Dual Battery Setup

weejub · Feb 7, 2016

Updated pics of system with fuse on ACR ground:

Board for the back in bed:

GLTHFJ60 · Feb 7, 2016

That's a pretty cool gizmo. It's a battery isolator that's got a 110v battery tender built in. Sweet!

It's got a limitation that the alternator can only put out 65A, burst up to 115A. Your alternator puts out more than that, no? I doubt this relay will see that many amps unless you wire in your winch so that winch power is drawn through this device.

I suppose you'd wire your winch so that it's drawn off of the starting battery, because the secondary battery would be really low capacity and amp hour rating.

Why not use something along the lines of a ford starter relay with some moderately sized wires from the main battery to the back. Make it so that if the engine is running that the relay is closed and feeding current to this smaller battery, so that when the truck is shut off it's isolated. That would be cheaper than the proposed setup and it doesn't look like there will be any other downsides, other than losing the battery tender feature.

Cruiserdrew · Feb 7, 2016

You can do what you want with this and some cable:

Amazon.com : Blue Sea Systems SI-ACR Automatc Charging Relay - 12/24V DC / 120A : Boating Battery Switches : Sports & Outdoors

Great piece of kit, easy to install(literally 3 wires), warranty for life. And surprisingly inexpensive, not much more than a big relay based system.

Not fancy, but it perfectly combines the batteries when it detects charging voltage on either side, and isolates them when charging voltage is taken away. Much more sophisticated than a relay based system, and can handle 200 amps for 5 minutes and 120 amps continuous. Given how small your #2 battery is, it will never see close to that.

The fact that it can detect charging voltage on either side, means that say you're parked and have solar hooked up to the "house" side, as that voltage rises with charge, it will combine at 13v and also top off your main.

For keeping things topped up in the driveway, a BatteryMinder is a nice bit, and applies a computer controlled stepped charge with a temperature compensated float. That way you've separated duties, and if either one goes down, it doesn't kill your $300 device. Some ideas:

Rubicon Solar Power Experience

Camp Solar Setup

weejub · Feb 7, 2016

Too much beer and too dumb to digest all of the above tonight. I will take a run at this again tomorrow!

The main concern for the little 2nd battery is not overcharging. If the charge is coming from the alternator through a solenoid, relay or something like it, I should be good, right?

Cruiserdrew · Feb 7, 2016

Over charging or not will be determined only by your voltage regulator.

GLTHFJ60 · Feb 9, 2016

Cruiserdrew said:
Over charging or not will be determined only by your voltage regulator.

Completely agreed. If it's a 12v battery then you won't ever overcharge, unless your VR goes out and put out more than 14.4v.

weejub · Feb 9, 2016

K thanks guys. So given my alternator puts out 85 amps, I would still just install a wire from the starter battery to the ACR with an inline fuse and then from the ACR to the second "house" battery, also with a inline fuse.

http://assets.bluesea.com/files/resources/instructions/990170140.pdf

Confused about the AWG I would need and the fuse I would need in line. My alternator puts out around 85 amps.

I don't have to run a 0-4 AWG all the way to the back of the truck for the second battery do I?? 120 amp fuses?

SeanLX · Feb 9, 2016

weejub said:
K thanks guys. So given my alternator puts out 85 amps, I would still just install a wire from the starter battery to the ACR with an inline fuse and then from the ACR to the second "house" battery, also with a inline fuse.

http://assets.bluesea.com/files/resources/instructions/990170140.pdf

Confused about the AWG I would need and the fuse I would need in line. My alternator puts out around 85 amps.

I don't have to run a 0-4 AWG all the way to the back of the truck for the second battery do I?? 120 amp fuses?

You're welcome to check out my install with the ACR. It's working great so far and I think a great solution for a second battery. You probably will never get the full 85 amps to the 2nd battery but the farther away you put it the bigger the wire will need to be. Plenty of charts online for current vs. wire size but you'll have to make the call on how small you want to go. (4ga, 8ga, etc.)

I have 4ga to the back in mine to power an inverter & amplifier + anything I want in the future.

Cruiserdrew · Feb 9, 2016

Size your fuse to your wire. Nothing else. In your case, I would likely run 2 gauge. That would suggest a 150 amp fuse.

Remember you need to ground the 2 batteries. So run a ground cable (same size as the +12V cable) between the #2 and #1 ground terminal.

JohnVee · Feb 9, 2016

Chart.

GLTHFJ60 · Feb 10, 2016

Size the wire to the load, then size the fuze to the wire.

weejub · Feb 10, 2016

Thanks for all the advice and info, gentlemen!

Sean - Would love to see your set up.

CruiserDrew and John - Thanks for the help with the wiring sizes!

Johnny - Thanks for the process!

Given the ACR can dynamically send electricity to either battery, I guess I would have to plan for full current both directions at any point?

Wire size should be based on peak current over it? So in theory, that second "house" battery could get up to 85 amps? Given I am pulling off the starter battery, would there be some loss to the ACR? Should both sides of wire coming out of the ACR be the same size, or is it more a matter of each side of wire should be optimized for length?

I will have to really measure out the length, but looks like I am right on the borderline of 4 AWG and 2 AWG.

I used a calculator online and it said at 15' of length, I would have a 5.4% drop in volts with 4 AWG and 3.4% with 2 AWG.

