How to make a cheap isolated dual-battery setup for $50

another_mike

Adventurer
Interesting.. Thanks for bringing this to our attention.
My catalogue of Cole Hersee is thru Littelfuse, makes no mention of mounting orientation.
In all fairness, that solenoid shown is Cole Hersee.
I knew I read it somewhere when installing mine. No idea the reason behind it.
 

ohiobenz

Member
Working on a balanced 24v dual battery system for my big rig.
The Smart isolator has performed well on my FJC for several years and lots of driven miles. It has also saved my hide several times due to how I split the system - separating the starter battery from everything else.
So here's what I'm in the process of now... The Bussman provides a 100A 12v tap and equalizes the draw on that bank's batteries.
The 24v starter always draws from the other bank, unless the Smart isolator is closed in which case it draws from all 4 batteries.
In addition there is a 60W 24v PV panel and Victron 75/15 as a battery maintainer for when parked extended time.
This is totally separated from any part of the habitat electrical, or the 28v 3.5k diesel genny.FB_IMG_1595551240951.jpg
 

ohiobenz

Member
Bussman implies the it has to be a DPST switch. That part of their schematic isn't showing it that way, but their paperwork clearly states that if you are using a disconnect, it has to be for both batteries.
I'm using marine circuit breakers instead of fuses, so if I really wanted to I could trip both breakers for the same effect.
 

ohiobenz

Member
That still makes no sense. If its an ordinary DPST, when ”B” battery switch is closed as it must be for normal operation, ”A” battery is also closed bypassing the equaliser.
DPST switches with form ”a” and form ”b” segments do exist, but one of high current capacity, a very specialised item.
The easy solution is use DPDT two position switch. Each switch segment wired accordingly.
Sounds like Bussman needs to revise their documents..?
Further, the only purpose I can envision that ”A” battery switch doing is serve for a limited time 12V loads if for some reason the equaliser quit working.
The disconnect switches are optional and not part of the Bussman operating needs.
Their directive is that IF you want a disconnect, you MUST disconnect both in order for the equalizer/converter to work properly.
I'm not going to argue with Bussman, they have 1000's of these units out there in operation.
It was my choice to use since my rig requires both 12 and 24v. This provides balanced batteries plus a bonus 100A 12v tap.
How it will work in combination with the Smart Isolator and the Victron MPPT unit remains to be seen.

BTW, this is a 500A DPST disconnect, readily available.

20200726_120104.jpg

I'm only using one of the switches - to interrupt power to the starter motor. At some point in the future I may choose to interrupt the 24v load to the cab.
 
Last edited:

faulke

Member
I ran this cheap dual-battery setup for about 7 years. At the time of install, I was a broke grad student with very little electrical experience, so it was a welcome sight to find something so cheap and easy. It worked almost flawlessly. Only hiccup was I had to swap out a busted solenoid about two years ago.

My only recommendations are spend some time building quality cables and keep an eye on the solenoid connections for corrosion, dirt, grime, rust, etc. Also, invest in a good multi-stage charger to top off your secondary battery once a month or so. The alternator alone won't do the trick and you may lose battery life. I ruined a nice Sears DieHard AGM in about four years by not doing this. You live and you learn.

Recently upgraded to an ML ACR and couldn't be happier, but this was a great way to get started. Thanks!
 

Peck

Observer
OK, bringing this one back to the top. But after about 28 pages I couldn’t make myself ready anymore. Especially since some of the posts were taking up half a page. Looking to do a dual system in my 2012 T4R. Here my questions...

I want to stick my house battery in the back of the truck near the fridge. What’s the longest run I can install a Bluesea ML-ACR?

Is this too long of a run to tie both batteries together for more winching umph?
 

evldave

Expedition Trophy Winner
OK, bringing this one back to the top. But after about 28 pages I couldn’t make myself ready anymore. Especially since some of the posts were taking up half a page. Looking to do a dual system in my 2012 T4R. Here my questions...

I want to stick my house battery in the back of the truck near the fridge. What’s the longest run I can install a Bluesea ML-ACR?

Is this too long of a run to tie both batteries together for more winching umph?
For reference I used a set of 4ga 16+' Jumper cables mated to my trailer that had another 6+' of 4ga welding cable to charge my trailer battery. Total length a little over 25' with a solenoid and Anderson connector between the 2 batteries. Haven't had an issue in 10+ years

Sent from my SM-N986U using Tapatalk
 

Peck

Observer
Awesome! Thanks!

Plan was to use 4ga welding wire along the driver side frame for positive and negative. Reason I want it back there is closer to the fridge and easier to mount solar controller back there. Plus inverter and other electrical needs.

Doubt I’d ever tie the batteries together for anything other than a jump but i like to think ahead. Thanks again!
 

justbecause

perpetually lost
I have been running this basic setup for years but now I'm curious. If I don't have a load on my second battery and it is constantly being charged when the vehicle is running what keeps that second battery from being overcharged exploding catching on fire etc
 

DiploStrat

Expedition Leader
I have been running this basic setup for years but now I'm curious. If I don't have a load on my second battery and it is constantly being charged when the vehicle is running what keeps that second battery from being overcharged exploding catching on fire etc
Short answer is “voltage regulator.” Basically, when the relay is closed, then the alternator “sees” both batteries as one, single battery.

EDITIED TO ADD: And the same is true of the load(s).
 
Last edited:

justbecause

perpetually lost
Short answer is “voltage regulator.” Basically, when the relay is closed, then the alternator “sees” both batteries as one, single battery.
They are balancing through the connecting wires? So it's not like my car battery is running my car and all it's stuff while the 2nd battery is doing nothing? The 2nd battery is doing an equal amount of work by being connected via the solenoid?
 
Top