have a fuse at the positive terminal of each bank?
I can see why I ”need“ one when adding an ACR to my Banks. And fuse at 75-100 AMPS.
And installing a double post block at each terminal is easy enough, but what size fuse would I use for the positive of each battery bank with an ACR added (and its fuse at 75-100 amps)

 

[dlochner]

Each battery bank should have a fuse within 7 inches of the battery terminal. If the house bank has 3 batteries wired in parallel, then there is only fuse at the positive end of the bank. In your case you would need 2, one for the start battery and one for the house bank. Yes, ABYC does not require a fuse for the starting battery, nonetheless it is a good idea and so long as the fuse is large enough ~250amps there should not be a problem with it blowing.

The instructions for an ACR also give some information on wire sizes and fuses.

The fuse size is based on the cable size. Look at an ampacity chart and then size the fuse to the cable. Here are a couple of charts:

Max Ampacity Chart - Non-Bundled Wire by Compass Marine How To

pbase.com

Marine Wire Size and Ampacity | West Marine

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[sailcapt340]

Thanks Dave for the guidance
The double MRBF terminal post seems to be a good fit for me, for each bank. I have 2/0 AWG 75C Battery Cables from the switch to the batteries. The distance from the ACR is about 2 feet to each positive terminal.
And the double holder will let me use a 100 AMP Fuse for an ACR and a 170 AMP Fuse for each Bank.
Thanks again

 

[dlochner]

Thanks Dave for the guidance
The double MRBF terminal post seems to be a good fit for me, for each bank. I have 2/0 AWG 75C Battery Cables from the switch to the batteries. The distance from the ACR is about 2 feet to each positive terminal.
And the double holder will let me use a 100 AMP Fuse for an ACR and a 170 AMP Fuse for each Bank.
Thanks again

One other thought, many people incorrectly wire batteries in parallel. You didn't describe how you wired yours, so you may well have them wired correctly. The incorrect way is the positive and negative cables come off of one battery with the others lined up behind them. The correct way is to have the positive come off of one end of the parallel circuit and then negative off the other end. This keeps the batteries in balance.

 

[sailcapt340]

Mine are wired:
Positive Cable from switch to the Positive Terminal on the 1st Battery of the Bank.
A jumper cable from the Positive of Battery 1 to Battery 2 Positive and another jumper cable to Battery 3 Positive.
Negative Cable from Ground Bar to the Negative Terminal of Battery 3 and jumper cable from Negative of Battery 3 to Battery 2 Negative and another from Battery 2 Negative to Battery 1 Negative.
Maybe a better way to describe it is: (this looked confusing to me when I wrote it)
Positive to Battery 1 to 2 to 3 and Negative to Battery 3 to 2 to 1.
Sound right to you?

 

[dlochner]

Yep that sounds right. The little electrons have to visit all the batteries to complete the circuit.

 

[sailcapt340]

Thanks Dave,
This battery stuff can get very confusing very fast. You put it in plain language for the rest of us.

 

[Scott T-Bird]

I always wonder what posters really mean when they say "Bank 2 Starting battery". Bank 2 is really just an auxiliary battery. It's not really being used as a starting battery on a normal basis assuming your normal setting is on Bank 1. I'm curious about why you added a 3rd battery to your primary bank and why did you install an ACR? You describe your battery use as light and you are on a mooring. How are you charging ... with solar?

The MRBF fuse is fine. What gauge cable are you running from the ACR? That would determine your fuse size. You might get away with 170 amp fuse for the 2/0 gauge without trouble for each bank, but you might be better off with 250 for each bank. Your engine starter is powered by either by Bank 1 or Bank 2, depending upon which bank is selected.

 

[sailcapt340]

Scott,
I think, at least for me, I call it a Starting Battery because when we bought the boat in 2000 and had it commissioned by the yard they called it a Starting Battery. So it stuck and instructed us to turn the battery switch to the number 2 position, start the engine and turn back to both while running. A lot of work, right. Lets not even talk about the Human factor of forgetting.
As far as the 3rd battery, I have a friend who bought his batteries along with me 2 years ago. Same batteries, size etc. Decided he no longer wanted to be in boating and sold it, minus 1 battery, which mysteriously appeared in my cockpit one day. So why not wire it in.
The ACR came when we thought we might like to be on a dock. Just planning ahead. But in reality, mooring is the way to go. Quiet, alone, all around peaceful.
Now it’s a selling point for the future. But it does work while running and plugged in.
I do have a portable 20 Watt Solar panel I use and it helps but in no way to the level of bigger panels or plugging in at the dock. I’ve had no issues with the batteries holding a charge for a season, given occasional overnight time spent at a dock and motorsailing charging.
It all seems to work for us.
There’s no real science to why we have what we have and it all works for us.

 

[Scott T-Bird]

If it's been working since 2000 it can't be all wrong, right? I think there is no reason why you shouldn't just start the boat with the selector switch on position 1 and never even bother with position 2. OTOH, if you have 4 batteries, all the same size and type, I think I would be inclined to put 2 batts on each bank and simply alternate use. I'm sure you are aware that the reason for switching to BOTH is so that both banks are being charged off the alternator while you are running the engine. (No need for ACR in this instance). I hope you are not leaving the switch on BOTH when you are using house loads and not charging. For the reason that you stated, starting the boat on Bank 2 and then switching to Bank 1 for house loads, I am not a fan of the 1 + 2+ B switch. I don't know why you wouldn't just go to a 2 switch or DCP switch alternative if you really want to start the engine on a different bank. Your solar is probably keeping your batts healthy if they have enough time to re-charge. If you rely on the alternator, you would probably be under charging your batts, leading to early death.

 

[heritage]

As far as the 3rd battery, I have a friend who bought his batteries along with me 2 years ago. Same batteries, size etc. Decided he no longer wanted to be in boating and sold it, minus 1 battery, which mysteriously appeared in my cockpit one day. So why not wire it in.

You need to make sure you have enough charging capacity for that extra battery. If not, none of the batteries will be fully charged and will suffer from battery sulfation. Your 20 amp panel seems a little small for over 400 amp/hour battery bank if you consider cloudy days and some shading.

 

[Ward H]

If it's been working since 2000 it can't be all wrong, right? I think there is no reason why you shouldn't just start the boat with the selector switch on position 1 and never even bother with position 2.

I agree with Scott. Keep it simple.