Adding a Dual Battery

Vuk · November 18, 2019

This is the battery tray, secondary auxiliary relay, and the optional 3 fuse box and cover for older model years if you want to replace the single fuse box on the aux battery harness:

This is the factory harness between the primary battery and the X50A underhood fuse box:

This is the aux battery harness that replaces it and adds the relay for switching the battery isolator, cable running from the primary battery distribution block to the isolator, cable running from the isolator to the aux battery (including the battery terminal with a 125A fuse), the connector for controlling the isolator, and the connector to control the aux battery switching:

Here you can see the blank screw on the primary battery distribution block - the cable running to the isolator hooks up there.

This is the connector on the aux battery harness where ground, (+), and the control signal (run/ignition) need to get in:

This is the service terminals that connects to that connector. I ran black, violet/brown, red/white wires in the standard flex installation hose along the firewall (connector is on passenger side, my wiring goes back to the driver side)

Here is the battery isolator secured to the firewall. I pulled out the black wire right there to connect to the ground post.

This is the connector where you need to connect the red/white wire from the harness (the terminated lead wire is white)

This shows where things come together with the red/white wire going to the service connector:

Violet/brown can just be tapped into on the engine/body connector:

All done (except for the aux battery which is on order, so I left the cable going to it disconnected from the isolator just to avoid having to heavily isolate it).

Edited November 18, 2019 by Vuk

Grumpy Bear · November 18, 2019

What am I missing? Two identical batteries in parallel just doubles the amp capacity and capacitance. Literally acts like a battery with twice the CCA and reserve minutes. So...why isolate and use/charge independently? The alternator doesn't care and the ECU hasn't any idea how big the battery is. So I'm guessing there are other reasons for this???? Educate me...

KARNUT · November 18, 2019

My first diesel, 91 dodge had one battery. We definitely had it the longest, it just worked out that way. Never had an issue. Two batteries in diesels there after. Didn’t see the point.

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1SLOW1500 · November 18, 2019

What am I missing? Two identical batteries in parallel just doubles the amp capacity and capacitance. Literally acts like a battery with twice the CCA and reserve minutes. So...why isolate and use/charge independently? The alternator doesn't care and the ECU hasn't any idea how big the battery is. So I'm guessing there are other reasons for this???? Educate me...

Normally it is marketed for the plow guys. They can hook up to the 2nd battery all there accessories and with truck off if truck dies you can start truck. This is why the battery doesn't connect in cranking. It would only hurt the first battery if 2nd was dead.

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Grumpy Bear · November 18, 2019

19 minutes ago, 1SLOW1500 said:

Normally it is marketed for the plow guys. They can hook up to the 2nd battery all there accessories and with truck off if truck dies you can start truck. This is why the battery doesn't connect in cranking. It would only hurt the first battery if 2nd was dead.

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Ah!!! Thanks.

Vuk · November 18, 2019

1 hour ago, Grumpy Bear said:

What am I missing? Two identical batteries in parallel just doubles the amp capacity and capacitance. Literally acts like a battery with twice the CCA and reserve minutes. So...why isolate and use/charge independently? The alternator doesn't care and the ECU hasn't any idea how big the battery is. So I'm guessing there are other reasons for this???? Educate me...

In fact, ECUs can estimate the battery capacity by keeping track of the discharge rate through the current sensor, which these trucks have. I wanted an auxiliary battery for a 2-3kW inverter, but thought that it would also provide me with a secure start in case my primary battery dies (just of old age because the BCM seems to reserve the right to shut things off if the battery discharges below a certain point).

The fact that the factory fuse between the batteries is 125A explains that the aux battery is not meant for cranking.

Edited November 19, 2019 by Vuk

2003 SSEi · November 19, 2019

@Vuk Excellent Post/write up with Pictures. I will be collecting parts soon for this mod.

gmcdawg · November 19, 2019

Great write-up Vuk. I only wish my 2018 could use it so I could make my setup OEM like that. Unfortunately, looks like they've made some changes with the '18.

Vuk · November 19, 2019

1 hour ago, gmcdawg said:

Great write-up Vuk. I only wish my 2018 could use it so I could make my setup OEM like that. Unfortunately, looks like they've made some changes with the '18.

All the wiring info is from the 2018 builder's manual. The part numbers I got printed out from a dealer, so not sure if the battery tray or the harness would be the same.

Edited November 19, 2019 by Vuk

gmcdawg · November 19, 2019

4 minutes ago, Vuk said:

All the wiring info is from the 2018 builder's manual. The part numbers I got printed out from a dealer, so not sure if the battery tray or the harness would be the same.

Haven't had time to pull the cover off the primary distribution block on top of main battery, but it looks to be different than yours. I also read in another post, I think, that the '18 changed. Looks like I have a small ECM on top of my battery, and the cover/charging/jump post looks different. I'll have to get that cover off to verify.

Vuk · November 19, 2019

Could be that it has changed at some point during 2018; I am not knowledgeable enough to say anything about that.

Grumpy Bear · November 19, 2019

18 hours ago, Vuk said:

In fact, ECUs can estimate the battery capacity by keeping track of the discharge rate through the current sensor, which these trucks have.

This has my interest. How can a current sensor tell the difference between a 30 amp draw on a 600 CCA system v a 1200 CCA system? Unless I don't understand what is meant by 'rate'.

1SLOW1500 · November 19, 2019

This has my interest. How can a current sensor tell the difference between a 30 amp draw on a 600 CCA system v a 1200 CCA system? Unless I don't understand what is meant by 'rate'.

It guesses. But really it has no clue if there is one or a million batteries. My understanding is it sees voltage and current draw. Nothing fancy just a sensor. And thinks it is low it shuts off circuits. Much like regulating the voltage output on the alternator based on the voltage.
There was a post long ago when we talked of getting the sensor on both batteries so it can learn. But no one tried it. I don't have it and never a issue because vehicle has no clue.
But now you have me thinking. If I grabbed the signal wires and just feed my relay it would work like yours then swap signal wire and the connect on key on. [emoji2962]

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Vuk · December 6, 2019

I picked up a 48PG battery today and installed my three-fuse modification. The original negative auxiliary battery cable 84354708 seems like it would never be delivered, so I picked up 22846471 (at about the same price) and removed the sensor. This one gives an additional ground lead of decent gauge that can be used for accessories, which is even better.

I've replaced the 175A fuses with 250A as a preparation for a 3000W inverter running off of the aux battery. I will post pics once the battery lays down in the next couple of days.

Edited December 6, 2019 by Vuk

Vuk · December 8, 2019

My setup is complete now with nice factory-looking points to hook up an inverter. Should I ever need to jump start the primary battery, I can always run jump cables from aux battery to the primary.