Dual battery solenoid options

[MDH33]

After 16 years of reliable use, my National Luna dual battery solenoid died on my recent Utah trip. I need an easy replacement and was wondering what folks are doing in 2025? I already have all the cables, so would like to just replace the solenoid and in cab controller. I wouldn't mind something simpler that I can easily connect the 2 without a timer, etc.. Any suggestions?

 

[MDH33]

Also, could I just delete the solenoid and connect positive to positive on the batteries to keep the second charging in the meantime?

 

[Corbet]

I’ve gone solar. 80 watt panel takes care of my camp needs with just one quality AGM battery.

 

[mikedrom]

I use this to isolate my winch: https://a.co/d/90fDHUp

 

[Cruisertrash]

I have a very simple system running a Blue Sea ml-ACR (7622) and it works great. Connects and disconnects automatically when it needs to. You can force connect or disconnect on the unit itself or remotely with the in cab button. Enough current capacity for a starter or winch - I think it’s very 500A for some decent duration.

@Crash runs a BEP unit that’s very similar too. I know that’s been solid for him.

 

[MDH33]

I have a very simple system running a Blue Sea ml-ACR (7622) and it works great. Connects and disconnects automatically when it needs to. You can force connect or disconnect on the unit itself or remotely with the in cab button. Enough current capacity for a starter or winch - I think it’s very 500A for some decent duration.

@Crash runs a BEP unit that’s very similar too. I know that’s been solid for him.

That looks like a solid choice. Where did you mount it? Did you do anything for in cab voltage monitoring?

 

[Cruisertrash]

That looks like a solid choice. Where did you mount it? Did you do anything for in cab voltage monitoring?

I mounted it on the left side inner fender. The 60 Series battery trays and hardware are ambidextrous since they were used for 24V trucks. I installed new OEM stuff (only the diagonal support brace is unavailable) for that on the left side, and the ml-ACR is mounted just behind that.

For monitoring I took an Amazon dual voltmeter, extracted the electronics, then stuffed it into a factory switch blank that I had cut an opening in.

Currently the second battery is monitored directly from the positive post, and the primary battery is monitored off the cigarette lighter hot wire (so it’s ignition-dependent). I had to make a small modification to the circuit board of the voltmeter by removing one diode - in its original design the voltmeter stays on if it gets voltage from either battery. I wanted it to come on only when it received power via the ignition. I have a write up for it on Mud and I can send the link and walk you through. Both positive feeds to the meter are fused of course!

Or … get the National Luna dual voltmeter, more of a bar graph style thing, looks pretty slick.

 

[Cruisertrash]

Honestly, I kind of want three voltmeters. One directly from each battery positive post, and one tapped off the interior fuse panel. So I could read each battery’s state of charge directly, plus I could read the “system” voltage which accounts for voltage drop from both wire resistance and electronics being on. The dummy gauge still works but I don’t look at it much.

 

[Crash]

@Cruisertrash thanks for chiming in for me. I was waiting to get a chance to find the paperwork for my BEP as I couldn’t remember the name. @MDH33 I have found the unit to be super simple in operation and efficient in usage. I have the 500 amp version but I know of a couple of trucks with the 150 amp that have worked great. It would be an easy install if you have the wiring already in place and in cab switching is available. I don’t run voltmeters - too much information. Mine is mounted on the driver side fender well and easily switched manually so I don’t have it wired to switch in cab.

 

[Crash]

Looked at BEP isolators online and I'm not sure I can recommend them anymore. The new ones are about 1/3 the cost of the one I bought 13 years ago. Not saying they aren't recommendable just that I don't have any experience with what appears to be a very different product than what I have.

 

[nakman]

Also, could I just delete the solenoid and connect positive to positive on the batteries to keep the second charging in the meantime?

yes. would be as if your solenoid combined them. Maybe not ideal long term you risk running them both down, and your alternator might not enjoy charging them both at the same time, but yeah it would work.

I also did the Blue Sea ACR in the 60... my voltage meters were inside the Tuffy, along with the switch to combine batteries.

most of it is in the build thread for that one: https://risingsun4x4club.org/xf/threads/nakmans-60-build.30021/page-6

 

[Inukshuk]

Maybe not ideal long term you risk running them both down

Correct as this is THE purpose of isolators

, and your alternator might not enjoy charging them both at the same time, but yeah it would work.

Alternators always charge both once combined. Why do you say this would be an issue in a system with two batteries in parallel?

