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Noob Vehicle 12v electrical: best way to run a second battery?

Sandy Johnson

Sandy Johnson

Harry Member
Joined
May 19, 2020
Member Number
247
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2,849
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Spreckels, Ca
The plan is to use this battery to run some auxiliary things like a plug in cooler, some rock lights, charging ports, and the stereo so I don't kill my starting battery with them sitting in the desert all day with the motor off. I've got a vehicle with no room for a second battery under the hood, but I do have room out back. I am the opposite of an electrical genius.

I've got a battery isolator I plan to use between the two. I'm probably looking at a 6 to 7 foot run of cable. Does the size of the wire that I need go up with a longer run? Should I put a breaker at the beginning of the run in case of a short? What is my dumb self missing?
 
Harry Johnson said:
The plan is to use this battery to run some auxiliary things like a plug in cooler, some rock lights, charging ports, and the stereo so I don't kill my starting battery with them sitting in the desert all day with the motor off. I've got a vehicle with no room for a second battery under the hood, but I do have room out back. I am the opposite of an electrical genius.

I've got a battery isolator I plan to use between the two. I'm probably looking at a 6 to 7 foot run of cable. Does the size of the wire that I need go up with a longer run? Should I put a breaker at the beginning of the run in case of a short? What is my dumb self missing?
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You need to use a really big cable or the aux battery won't get charged fully while loads are being pulled from it. A few tenths of a volt makes a pretty big difference in SOC. Ideally you'd have the isolator near the alternator under the hood, and on the aux contact for it you would have two cables, one going to the battery, and another going to the aux loads.
 
I always wondered on lots of factory wiring, the charge wire is MAYBE 10 guage. How the hell doesn't that melt down with a dead battery and the alternator at full pull. 10 guage is good for what, 30 amps?
 
I would mount it where ever. Hook the ground to the frame of the second battery. Use a heavy 2 ga or bigger from the positive to a flaming river on of switch then to the battery up front. Hook all you accessories to the back battery. Turn switch off when you want to save main turn it on to charge/help main.
 
On my flatbed I dropped in a 200 amp alternator and ran 4 gauge OFC cable from it to a 500 amp constant duty relay that's powered by the ignition so when the engine is running the batteries are being charged. When it's off the systems are isolated from earth other.

From the relay I ran 4 gauge with 200 amp fuses on each leg, 1 to the stock location battery and 1 to the rear battery.

I have a dual voltage gauge that only reads when the ignition is on so I know what each system is doing when being used.


4 gauge in automotive cabling wouldn't be shit but stereo cables are much better quality.
 
Harry Johnson said:
Is that so when the motor is running the aux equipment isn't detracting from charging the battery?
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most isolators have a slight voltage drop to the aux battery. You magnify that when you put the isolator at the long end of the battery cable that's already induced a voltage drop.
 
DWT said:
On my flatbed I dropped in a 200 amp alternator and ran 4 gauge OFC cable from it to a 500 amp constant duty relay that's powered by the ignition so when the engine is running the batteries are being charged. When it's off the systems are isolated from earth other.

From the relay I ran 4 gauge with 200 amp fuses on each leg, 1 to the stock location battery and 1 to the rear battery.

I have a dual voltage gauge that only reads when the ignition is on so I know what each system is doing when being used.


4 gauge in automotive cabling wouldn't be shit but stereo cables are much better quality.
Click to expand...
4 guage is only good for 85 amps though.
 
AKnate said:
4 guage is only good for 85 amps though.
Click to expand...

It's a function of length of run, acceptable voltage drop, amount of current actually flowing, and allowable temperature though. There are rules of thumb for car wiring gauge, but it's not like household code where things are regulated to meet known safety requirements.
 
Fuse or circuit breaker near each end of a long cable run.

Noco isolators claim to need identical batteries. I melted one down trying to run a peltier cooler off a typical tractor trailer size battery. Should have had a low voltage cutoff to the cooler.
 
SomeGuy said:
It's a function of length of run, acceptable voltage drop, amount of current actually flowing, and allowable temperature though. There are rules of thumb for car wiring gauge, but it's not like household code where things are regulated to meet known safety requirements.
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Yup and couple that with using QUALITY wire makes it work. Stereo 4 gauge will flow better than automotive grade 0 gauge.
 
SomeGuy said:
It's a function of length of run, acceptable voltage drop, amount of current actually flowing, and allowable temperature though. There are rules of thumb for car wiring gauge, but it's not like household code where things are regulated to meet known safety requirements.
Click to expand...
Make sense. Not sure why I was thinking car wiring = house wiring. Would be nice to have rules that are similar though. I just looked up 12v amperage charts and they are ALL over the place for what is ok.
 
I justed tied in a second battery like on a Diesel truck:homer:. Didn't see a reason for a isolator
They are both on a 1,2 1+2 battery switch
 
ANGELO said:
I justed tied in a second battery like on a Diesel truck:homer:. Didn't see a reason for a isolator
They are both on a 1,2 1+2 battery switch
Click to expand...
I don't want to think. I want it to charge when driving.

I don't want the running aux equipment to kill the starting battery if I leave it on for an extended time while parked.

Basically sitting here for several hours keeping my beer cold without ice:
1661983243587.png



Also the one thing I already have sitting here is a spare isolator.
 
Run a battery isolator between the battery. The same way you'd run a boat and RV.

I would not run a solenoid between the two batteries for anything other than an emergency. Flash charging the accessory battery from the main battery will end up killing both batteries since it's a low voltage / high amperage discharge.

Here's a diagram in MSPaint that's worthy of scientific publication:

1661983533117.png
 
FordFascist said:
Run a battery isolator between the battery. The same way you'd run a boat and RV.

I would not run a solenoid between the two batteries for anything other than an emergency. Flash charging the accessory battery from the main battery will end up killing both batteries since it's a low voltage / high amperage discharge.

Here's a diagram in MSPaint that's worthy of scientific publication:

1661983533117.png
Click to expand...
What about larboc 's suggestion of running the aux equipment straight off of the battery isolator post?
1661984105280.png
 
I use one for these mounted in a plastic wall switch box.

that interrupts the power. I run a ground back to the battery. 4 gauge is plenty. Run the hot in plastic conduit/wire loom, pex scraps, etc.

recently I attached a small Prius agm battery to an Engel fridge and used a lighter plug to charge it when the truck was running. Worked great but not cost effective unless you have recently parted out a Prius.
 
DWT said:
Yup and couple that with using QUALITY wire makes it work. Stereo 4 gauge will flow better than automotive grade 0 gauge.
Click to expand...

Yeah, but it's shit for uv tolerance and corrosion eats the thin strands quickly, ask me how I know.
 
I use 2/0 to 4/0 depending on length of run or draw on all 3 of my batteries.

1 & 2 used as starting batteries and 2 & 3 used as batteries for 24/28v radio batteries, with switchable charging when 24/28v isnt used.
 
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