Dual Battery Diagrams & Ideas


This is the typical dual battery install.


This is a quick way to wire dual batteries so both don't become discharged when an appliance is left on or over used.
#1 is the factory starter solenoid,
#2 is a heavy duty accessory solenoid (also called a continuous hold solenoid),
#3 is a 10 PSI oil pressure switch (NAPA p/n ECHOP6624 ),
#4 is a momentary push button switch (momentary meaning it automatically shuts off when you let up on the button).

The second battery is isolated from the primary circuit when the engine oil pressure drops, like when you turn the engine off... (switch #3)
To use the second battery for self jump starting, just push the momentary switch (switch #4) and crank the engine like normal.
I've built this system several times, everyone that has it seems to be happy with it, and I've never been stranded with it.


This is the beginning of a somewhat elaborate battery Separation & Isolation system.
Off roaders, in particular ones that work or do trails by themselves need the capacity to not only Isolate one battery, but to be able to 'Self Rescue'.
This shows the basic design for just such a system, using 'Off The Shelf' parts from any discount auto parts store.
Fancy aftermarket toys are nice for the guys that are more worried about color matching their rigs than actual performance on the job, but this system is designed to do WORK!

As you can see, this system has TWO batteries, and TWO starter solenoids.
This is what is known a 'Integrated Operational Redundancy', meaning if something fails, you may not even know there was a failure until you are doing maintenance on your system, and it WILL NOT leave you stranded with a single starter solenoid or battery failure.

I have included some wiring for the solenoid that you should study and be familar with, since it will come into play later...

Now comes some of the added wiring,
First, we have to do something about charging. Since the batteries are actually isolated from one another when at rest, we have to do something about discharging and charging!

Provision has been made for the high current drain to feed the starter through the dual Starter Solenoids, but there is NO provision for Common low current discharge, or common charging of this set of batteries.

The change to this diagram shows the addition of an oil pressure switch that links the two batteries together for common, low current, charging and discharging.

If you are running a 'Normal' alternator (around 60 amp output) then you can probably get away with an oil pressure switch for connecting the two batteries.
I use one designed for a reasonable amprage load, like for a choke heater switch ( NAPA p/n ECHOP6624 without 'Idiot' light, NAPA p/n ??? with idiot light).

By doing things this way, the charging of the battery is automatic when the oil pressure is 'UP', and if the engine dies, or you shut the engine off, the second battery is automaticlly disconnected from the rest of the vehicle.
No switches to flip, No extra load on the already over worked ignition wiring, No 'Forgetting' to turn things off, No accidental turn ons from kids or dogs, ect...

Completely Seamlessly Intergrated.

The next version is about the same thing, but will allow for a 'Self Rescue' switch in the event of the 'Primary' battery being so run down it can't activate the ignition switch.

The dash swtich simply supplies power from the second battery to the charge/accessory relay to activate the vehicle ignition switch.

If you use a regular switch that stays on, the connection between the batteries will remain 'ON' and both could become discharged.
By using a 'Momentary' switch, after you let go of the switch, it will return it's self to the 'Open' or 'Off' postion, so there won't be an 'Accidental' secondary battery dischargin going on!


This is for guys with Winches, Inverteres that are going to deeply discharge the batteries, or if they have 'High Output' alternators, that might over amp the oil pressure switch contacts.
In this diagram, I'm using a heavy duty automotive relay with a 70 amp capacity.
99% of the population will be able to get by with a common 20 or 30 amp accessory relay off the shelf at the autoparts store or at Radio Shack...
But I winch and weld off my batteries, so they get pretty well discharged some times and can draw a lot of amprage from my 78 Amp rated alternator.

This particular relay is rated at about 120 Amps, but will only handle about 70 Amps continously at 12 volts.
This is the same relay Harley Davidson uses as a Starter Solenoid, so if it will start a Harley, it should work for just charing my batteries...

