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Old 06-08-2020, 13:51   #1
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FET Isolator vs ACR for simple charging systems

I am replacing my battery charger and want to take the opportunity to re engineer my charging system. I believe my simple setup must be a common situation. A weekend sailor mostly hooked to shore power. One factory alternator, a house bank (3 Flooded 100AH), a cranking battery (1 x 100AH Flooded) and a multi output smart charger.

I am considering replacing my (1,B,2,Off) switched system with an FET Isolator and an on/off switch to create a true house/cranking separation. I have found many posts with complicated schematics to include many other optional in and outs. I like the FET isolator option better than using an ACR.

I attached what I think is the correct wiring. At least the starting point before I add the fuses, breakers, switches etc.

I plan to get a certified marine electrician to help but want to make sure I have all of the parts and supplies before we start.

Am I started in the right direction?
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Old 06-08-2020, 19:41   #2
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Re: FET Isolator vs ACR for simple charging systems

You need a switch on the engine as well. and it’s good to have the abiliby to parallel or cross connect. Which is where the blue sea dual circuit switch comes in handy.

If I had a 35a alt. I wouldn’t bother rewiring it and just add an Acr instead. If you were say upgrading to a 100a and needed to redo the wiring anyways. Then Yiu could look at different options.
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Old 07-08-2020, 06:01   #3
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Re: FET Isolator vs ACR for simple charging systems

I have a FET isolator. My charger and alternator both connect at 1/2/both/off switch and then feed to the IN on the isolator when the house bank is also connected. That then feeds across it to the start battery circuit loads and on/off switch.

This way my house battery feeds my start battery circuit, but the start battery can not feed the house loads.
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Old 07-08-2020, 07:30   #4
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Re: FET Isolator vs ACR for simple charging systems

Thanks smac99 for the reply.

I agree that rewiring the 35 amp alternator will be a bigger job. This project was started because my battery charger died. It was not correctly wired so I decided to start with a blank slate. I may take your advice and simply replace the charger (wired correctly this time) and then wait to do an isolator when I need to replace the alternator. (Option B) I can live with the downsides of the 1/B/2/O switch for a while. And yes, an ACR option would be less wiring and be better than just the 4 way switch.
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Old 07-08-2020, 09:32   #5
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Re: FET Isolator vs ACR for simple charging systems

A battery isolator, FET or not still has a voltage drop. I'm guessing that the alternator has an internal voltage regulator so the voltage output is measured at the alternator.

If you were out cruising that would be a poor choice of a setup but as a weekend sailor the battery charger is more important. If you were on a mooring or out cruising for long periods of time an ACR would be better. Also a larger alternator with an external regulator that measures voltage at the battery.
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Old 07-08-2020, 10:01   #6
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Re: FET Isolator vs ACR for simple charging systems

Not only is an ACR a better choice for the reasons stormalong posted but it lends itself more easily to multiple charge sources. Solar or any other added source will charge both banks with an ACR. An isolator, in addition to its loss of voltage, is designed to work with an alternator only. An ACR is both more effective and less money. There are no downsides to an ACR.

And yes you do need a switch for the start battery.
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Old 07-08-2020, 12:11   #7
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Re: FET Isolator vs ACR for simple charging systems

Thanks for the great feedback. I was actually committed to the ACR option until I read an article about nuisance tripping that can happen when the ACR is connected to two battery banks of different sizes. The article described fuse tripping that can occur when one of the banks is full and the other is deeply discharged. When combined, the current rushes from the charged battery to the depleted battery tripping the fuses along the path. Maybe this is not an issue with my battery configuration.

Besides the benefits you mentioned, it would require less wiring changes on my boat.
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Old 07-08-2020, 12:18   #8
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Re: FET Isolator vs ACR for simple charging systems

Why are there two outputs from the charger? Normally one output. If there are two, are they separately programmable? Assuming you have one output from the charger or that even if there are a couple or 3 terminals they are probably all connected together inside the charger. I also assume your batteries are all the same chemistry, no Lithium and lead acid combination.
My suggestion for your diagram:

Delete the wires that you show connected to charger terminal #1 and charger terminal #2. Put a single wire from charger #1 to the FET isolator input, the same terminal that the alternator connects to.

This way your batteries are isolated yet still will be charged either from the charger or the alternator. It's OK to combine these output currents so you can run the engine at the dock with the charger plugged in and operating.

