ACR Start Isolation Feature Question

LeslieTroyer · Feb 21, 2018

@Ward H i think you answered your own question. It only benefits folks like me that have separate house and start batteries. You will derive no benefit and it won’t hurt the emergency battery to be disconnected for a short period.

Mahalo

dlochner · Feb 21, 2018

@Ward H since you are using a 1-2-both battery switch, connecting the Start Interrupt wire will only matter if the engine is started when the switch is on Both. The only time it would be necessary to start on the Both position is when both banks are low and you need all the power in both batteries.

marchem · Feb 21, 2018

Ward H said:
The only advantage I see is that by isolating the batteries your ensuring the starting current is only drawn from the house bank.

Of course the batteries will only be connected by the ACR when charging with the shore charger if the engine is not running.
Is the issue with starting the engine while charging with shore power?

I'm in the middle of my DC upgrade and am ready to run a wire for the SI feature if I will benefit from it. Just don't see how I will.

I know I am missing something so appreciate someone helping me understand this.

If you are using your house bank for both starting and the house, then there's no reason for it. Like Leslie said: only of value with separate banks that you want to keep separate, except when being charged. As you said, this would only be when starting while charging with shore power, although I'm sure there might be some other circumstances when it might help.

I didn't use it on the Catalina 27 I rewired, but I did on the current boat. Given everything else I was doing at the time it was a small amount of extra work, and gave me a chance to make a more secure connection at the starter connection.

marchem · Feb 21, 2018

dlochner said:
@Ward H since you are using a 1-2-both battery switch, connecting the Start Interrupt wire will only matter if the engine is started when the switch is on Both. The only time it would be necessary to start on the Both position is when both banks are low and you need all the power in both batteries.

Dave, not true. The ACR will combine both banks when charging. If you start the engine when it is combined both banks will be drawn on. And depending on how you wire the battery switch when it is in the Both position the ACR is superfluous, because the battery switch is now combining both banks.

Scott T-Bird · Feb 21, 2018

Ward, I'm not sure about that either. It seems to me, that if you have sensitive electronics that you want to protect from a starting surge, then you would have to have them wired to one battery that is normally on the house bank but must be isolated when starting. I don't know how you do that and it probably doesn't matter since you have combined 2 - 6V batts for your house bank. As you know, I have dedicated house and start banks with DCP switch (electronics wired to house), which isolates the banks in normal operation, so I don't see any need to use the start isolation, either.

Scott T-Bird · Feb 21, 2018

Actually, reading a bit more carefully, it explains that it temporarily prevents current from flowing thru the ACR to affect the house bank during a start current event. I'd guess that you can't protect any sensitive electronics that are wired to your house bank because you are starting with your house bank. It seems that I don't have that isolation protection (that I thought I have - not that I have sensitive electronics) unless I use the isolation wire led to the starting circuit.

Scott T-Bird · Feb 21, 2018

Ward H said:
@Scott T-Bird
My understanding from the 1/BOTH/2/OFF Switches Thoughts & Musings thread is that if you wire for low voltage drop, have a beefy, healthy house bank and a small aux diesel like found on our size boats, voltage drop during starting should not be enough to restart electronics.
If a voltage spike takes out my 1993 electronics I'll just have to buy new.

Sounds like a plan!

How about the 70's and sun today! Leaving for Montana next week, take care of my baby! (Windows might be delivered next week)

dlochner · Feb 21, 2018

marchem said:
Dave, not true. The ACR will combine both banks when charging. If you start the engine when it is combined both banks will be drawn on. And depending on how you wire the battery switch when it is in the Both position the ACR is superfluous, because the battery switch is now combining both banks.

I agree, the SI circuit when using a 1-2-Both switch is superfluous. If the ACR is combining the batteries and the switch is in the Both position, then the batteries will be combined through 2 separate parallel circuits. Not entirely sure under what conditions that would occur other than if one of the banks was being charged, for instance at the dock with shore power or with solar.

Thinking out loud...

Let's say both banks are low and being charged, the ACR will allow current to pass through to charge both batteries. When the switch is placed in both then current can flow through 2 circuits, the switch and ACR. The switch will not have voltage limits while the ACR does have upper and lower voltage limits. There will be 2 parallel circuits. The starter will be on one branch of the circuit, the branch that goes through the switch. When the starter is engaged, we want all the power to go to the starter, especially if there is any other house load like refrigeration. When the starter engages the SI circuit isolates the 2 banks and and all current then flows through the switch to the starter.

Those conditions would be pretty rare and the difference the SI makes, may be a difference that doesn't really make a difference.

Stu Jackson · Feb 21, 2018

dlochner said:
The only time it would be necessary to start on the Both position is when both banks are low and you need all the power in both batteries.

Dave, you've been around for a while. This is absolutely the WORST time to combine a bad and good battery bank. The bad one will drain the good one down.

If one bank can't start your engine, then use THE OTHER BANK. That's what it is there for, and should be sized properly to do just that.

And good management shouldn't allow both banks to "get too low." Indeed, the use of a properly sized house bank and a separate reserve (or start) bank should preclude that from happening.

