Alternator, ACR, and MRB re-wiring

[Rudderly Confused]

Hello all, especially any electrical gurus out there.  I'm in the process of re-wiring and updating the 40 year old mess that is Valhalla's 12V electrical system.  I want to re-direct the alternator output wire to the house battery bank, and use a Blue Sea 7610 ACR to charge the start battery. My plan on paper was to run the alternator output to a Blue Sea #6005 switch for a service disconnect, from the switch to a Blue Sea MRBF 5196 closely connected to the house bank. The ACR would also be connected to the  house battery bank through the same MRBF fuse, and to the start bank through another MRBF fuse at the start battery.  However, when I started installing (but not yet wiring) the components, I saw my service disconnect switch and the ACR are within inches of each other.  Then I began to wonder if I could just run a short wire from the alternator switch output to one of the terminals on the ACR and run one wire from that terminal to the house bank MRBF?  My thinking was the alternator output wire and the ACR wire to the house bank would have been connected at the MRBF anyway, and I could use one 5' wire run instead of 2.  The attached diagrams may be clearer than my words.  My concern is if running the alternator output through the ACR would affect the voltage sensing inside the ACR, and therefore affecting when it combines charge to both batteries?  Plan A or plan B?  Any thoughts, or anything I'm not seeing clearly?  Thanks.

[PeteW]

Based on what you are trying to do, the ACR looks like  the wrong device. If you want to isolate your alternator current and direct it to 2 battery banks based on demand I recommend using a Pro Split R from Sterling. The alternator regulator sense wire if you have a dedicated external alternator regulator goes to the start battery. The Pro Split R will sense which battery bank is in the lowest state of charge and directs charge current to that battery bank. And so on.

https://sterling-power.com/products/volt-drop-alternator-splitting-system-pro-split-r

But if you want to use the ACR I recommend following the diagram in the installation guide. That means that the alternator gets hard wired to the start battery. And the start battery is hard wired with AWG 0 to the starter motor. With the Pro Split R there is a dedicated connection for alternator charge current.  I see no need to switch charge current with the ACR. It will charge both batteries just the same.

http://assets.bluesea.com/files/resources/instructions/990170140.pdf

The use of a battery switch is for selecting starter current (750 amps) not charge current. Start current is a different circuit with much heavier wiring than the 60 amps that the ACR is designed to handle. So that switch on your alternator serves no purpose. if anything in that alternator leg, you would be better served fusing it to protect you from fault current due to  a shorted diode. The fuses on the ACR (60 amps) are for charge current only.

There are schematics on the site.   Pete

[Rudderly Confused]

Hi Pete,

Thanks for taking the time to help me think this through.  Perhaps I didn't explain my reason for the disconnect switch very well.  The only purpose is to disconnect the alternator from the house bank in the event the alternator ever needs to be worked on.  At all other times the switch will be in the "ON" position.  I'm trying to follow this advice:

http://www.pbase.com/mainecruising/image/158793046

It is apart from, and in addition to, my main battery selector switch.  Without a disconnect switch, wiring the alternator to any bank would make the alternator B+ hot at all times, even when the engine was off, and even if the main battery selector switch was off.  To safely do any work on the alternator I would have to physically disconnect it from the bank by removing the cable, or the fuse.

From what I've read, connecting the alternator directly to a battery bank, as opposed to going through the starter or battery selector switch, provides a more efficient charge to the bank.  Less voltage drop, more efficient charge.  The purpose of the ACR is to divert some charge to the start bank once the house bank reaches a certain voltage.  I've had a series of email exchanges with Blue Sea, and I have the installation guide shown in the link you provided.  The Blue Sea technician suggested I follow those guidelines as well, but never gave an answer if my Plan B (above) would work.  He also did not comment either way on the alternator disconnect switch.  As of now, that is how I will do it, my Plan A. 

One part of your reply I did not understand is why you would wire the alternator to the start battery rather than the house bank?  The Blue Sea installation guide shows 4 possible places to direct the alternator wire, starter, main battery switch, start bank, or house bank. It goes on to say connecting the alternator to the larger bank is most efficient.  In my case, and I assume in most 2 bank installations, the house bank is the larger bank.

Chuck

[PeteW]

I'm not a big fan of the Ping-Pong relay charge controller devices. Sort of old school and low tech. You would be better served wiring in an A-B-Both switch. At least with that you would be able to start your motor from the house battery in an emergency. If smart charging, efficiency and isolation of multiple battery banks is your requirement I stand with my recommendation of the Pro Split R product from Sterling.

I have never seen anyone recommend a service switch on a alternator. I does not strike me as being useful. In fact disconnecting a running alternator from its battery load will cause overvoltage spikes that could blow out the alternator and depending on how you have you boat wired every piece of electronics on board. Fusing your alternator at 150% of its output is what I have seen recommended. If you install a switch it should be a breaker.

You can wire your alternator to the house battery if you like. With the pro split R you don't wire it to any battery. The smart device figures that out for you based on battery bank voltages. When the ignition is off the alternator is isolated from all batteries.

I'd be remiss if I did not add that the key to efficient battery charging is actually based on having a smart alternator controller. I've had several and have found the Gasser Smart regulator to be the best. A must if you have AGM batteries. The manual float mode switch is will keep you from frying some expensive batteries.

Sounds like you have already made you mind up on what you want to do. Let us know how it works out.

Pete

[Rudderly Confused]

Pete,

My battery selector switch is an On/Off/Both, which isolates the house from the start circuit, but can combine both banks for emergency starting.  If that ever fails, I will go to the A/B/Both style switch.  I'm also aware that switching off a running alternator will cause big problems.  My disconnect switch is not near the main battery selector switch, or the DC distribution panel, so hopefully it won't get turned off by mistake.  It's also well labeled.  In the past I've had issues with my alternator and tach connections, so I thought the disconnect switch would be handy if I have to get in there to work on those.

The Prosplit R looks interesting, I hadn't heard of those before.  I already have the ACR in place, but not yet wired, but if for some reason my plan doesn't work out, I'll look deeper in to the Prosplit.  Either way, when Spring finally gets here and I have the batteries back on the boat, I'll try to remember to let the board know how things worked out.

Thanks again,
Chuck

[Della and Dave]

At the possible risk of re-opening Pandora's box.  There is a pretty good string on electrical wiring on this site from a few years ago.  See. http://www.pearson365.com/forum/index.php?topic=1293.0

I learned a lot from it about good wiring practices