If you ask about ACR, yes the FET isolator is a much better solution for battery isolation. I have many years troublesome experience with mechanical relays on my boat. They gradually build of resistance in the contacts. You will only know this if you monitor the voltage drop across them at high charging current. And the coils use up power and get hot. It's all bad IMHO.
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Old 07-08-2020, 12:30   #9
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Re: FET Isolator vs ACR for simple charging systems

The other concern I had with the ACR was possible interference with the dual output charger??? It is trying to charge two banks separately that are now combined?
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Old 07-08-2020, 12:37   #10
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Re: FET Isolator vs ACR for simple charging systems

Hello Waterman46. Thanks for the detailed input. A quick answer to your question is "Yes, the ProNautic Charger 1220P has three outputs". It is supposed to intelligently charge up to three banks depending on their needs.
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Old 07-08-2020, 12:39   #11
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Re: FET Isolator vs ACR for simple charging systems

Quote:
Originally Posted by waterman46 View Post
Why are there two outputs from the charger? Normally one output. If there are two, are they separately programmable? Assuming you have one output from the charger or that even if there are a couple or 3 terminals they are probably all connected together inside the charger. I also assume your batteries are all the same chemistry, no Lithium and lead acid combination.
My suggestion for your diagram:

Delete the wires that you show connected to charger terminal #1 and charger terminal #2. Put a single wire from charger #1 to the FET isolator input, the same terminal that the alternator connects to.

This way your batteries are isolated yet still will be charged either from the charger or the alternator. It's OK to combine these output currents so you can run the engine at the dock with the charger plugged in and operating.

If you ask about ACR, yes the FET isolator is a much better solution for battery isolation. I have many years troublesome experience with mechanical relays on my boat. They gradually build of resistance in the contacts. You will only know this if you monitor the voltage drop across them at high charging current. And the coils use up power and get hot. It's all bad IMHO.
I have to disagree with this. A battery isolator has a voltage drop so the batteries are never getting a full charge. The ACR has no voltage drop. I have had a Yandina ACR in use for about 20 years now without problems and they come with a lifetime warranty. The relay coil uses power when you have power to burn. It draws power only when there is a charging source.

Many battery chargers have multiple battery outputs these days. They are isolated but not individually regulated so different battery chemistries still need separate chargers.
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Old 07-08-2020, 12:45   #12
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Re: FET Isolator vs ACR for simple charging systems

Quote:
Originally Posted by gpsatsea View Post
The other concern I had with the ACR was possible interference with the dual output charger??? It is trying to charge two banks separately that are now combined?
That is not an issue with same chemistry batteries. The battery charger has multiple isolated outputs but one charge controller. The bit of advertising about each battery getting what it needs is the same thing that happens anytime multiple batteries are charged at a set voltage. The more discharged battery has a lower internal resistance so more power goes to it until both batteries are at the same voltage.
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Old 07-08-2020, 14:34   #13
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Re: FET Isolator vs ACR for simple charging systems

Quote:
Originally Posted by gpsatsea View Post
Thanks for the great feedback. I was actually committed to the ACR option until I read an article about nuisance tripping that can happen when the ACR is connected to two battery banks of different sizes. The article described fuse tripping that can occur when one of the banks is full and the other is deeply discharged. When combined, the current rushes from the charged battery to the depleted battery tripping the fuses along the path. Maybe this is not an issue with my battery configuration.

Besides the benefits you mentioned, it would require less wiring changes on my boat.
I have not ever come across the issue you mention and I have installed dozens of ACRs over the years.

The only issue with an ACR usually occurs when the manual isn't read before installation. All charging sources should go to the larger of the 2 battery banks. If they go to the smaller battery what happens is the following. When the small battery hits 13 volts the ACR combines. When this happens the current from the smaller battery causes an inrush to the larger bank if it is at say 50% SOC. Because of this the voltage drops and the ACR then closes. This will repeat until the voltage rises enough to keep the ACR combined. It is called chattering by some. If the charge sources go to the larger bank there is no issue.

Even in the above case as long as the correct wire size is chosen and properly fused there is no problem, just a click as the ACR closes and opens.

In your case leave the Pronautic output 1 in place. Eliminate output 2. Wire the alternator output to the house bank. Wire the ACR between the battery banks, essentially replacing the isolator in your schematic. Any additional charge sources such as solar wire to the house bank. Both banks will charge automatically whatever the source.
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Old 07-08-2020, 14:55   #14
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Re: FET Isolator vs ACR for simple charging systems

I recommend that you go on U-tube, search for "Pacific Yacht Service" There is a whole series on this and related issues! It covers it all.
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Old 10-08-2020, 09:48   #15
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Re: FET Isolator vs ACR for simple charging systems

My original post was made before I looked up the specs for your Promariner charger. My bad. The charger actually has programmable outputs for up to 3 different banks, so they must be isolated as far as the charger is concerned. Your wiring circuit showing the FET isolator looks good to me.
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