This is much like this discussion from Maine Sail:

AGM Battery Issues and the Blue Seas Dual Circuit Switch (from Maine Sail) "DARN AGM Batteries"
https://forums.sailboatowners.com/index.php?threads/darn-agm-batteries.133773/

marchem · Feb 21, 2018

dlochner said:
I agree, the SI circuit when using a 1-2-Both switch is superfluous. If the ACR is combining the batteries and the switch is in the Both position, then the batteries will be combined through 2 separate parallel circuits. Not entirely sure under what conditions that would occur other than if one of the banks was being charged, for instance at the dock with shore power or with solar.

Thinking out loud...

Let's say both banks are low and being charged, the ACR will allow current to pass through to charge both batteries. When the switch is placed in both then current can flow through 2 circuits, the switch and ACR. The switch will not have voltage limits while the ACR does have upper and lower voltage limits. There will be 2 parallel circuits. The starter will be on one branch of the circuit, the branch that goes through the switch. When the starter is engaged, we want all the power to go to the starter, especially if there is any other house load like refrigeration. When the starter engages the SI circuit isolates the 2 banks and and all current then flows through the switch to the starter.

Those conditions would be pretty rare and the difference the SI makes, may be a difference that doesn't really make a difference.

The SI circuit is only superfluous if the battery switch is in Both. And as Stu said there are good reasons to never use the Both position.

Also, I'm not sure that the starter will be on the branch that goes thru the switch; it depends on how things are wired. It's not for my wiring scheme.

dlochner · Feb 21, 2018

Stu Jackson said:
Dave, you've been around for a while. This is absolutely the WORST time to combine a bad and good battery bank. The bad one will drain the good one down.

Stu, I didn't say that's what I would do. The first project on my last 2 boats was to separate the house and start batteries. Also, I wasn't thinking big differences, say one bank at 12.5 v and the other at 12.2 volts.

dlochner · Feb 21, 2018

Ward H said:
I'm actually thinking about putting a red circle with a line through on the word "Both". May help to keep the Admiral and helpful guest from putting the selector on that position.
Or replace with switch with a new Blue Sea 9001e. I think it will fit the same position and has the "Both" position on the bottom so you do not have to go through the Both position to move from 1 to 2. $35. If I'm replacing everything in the system but the starter, whats another $35 for a new switch.

Or better yet, a Dual Circuit Plus, e-Series Dual Circuit Plus™ Battery Switch - Blue Sea Systems It's only money.

Ron20324 · Feb 21, 2018

Stu, you missed the part where Dlock says BOTH batteries are low. It seems to me previous posts point out the ACR is to make up for people who do not use a 1-2-all switch properly, or forget when to change it's function. ACR puts the charging regimen in the power lead that provides a voltage higher than the batteries hold. It the charge voltage is higher than the batteries (and above the ACR's trigger voltage), the ACR operates to connect the house to the start- provided the charge source(s) IS wired to the house. I will point out do not have an ACR as I have 3 individual battery switches.

dlochner · Feb 21, 2018

Ward H said:
But the DCP switch fully isolates the two batteries UNLESS you put it in the combine position.

Correct.

Ward H said:
What you cannot do is deplete the house bank, then switch to the reserve bank to continue using house loads (instruments) and for starting.

To make this work correctly, each bank needs a separate on/off switch prior to feeding the DCP switch. For example if the house bank is dead, turn it off, switch the DCP to the combine position, and the other bank will now feed the starter and the house. The ACR will allow the house battery to be charged even though it has been isolated.

Ward H said:
I've even considered buying a new switch without the "Both" position.

That leaves no fall back position. A few years back the boat was on the hard, far away from any electricity, and needed winterizing. The start battery low enough, that it wouldn't start, the house battery wasn't strong enough by itself to start the engine, but combined there was enough power to get it started. Once winterized, I left the batteries to the next owner to deal with.

Ward H said:
If I find both banks too low to start the engine, I'll use my Tow Boat US insurance.

Or just sail it back!

Maine Sail · Feb 21, 2018

Ward H said:
Or replace with switch with a new Blue Sea 9001e. I think it will fit the same position and has the "Both" position on the bottom so you do not have to go through the Both position to move from 1 to 2. $35. If I'm replacing everything in the system but the starter, whats another $35 for a new switch.

If you want to go from bank 1 to bank 2 the best pathway is through BOTH. No drop out of electronics and no blown diodes and the resultant transient to house loads if the alts still flows through the battery switch.

There is NOTHING WRONG with passing through the BOTH position and you don't drop instruments or such by doing so...

The SI feature is a nice addition, even with a 1/2/B, because it prevents the high starting currents from being carried across the relay when you have alternative energy or are starting at the dock while still charging and the relay is in combine mode...

Some may be interested in this:

Voltage Transients In Marine Electrical Systems

Scott T-Bird · Feb 21, 2018

Maine Sail said:
The SI feature is a nice addition, even with a 1/2/B, because it prevents the high starting currents from being carried across the relay when you have alternative energy or are starting at the dock while still charging and the relay is in combine mode...

Some may be interested in this:

Voltage Transients In Marine Electrical Systems

Thanks for this discussion, Ward. Even after reading all the posts on these battery switch topics, I missed the point that high starting currents can be carried across the ACR. I thought that by adding the 2 on-off switches with my DCP, that I finally had true isolation in both banks.

I will now have to wire the SI feature.

(Even though it would be an extremely rare event when it would be necessary, it seems)

Except for the alternator output, I am left wondering why we need the ACR, though. It seems that our battery charger (you now have the same one as mine) could simply be wired to both banks. But I guess when you consider the AO, we might as well leave it in place.