The socket for the relay is from an electric compressor company, and it's QUITE IMPRESSIVE!
The primary leads are 8 Gauge automotive rated wire, and 8 Ga. has an amp loading capicity of 46 amps, and that is more than half of the rated output of my alternator at it's highest output!
Remember, we are dealing with two batteries here, so you can take the rated output of the alternator and devide it by 2, since both batteries are going to be charing, and only one is on this circuit...

NOW! most of us have only one oil pressure gauge, and it's electric, with a big stupid sending unit on the engine for it...
If you are only adding one accessor to the oil delevry hole that sending unit is screwed into, you can use a 1/8 NPT 'T', one end male, one end and one side female,
And that will give you a second accessory port for your pressure swtich for the relay, and for the 'Idiot' light, dipending on what type of switch you use (two or three terminal).

I'm not a 'T' kind of person, I think they look tacky and unprofessional.

Back in my racing days, I learned to have redundant gauges under the hood, ususlly most of a full set, so you didn't have to run around to see what the gauges were doing all the time.
I still like having that, so when I did my Isolated battery and Self Rescue system, I continued that tradition of having gauges under the hood.
This is what I came up with,

Now, this is just a piece of scrap aluminum I had lying around here, but it could just as well be steel.
This one takes the oil pressure from the black line, and distributes it to the Pressure Swtich, The Pressure Gauge, and my Mechanical Oil Presssure line (which is very hard to see, but coming out of the 'Top' of the block).
There is expansion room if I need to connect any more devices, like a choke heater, ect.
And, this is just as easy to make for an Electric Oil Pressure Sender.

I started with a chunck of aluminum, I think it was about 1" thick, 3" long, and about 1.5" wide, but you can go smaller if you like, it's just what I had lying around.
I used a letter 'R' (0.339") drill, and drilled 2.75" into the block (3" long block) from one end.
Leaving about 1/4" or a little more is always prudent, but if you drill all the way through your material, just thread and plug the unused opening for later use.
That same drill for one hole from the 'Side' facing the engine for the 'Feed' line, Two holes on 'Top' for the gauge and switch.,
If you are running an electric gauge, you may want to plug the top hole and make an extra hole on the side for the sending unit, which are usually pretty large.

Tap all 'R' drill holes 1/8" NPT and chamfer the tops of all holes...
PLEASE buy the correct tap and drill if you don't already have them!
This simple $7 or $8 dollars, along with some cutting/tapping fluid, will make this project SO much easier!

Here you can see the black flexable line that absorbs the engine vibration, and the hard copper line that will feed my mechanical oil pressure gague.

This is a drawing of the block in general terms. Notice there is a second side hole on the version for an electric gauge sending unit... That's so the sending unit can hang over the inner fender lip.... That sucker is BIG!

Opinions vary, but I prefer the copper once the oil pressure sample is off and away from the engine.

Attached to the engine, the vibration makes the copper brittle at the fittings, and it breaks.
I won't use that plastic stuff unless I absloutely have no other choice!
It melts through easy, it gets brittle and cracks easy, if it does fail, you can't just kink it to stop the leak like you can with copper line...
Not 'Field Fix' frendly.

For the line up, I priced some hydraulic line, rated for petrolium and over 100 PSI, with 1/8" NPT fittings at both ends, and about 18" long.
The cheapest I could find one custom made was about $30!
And that was nearly a deal buster for just my old Farm/Trail jeep!

After throwing more mental effort at the problem than I care to admit, I came up with a low budget sloution...
A 'Whip' hose from a grease gun... (it's not called 'Junk Yard Genius' for nothing!)

High pressure rated, petrolium rated (DUH! It's for grease!), 1/8" NPT fittings already crimped on both ends! All for $5!
The only thing it doesn't have is a swivle end, so you will have to screw it into the engine block, then screw the pressure manifold onto it, but for $5, I can live with that since I only have to do it once!

The expensive part of this system is the Gauge.
Gauges under a hood, especally of an off road vehicle, MUST be water proof, something most pressure gagues ARE NOT.

