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

[ohiobenz]

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.

 

[john61ct]

Maybe the "24V vs 12V" switching idea is getting muddled in?

 

[ohiobenz]

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.



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.

 

[john61ct]

Probably would be worth documenting these more complex ideas in a separate thread

to keep this one more useful for those looking for the OP / title concept.

 

[faulke]

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!

 

[evldave]

Glad it worked out for you. Hopefully you drank plenty of beer

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[Peck]

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]

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

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[Peck]

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!

 

[john61ct]

If using proper sized conductors, there is no length limit.

Well once the copper starts to cost or weigh too much

then maintaining a secondary bank + DC-DC charger dedicated to the remote load becomes more practical.

 

[justbecause]

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]

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).

 

[justbecause]

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?

 

[john61ct]

Lead batteries FLA type are just fine with constant charging at Absorb (CV) voltage, so long as you keep topping the water up.

GEL or AGM are going to lose electrolyte and cycling lifespan unless voltage is dropped down to Float level once they get to Full

which if they are being cycled (discharged) much will take a minimum of 5-7 hours, no matter how high the C-rate.

Any owner with a clue is tracking the wear factor, dropping SoH% over the years

and proactively replacing the House bank long before there are gross symptoms indicating EoL approaching

much less danger of explosion.

 

[Lance4me]

It's nice to see this still going on after 10 years. Just wondering how many of you switch to Lithium instead of using Lead Acid, AGM.....