Honestly, I don't see me stuffing 2 AWG under my carpet/housing - - it is tight in there already and I have my antenna in there, too, for the CB. I may have to some cutting under there

Cruiserdrew · Feb 10, 2016

You won''t really have much in the way of loss in the ACR. They are extremely efficient and use only 0.175 amp to keep the relay closed.

Given how small your #2 battery is, I doubt it would ever see 85 amps, but that isn't a bad place to start planning, especially because once you find out how useful and good it all is, you'll want to up size your system.

In general, in a critical system, you aim for 3% voltage loss or less. Also remember that 15 feet is really 30 feet electrically because you need to figure the return path is part of the circuit.

All the wires should be the same size.

But let me say, I would highly recommend you install the battery under your hood. It makes the wiring and everything else much easier. It also means you can run much smaller wiring like 6 or 8 gauge to the back of the truck. But that may not be practical for you.

As long as you have appropriate fuses on any wire leaving the pos(+) terminal of any battery, you will end up with a safe system.

I like the following recommendations from Blue Sea. John Vee's chart above is safe, but it seems to ignore 2 gauge and 1 gauge wires and jumps from 4 gauge to 1/0. Looking at your set-up, and a likely 30 amp load, I came up with the border between 4 gauge and 2 gauge. So I'd use 2 gauge to allow for future growth!

http://assets.bluesea.com/files/resources/reference/20010.pdf

SeanLX · Feb 10, 2016

^ I agree installing under the hood makes the wiring easier. The 30 amp guess is pretty good too. I don't think you'll ever get 85 amps from the alternator to just the rear battery, the starting battery, engine, A/C, radio, and anything else will be pulling out of that too.

I also agree what's been said about fuses, size so you don't melt the wire and put them near the positive post of any battery. The factory gets away without it because of lots of time and testing on well planned out wire routing with clip points.

There will be debate about the ground. 2 schools, 1 is run the wire, 2 is good solid chassis/frame grounds. (I went with frame grounds)

Rice · Feb 10, 2016

The points from @Cruiserdrew , @SeanLX, and @GLTHFJ60 above are solid. The only thing I did not see (and perhaps missed) is to super protect your cables from chaffing on your front-to-rear run.

(Pic stolen from web)

Here is just one more chart for you to look over. Calculation is showing the Voltage drop for a 20 foot run at 75 amps. As mentioned it is highly unlikely that much current will ever travel between your two sources and, in fact, I would begin looking for a problem if it ever occurred. The 30 Amp fuse Sean mentioned should be about right.

GLTHFJ60 · Feb 10, 2016

Rice said:
The points from @Cruiserdrew , @SeanLX, and @GLTHFJ60 above are solid. The only thing I did not see (and perhaps missed) is to super protect your cables from chaffing on your front-to-rear run.

Very good call.

@weejub I still have a bunch of the loom and abrasion-resistant sheathing that @fourtrax gave me. You're welcome to have some of it for this project!!

Cruiserdrew · Feb 10, 2016

Great pic @Rice and the poster child for good fusing practice. Wires are going to get chafed and abraded. It's normal. Unlike a breaker, fuses are essentially fail safe and don't wear out.

weejub · Feb 10, 2016

All great input - thanks!

Sounds like it would be a lot safer, cheaper and easier putting the house battery under the hood and then running a hot line to the back. This would give me power for needed stuff back there like a fridge, charging stuff, etc.

Given the way the ACR works, I would guess I would still need a similar sized wire and protection and fuse to the back. If it was "hot" I could also plug in a charger directly to it via Solar or 120v AC via my existing battery tender as needed, right - like old-school cigarette lighters?

So if I went up under the hood, maybe go with a little bigger battery and just charge my little battery in the rear off of the bigger house battery, as needed??

And I thought the $$ were already spent once I bought the little battery!!

Cruiserdrew · Feb 10, 2016

Actually no. Your wire to the back can be sized to the load in the back. So if it's a fridge, radios and a charging station, that isn't much power. I don't think you'll need that small battery in the back. I would run 8 gauge from your "house" battery to the rear somewhere, but fused with it's own fuse (60 amps, at the battery) to a fuse panel in the rear. From there, run your individual circuits to your devices. That way everything is fused and safe, and it also give you a very flexible set up.

I'll warn you though, I've measured my 24 hour fridge draw and it's around 20 amp hours. That's enough to discharge the small battery you show. My house battery is 100 amp hours. Just one more thing to consider.

weejub said:
And I thought the $$ were already spent once I bought the little battery!!

That's what you tell Mrs. weejub.

weejub · Feb 10, 2016

The little battery runs my CPAP at night - - it will go two nights if it is fully charged. Would be great to simply use the hot line to charge it off the second battery, only as needed.

If I do what you say and put in a panel in the back, I can run SAE connections out to Fridge, Battery, etc. - - would be great.

Dual Battery Setup

weejub

GLTHFJ60

Rum Runnin'

Cruiserdrew

On the way there

weejub

Cruiserdrew

On the way there

GLTHFJ60

Rum Runnin'

weejub

SeanLX

Cruiserdrew

On the way there

JohnVee

GLTHFJ60

Rum Runnin'

weejub

Cruiserdrew

On the way there

SeanLX

Rice

GLTHFJ60

Rum Runnin'

Cruiserdrew

On the way there

weejub

Cruiserdrew

On the way there

weejub

Share this page

Connect with us

Dual Battery Setup

Rum Runnin'

On the way there

On the way there

Rum Runnin'

On the way there

Rum Runnin'

On the way there

Rum Runnin'

On the way there

On the way there

Log in

Share this page