Discharge through blue sea ACR

Wrathchild

Active member
Just wrapped up my dual battery and solar setup. The starting battery is connected to a northstar group 31 AGM via the blue sea ACR and disconnect.

On my Victron monitor I am seeing about 100 - 200 mA discharge from the AGM into the truck battery (solar and all loads disconnected from the house battery). I know for a fact it’s discharging into the starter battery because I can make the draw go to zero by switching the disconnect to off.

The ACR is functioning correctly. But it would take a really long time for it to isolate since at rest my starting battery is above the 12.75 volt threshold.

The starting battery is in good shape, only about 4 months old. I replaced it in anticipation of the dual battery and wanting both batts having similar strength.

Should I be concerned about this small discharge?

I assume this is just one of the pitfalls of having slightly dissimilar batteries wired in parallel.

It’s not a big deal to me if I need to connect them when I want a good bulk charge from the alternator. But I would prefer to have them connected and just do their thing without my input.
 

rayra

Expedition Leader
doubt it's the dissimilar batteries, or the ACR's fault per se (but I'm not 100% sure of their operating parameters). It most likely reflects a parasitic drain in the factory side of things, which is typical in the multiple-computerized systems in recent decades. I had one occasion with my Sub that took a long time to suss out and eventually figured out it was my analog OnStar trying to phone home for help*, after a mild vehicular impact. Had to kill all power to reset everything.

eta


What is an ACR, and how does it work?
  • What is an ACR?
    An ACR parallels (combines) batteries during charging, and isolates them when charging has stopped and after battery voltage has fallen. An ACR is intended to keep a load from discharging both of the batteries.
  • How does an ACR work?
    An ACR senses when the voltage of either of the batteries rises to a level indicating that a charge source is active (13.0V for 2 minutes). The ACR′s contacts then connect and the ACR applies the charge to both batteries. If the voltage on both of the batteries subsequently drops to 12.75V for 30 seconds, the ACR will disconnect, isolating the batteries.

--
so if I interpret that correctly, if both your batteries are over the 12.75 level that 'smart' ACR may be continuing to combine the batteries until one of them drops below 12.75V. And that could come from either the batteries settling down or from a drain-down. And it will take the 100-200mA draw a while to do that. what are the volt readings on your batts first thing in the morning, after they've been idle for hours?
So I'd basically figure that your ACR is working as designed.
And furthermore as long as your solar is running, your ACR is going to share that juice as it sees >12.75V as long as the solar charge controller is is providing juice.

as an aside, I'm getting a similar 'care free' combining / disconnect from my 'dumb' solenoid solution, they're only combined when the key is hot and not when not.




* hasn't been an analog OnStar system available to answer it for over a decade, it will literally try to phone home until your battery is dead. And you can't really disable it. Tied into everything, no discrete fusing.
 

john61ct

Adventurer
If it's just the starter batt who's lifespan will be slightly reduced

I wouldn't worry, next one check its Full V at rest is lower.

But I would contact Victron, that is a design flaw, maybe they have a fix.
 

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Wrathchild

Active member
Well I’m a giant dummy. Thank you @rayra i just did a parasitic drain test on the truck batt. Seeing about 100mA which is in line with all the electrical stuff in newer vehicles. I guess inadvertently my solar will be helping the truck battery too.

not too worried about it now I guess. Worst case a few aHs get pulled out of the AGM until they isolate. Which will all get replaced by the alternator or solar.
 

Wrathchild

Active member
@john61ct What would the design flaw be? Seems to me like the monitor and solar are working perfectly. It also seems like dumb luck that I connected the second battery ground to the starting battery. If not my monitor would probably not be seeing the discharge.
 

rayra

Expedition Leader
Now I wonder if the ACR stays closed / connected all day when the solar controller is doing its thing? Seeing >12.75.
It will stay connected while the sun is up and the rooftop solar is juicing the Aux and the ACR holds connected while the Starter battery is excited and then taking power from the solar thru the ACR connection. Until the sun goes down and the solar controller stops providing power, both batteries were getting bulk / float from the solar controller and that's holding the ACR in thrall until the factory side vehicle parasitic drain drops (both!) batts down to 12.75 at which point the ACR opens / severs.
At which point the Starter side creeps lower with the parasitic drain. Then the sun rises, solar does its thing, Aux goes up. Starter keeps draining, until you fire the vehicle up and the ACR sees ALT voltage at your Starter battery and combines both. You drive around, park it for the night and cycle repeats.


I was having something similar last year with my no-name PWM controller getting my Aux to 12.6-12.7. But my key-triggered (aux accessory fuse supply) solenoid severing things as soon as the vehicle switched off. And my GM/Bosch factory ALT only getting the batteries to ~12.7 on a good day. Then I had my OnStar go nuts and was likewise losing about the same draw as the OP. And driving only every 2-3 days I was seeing as much as .1-.2V divergence between the batts. Then I got a better / newer / Renogy PWM that zaps things at 14.6-14.7 and nets me a solid 12.9-13.0 on my Aux. But Start still lower as the factory alt was always holding lower and running at 14.1V. Couple months ago I got my aftermarket high output ALT and it (also) puts out at 14.6-14.7. And now I finally routinely get 12.9-13.0v resting at both of my flooded batts. Which is nice.
 