The only Common gauge you can be sure of being COMPLETLY SEALED to keep the mud, water, anti-freeze, blood and beer out is a 'Liquid Filled' gague.
If it will hold the liquid inside,..... It will hold the mud outside....

This one is from SummitRacing.com and ran about $20, 0-100 PSI read. It's the single most expensive part of this Oil Pressure Manifold project!

Swtiches, Wires, and Misclanious Parts...

Battery Cables.
Store bought battery cables are worthless.
NAPA carreis Welding Cable, and it's much better that any discount battery cable you can get off the shelf.
Use a 4 Ga. or larger WELDING CABLE for Batteries.
Battery cable has some real strong points going for it, 1. It's industral grade. Most 'Consumer' grade products are crap.
Industral grade wire is usually virgin copper, no alloys to make it less expensive, and carry less curren in the process.
2. It's fine strand. The finer the strands, the more of them they can pack into same size wire, and the more current it will carry.
Remember, Welding Cable is specifically designed to carry high AMPRAGE loads... Like your high amp starter.
Fine strnads also make the cable easier to work with.
3. The insualtion is rated for high tempratures. When it's in a welding enviroment, it's getting moleten balls of metal splattered on it, so it should hold up to your underhood heat.
4. The insualtion is 'Rubberized', not vinyl, so it will show plenty of abrasion resistance.
Remember, it's industral rated, so it has to survive being dragged across sharp, not metal and concrete floors.
5. The Insulation is CHEMICAL resistant... Like the Gas, Oil, Anti-freeze, Transmission Fluid, Brake Fluid, ect, it's going to come into contact with in your engine bay...

Battery and solenoid terminals...
Use solid copper with cadimum plating, don't use cheap lead terminals!
The solid copper terminals are only like $1.50 more each, and they will last you SO much longer.
Use the 'Crimp On' kind of terminals, and if you feel you can do it correctly, solder the terminals on after that with some Silver Bearing solder.
Don't forget to crimp in some 10 Ga. wires on the Negitave terminal so you can have some dedicatd ground wires!
Crimping in some 10 Ga. wires at the 'Starter Solenoid' terminal end of the positive battery cable will allow you to run accessories with a very clean, streamlined, professional look.

USE STAR WASHERS! Internal tooth type locking washers for electrical connections is ALWAYS a good idea!
As the electrical terminal expands and contracts from the heat of using the circuts and underhood heat, the joint will loosen up. Star washers will help keep things connected between routine maintiance rounds.

I use about the same oil pressure swtiches in most of these builds.
If you have an electric oil pressure gague, and don't want an 'Idiot' light, then use NAPA/Echlin p/n ECHOP6624.
It only has two terminals, and both of them are isolated from ground, and connect to each other when the oil pressure rises above about 10 PSI.

The one I use the most of is a 3 terminal unit that has an isolated, normally open switch, like the one above, but it also has a normally close terminal that goes to ground.
That third terminal is the one for the 'Idiot' light,
GM, Ford, Chrysler, AMC/Jeep all used this same light/electric choke switch somewhere in the 70's or 80's so it's not hard to find.
The GM p/n 364713.
The p/n PS 127 (Car Quest stores)
Everyplace I checked it was under $5. (US)

If you are that 1/4" NPT kind of guy, and some engines are 1/4" NPT instead of 1/8" NPT like most Jeeps, Then this is your part number, The GP Sorenson p/n OPS 46 (Advanced Auto stores)

If you are in the land of METRIC threads... (and I've run into this before...)
Try GP Sorensen p/n OPS 56 (Advanced Auto stores)
Stats of switch,
M10 x 1 Metric thread.
'A' terminal to ground, normally closed, opens at 3 to 9 PSI.
'B' terminal to 'C' terminal, normally open, closes at 11 PSI.

Copper line!
I got mine as a kit from Advance Auto of all places!
Good little kit, with good fittings, adaptors, and everything nessary to make it work, and it did!