Also, could I just delete the solenoid and connect positive to positive on the batteries to keep the second charging in the meantime?

Yes. This is simple wiring in parallel. Voltage remains at 12V. Combined amperage is now available.

I also did the Blue Sea ACR in the 60... my voltage meters were inside the Tuffy, along with the switch to combine batteries.

I have a Blue Sea ACR in Long Cruiser to isolate starting battery from House bank. I use the less-expensive and lower amp capacity isolator. If I needed to start or jumpstart myself from the house-bank I'd just have to use cables or fiddle with a few wires.

 

[nakman]

Alternators always charge both once combined. Why do you say this would be an issue in a system with two batteries in parallel?

My understanding is that the intent with these systems is to first charge the primary battery, then once a certain set of conditions are satisfied, flip a switch to then charge a second battery. Thereby giving the alternator the impression it's only really charging one battery at a time, and giving some mechanical sympathy to the charging system. The alternator only charges both when the first one is all topped off (>13.1v for 30 seconds, iirc).

To put 2 "dead" batteries in parallel then ask the alternator to just charge them both right now is more work for said alternator, and likely more load than it was designed for, at least how things exist in my head.

 

[Crash]

My understanding is that the intent with these systems is to first charge the primary battery, then once a certain set of conditions are satisfied, flip a switch to then charge a second battery. Thereby giving the alternator the impression it's only really charging one battery at a time, and giving some mechanical sympathy to the charging system. The alternator only charges both when the first one is all topped off (>13.1v for 30 seconds, iirc).

To put 2 "dead" batteries in parallel then ask the alternator to just charge them both right now is more work for said alternator, and likely more load than it was designed for, at least how things exist in my head.

With the BEP isolator both batteries are being charged automatically when voltage rises above 12.7v with the engine running and then automatically separated when voltage drops below 12.7v after the engine is turned off. Been doing this for 14 years on the 100 without issue. Ran the starting battery flat listening to CDs for two days and manually had to switch the isolator to combine both batteries and have run the aux battery low with the fridge running off it. Never flattened both batts at the same time. YMMV.

 

[Inukshuk]

Sure, isolators have some nifty battery health features, but they are by no means necessary to reap benefits of having multiple parallel battery.

My understanding is that the intent with these systems is to first charge the primary battery, then once a certain set of conditions are satisfied, flip a switch to then charge a second battery.

That "switch" combines in 10 or 90 seconds and opens in 10 or 30 seconds.
It is time and voltage based. It simply makes sense to not combine when the start battery is powering the starter and then put some juice into the just-used starter battery first. Nothing to do with state of charge.
Blue Sea MIL ACR Relay Instructions: https://d2pyqm2yd3fw2i.cloudfront.net/files/resources/instructions/980035700-001-ML-ACRs.pdf

Blue Sea MIL ACR Relay Instructions: https://d2pyqm2yd3fw2i.cloudfront.net/files/resources/instructions/980035700-001-ML-ACRs.pdf and https://www.bluesea.com/support/articles/1366/Automatic_Charging_Relay_[ACR]_Explained

Thereby giving the alternator the impression it's only really charging one battery at a time, and giving some mechanical sympathy to the charging system.

An alternator can only do its maximum. If you have one dead car battery on a 100 amp alternator, that alternator will output whatever the battery will accept up to 100 amps. If you have two or 10 dead car battery on a 100 amp alternator, that alternator will output whatever the batteries will accept up to 100 amps.

The alternator only charges both when the first one is all topped off (>13.1v for 30 seconds, iirc).

Just the same above time and voltage based method.

To put 2 "dead" batteries in parallel then ask the alternator to just charge them both right now is more work for said alternator, and likely more load than it was designed for, at least how things exist in my head.

Same as above: An alternator can only do its maximum. If you have one dead car battery on a 100 amp alternator, that alternator will output whatever the battery will accept up to 100 amps. If you have two or 10 dead car battery on a 100 amp alternator, that alternator will output whatever the batteries will accept up to 100 amps.