Wrathchild

Active member
Yeah I guess you’re right. Ground is ground. I’ve been working on electrical systems for a long time. But this is my first time “engineering“ one. Been a fun experience and know a heck of a lot more now than when I started.
 

john61ct

Adventurer
Yes that is what an ACR is supposed to do

so long as a charge source is active on either side, the two circuits are combined into one.

The design flaw is selecting a trigger voltage that does that even when no source is active.
 

Wrathchild

Active member
Now I wonder if the ACR stays closed / connected all day when the solar controller is doing its thing? Seeing >12.75.
It will stay connected while the sun is up and the rooftop solar is juicing the Aux and the ACR holds connected while the Starter battery is excited and then taking power from the solar thru the ACR connection. Until the sun goes down and the solar controller stops providing power, both batteries were getting bulk / float from the solar controller and that's holding the ACR in thrall until the factory side vehicle parasitic drain drops (both!) batts down to 12.75 at which point the ACR opens / severs.
At which point the Starter side creeps lower with the parasitic drain. Then the sun rises, solar does its thing, Aux goes up. Starter keeps draining, until you fire the vehicle up and the ACR sees ALT voltage at your Starter battery and combines both. You drive around, park it for the night and cycle repeats.
.
This is exactly what happens. It seems like I was giving the ACR more credit than its worth. Its really just a fancy safety switch. Everything is working normal and how it’s supposed to. Just surprised the discussion isn’t more common. Or are people not really diving into what’s going on with their batteries? Im not the only person that’s done a system like this in a newer vehicle that is power hungry.
 

Wrathchild

Active member
Yes that is what an ACR is supposed to do

so long as a charge source is active on either side, the two circuits are combined into one.

The design flaw is selecting a trigger voltage that does that even when no source is active.
Now I understand. Yeah it would be nice if the blue-sea had a disconnect voltage closer to 13. But it seems like now both batteries are benefitting from the solar accidentally on my part.
 

john61ct

Adventurer
The lower isolation setpoint allows tge two circuits to stay combined longer so you can tap some of the power in your start battery to run auxiliary loads.

It will still disconnect the two batteries early enough to prevent your start battery running down too low to start your engine, but it does give a bit more flexibility for smaller systems to use some of the power of the start battery.
 

Utah KJ

Explorer
Just wrapped up my dual battery and solar setup. The starting battery is connected to a northstar group 31 AGM via the blue sea ACR and disconnect.

On my Victron monitor I am seeing about 100 - 200 mA discharge from the AGM into the truck battery (solar and all loads disconnected from the house battery). I know for a fact it’s discharging into the starter battery because I can make the draw go to zero by switching the disconnect to off.

The ACR is functioning correctly. But it would take a really long time for it to isolate since at rest my starting battery is above the 12.75 volt threshold.

The starting battery is in good shape, only about 4 months old. I replaced it in anticipation of the dual battery and wanting both batts having similar strength.

Should I be concerned about this small discharge?

I assume this is just one of the pitfalls of having slightly dissimilar batteries wired in parallel.

It’s not a big deal to me if I need to connect them when I want a good bulk charge from the alternator. But I would prefer to have them connected and just do their thing without my input.
I've built multiple vehicles in a short period of time, using the ACR, AGM and LiFePO4 batteries, in each case it was a problem. In one case, we removed the 2nd battery, another we replaced the 2nd battery with another AGM with a PAC isolator, and the other with a Redarc isolator and DC to DC charger. The later was the best, and what we've done going forward.
I have a bunch of low-mileage ACR's sitting around if anyone is not doing anything particularly challenging.
 

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Wrathchild

Active member
@Utah KJ did you redo the systems because you were having problems with it? Or was it to ensure that both sides were 100% isolated?

After looking at it and learning more im not really seeing a negative. Worst case a few aHs get pulled out of my house battery, they isolate, then it all gets replaced rapidly once the sun comes out or I fire up the truck.

The REDARC stuff is super nice, but I can’t justify the expense Unless there’s a really tangible benefit.
 

DiploStrat

Expedition Leader
One more time with feeling and gross undersimplification:

-- A properly designed relay system will work perfectly if the vehicle alternator puts out the correct voltage for both batteries and the resting voltage is the same. That loosely translates to - if your starter battery and camper battery are both lead acid of some type.

-- If your alternator voltage is too low or you want to mix lead acid and lithium, a battery to battery charger is probably your best bet, assuming that your battery to battery charger has the correct profile for your camper battery. Typically this means trucks that charge at 13.9v or lower and large lithium battery banks that might overload the alternator.

We will, for simplicity, ignore double alternator setups and other exotics.

Bottom line, if this doesn't make sense, get good help.
 
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