Undervoltage lockout is 9.6 for Blue Sea ACRs. The point being if a battery only reads 9.5 it is likely damaged and should be treated differently than ordinary charging. https://www.bluesea.com/support/articles/1366/Automatic_Charging_Relay_[ACR]_Explained

"What does “Undervoltage Lockout” mean? As a safety feature, some ACRs prevent combining into a severely discharged battery. A dual-sensing ACR will monitor the voltage on both batteries and will not connect if either battery is below the undervoltage lockout level. Use caution when combining into a battery with extremely low voltage, because this might represent a faulty battery or a problem elsewhere in the system."​

 

[Inukshuk]

I use this to isolate my winch: https://a.co/d/90fDHUp

Mike's relay can also be used to isolate house battery from start battery.

Downside is a dumb relay combines the batteries as soon as you turn the key and the hopefully-fully-charged 12.8v starter battery feeds the starter AND the house battery that ran down to 12.2v with your stereo and fridge at last night's party. That 12.8v start battery would instant;y drop voltage more than if just on starter alone as it dumps a few amps into the house battery and starter while the alternator at 14.4 or so volts will start immediately pumping into both.

With delayed combining the starter battery does its thing to the starter only. The alternator supports just the starter battery and the starter. Then, 30 or 90 seconds later the alternator at 14.4 volts pumps into both batteries, each of which is at something lower than 14.4 and each which accepts volts based on their internal state of charge.

FYI that the confusingly named "Start Isolation" feature of Blue Sea ACRs does something similar but different than the basic combine and isolate.

 

[nakman]

Huh. so you're saying it makes no difference to an alternator to charge multiple dead batteries all at the same time, or to charge them sequentially one at a time? And yes I have run 2 batteries dead overnight #askmeaboutmyironmandrawerfridge lol. huh I say, huh.

 

[Inukshuk]

Huh. so you're saying it makes no difference to an alternator to charge multiple dead batteries all at the same time, or to charge them sequentially one at a time? And yes I have run 2 batteries dead overnight #askmeaboutmyironmandrawerfridge lol. huh I say, huh.

Yup. No difference to the alternator.
For a bank of batteries there can be a difference to the batteries depending on wire length and each batteries place in the system. I use the "good" method in LongCruiser

Now ready for Dave's reply

 

[Cruisertrash]

A couple of notes to add.

If you have a system with an isolator and run one battery down, then manually flip the isolator to combine the batteries, the battery with a better state of charge is going to dump into the run-down battery until they equalize. This happens instantaneously when they’re combined, so you have to make sure the cables are up to the job, because you could potentially see a LOT of current (and thus heat) depending on how low one battery is. Now extrapolate that to an isolator that immediately combines the batteries when you turn the key on, it’s the same thing. This is why most isolators are voltage-sensing. They allow the alternator to come on line before combining, which gets the main battery’s state of charge perfectly topped off, so you’re not just discharging the battery with the higher voltage.It’s supported by another device (the alternator) that can help supply the current demand of the run-down battery

Also … yes, an alternator will put out current up to its maximum ability when asked for it. It doesn’t care if it’s one battery or 100 doing the asking. However, when an alternator is putting out lots of current heat and motor drag increase. I manufacturers spec a maximum current supply for alternators, but I have never seen a spec for how long they can safely supply that current. A ridiculous example, but bear with me: if one alternator is seeing demand from 100 dead batteries it will put out it’s maximum current, and it will have to do so for a long time to charge all those batteries. At what point does it melt itself down internally? I don’t know. Certainly with dual battery system that risk seems low but it’s something to be aware of. Make sure you have the proper cooling fan behind the pulley and that the fan blades aren’t mangled up.

@MDH33 All of this technical and theory mumbo-jumbo aside, get the Blue Sea ML-ACR or the BEP unit, make sure the rating is above 500 amps, make sure your cables are fat, and go for it. You had a previous dual battery setup to go off of but if you need a diagram ping me. Also, fuse everything. I have a 300A slow blow on the positive cables running between the batteries, one for each battery about 6” from each positive terminal. The isolator is in series with that positive cable obviously.

 

[Inukshuk]

which gets the main battery’s state of charge perfectly topped off

What is a measure and defining of topped off?

Also, fuse everything.

100%. I put MRBF Terminal Fuses (Blue Sea or Bussman brand) on the battery at each end of the long run from engine bay to under Longcruiser rear seats. My short runs between batteries under the long cruiser rear seats are in PET Expandable Braided Sleeving inside flexible wire loom. I have a MRBF Terminal Fuse at the battery end of the long run from my other 80's cable (2 or 4 ga I think) from engine bay to ARB twin compressor in the cargo area. This is not because I anticipate heavy loads. This is to protect the wires from ground shorts.