Aux Battery Swap for H8 Main Battery

rpr · Mar 7, 2026

Well, I swapped the batteries and all was fine until the 2nd or 3rd restart. Got to destination but the CEL is back on and running volts are back up to 16V.

Seems like an alternator issue. Guess I’m off to the dealer unless anyone else has experienced this and has any ideas.

Anyone know if the code readers at Harbor Freight work on these trucks? Don’t know much about code readers.

Eighty · Mar 7, 2026

rpr said:
Anyone know if the code readers at Harbor Freight work on these trucks? Don’t know much about code readers.

OBDII is standard/universal across all platforms, so it will be fine.

gram10 · Mar 12, 2026

carlos33193 said:
Proceed at your own risk. My truck is L2+, which does not come with a PCR (therefore no F52 fuse).
I went back and re-did my AUX removal to fix my previous mistakes.
Seeing as how from everything I found the L2+ trucks don't have a PCR or fuse that can be easily bypassed, I went with my original approach.

I tied both IBS from main and aux - to the negative battery terminal, in my original removal I didn't realize i tied the aux battery - IBS to the main battery IBS output, so the main battery IBS was measuring the current draw of both which is what was probably causing my charging voltage to be 14.8 all the time. This was a mistake on my part. With this new method my charging voltage is 14.4 on startup and then drops to 13.8-13.9.

I then used a 50A fused wire link to jumper N3 (AUX Battery +) to N9 (Main Battery +), and disconnected the original N3 wire that runs to the AUX + and heat shrunk both ends just to keep it clean.

Everything is working as expected, with no OBD codes, and start-stop works also (although I will keep it disabled).

I used this to tie both IBS. https://www.amazon.com/dp/B09W24MCHD?tag=ramrho-20

Until a better, proven method comes out for the L2+ trucks I'll keep using this.

Is this still working well for you?

zmw · Oct 4, 2026

Just wanted to necro bump this thread, and give my experience. I have a 2026 RHO with the level 1, power boards, all the things.

what worked on mine (two battery sensors AND F52) may be different than yours so YMMV - Swapped to a single 80Ah LiFePO4 (group 49/H8) and pulled the aux battery. Posting because my truck doesn't match what the threads say. It has a sensor on each battery, which is supposed to mean no F52, but mine has F52 also.

The plain delete didn't work for me: aux out, N3 unbolted, F52 pulled, second sensor unplugged. The truck ran, but I got a CEL, an "automatic emergency braking unavailable" message, a pile of codes after clearing, the AC forgetting its setting every start, and the head unit doing a full reboot on every crank. The CEL did clear after a few starts, AC and big display reboot sucked.

The reboot gave it away. The relay isolates the N3 side during crank, and that little aux battery is what kept the head unit, climate and so on alive through the start. With it gone, that whole side goes dark every time you crank. It's unclear to me if the little aux battery is being used when the truck is off for the small parasitic draw. I had already removed the aux battery and wired everything up so I failed to measure that. My gut tells me with ignition off and when the truck 'sleeps' the aux power is flipped to the small battery to preserve the big battery for cranking.

What fixed it for me:
- 18" molded 2 AWG cable from N3 straight to battery positive. It's all under the fuse box cover and terminal guard. I did NOT fuse this, you could ALSO wire a jumper between N3 and N9 - but if you do that I'd fuse it with at least 50A and a minimum of 8ga wire. You really only need about 12" here 18" was the shortest pre-fabbed 2AWG cable I could find - if you're looking I went to walmart and bought a cable labled for switch to battery. It had molded crimped lugs on BOTH sides.

- Second sensor moved onto a brass multi-stud negative terminal, wired the same way the factory had it: one stud on the battery side, its own ground cable on the other stud. Don't put both ends on the same stud or it reads zero.

- F52 back in.

- Already had a start-stop eliminator installed (with in about 100 miles of purchase!) so take that for what it's worth.

Result: no CEL, voltage normal (13.6 settling at 13.8 with a full battery), screen and AC come right back after a restart. I logged the battery over Bluetooth overnight: 16.5 hours parked, it used 0.06Ah total, with one 2-minute wake-up at 2 AM. Sleep draw was below what the BMS can read.

One truck, so your mileage may vary. If your truck has two sensors and it's acting up after a delete, try feeding N3 before you give up on it. Let me know if you want some photos

-Z

gram10 · Oct 5, 2026

zmw said:
Just wanted to necro bump this thread, and give my experience. I have a 2026 RHO with the level 1, power boards, all the things.

what worked on mine (two battery sensors AND F52) may be different than yours so YMMV - Swapped to a single 80Ah LiFePO4 (group 49/H8) and pulled the aux battery. Posting because my truck doesn't match what the threads say. It has a sensor on each battery, which is supposed to mean no F52, but mine has F52 also.

The plain delete didn't work for me: aux out, N3 unbolted, F52 pulled, second sensor unplugged. The truck ran, but I got a CEL, an "automatic emergency braking unavailable" message, a pile of codes after clearing, the AC forgetting its setting every start, and the head unit doing a full reboot on every crank. The CEL did clear after a few starts, AC and big display reboot sucked.

The reboot gave it away. The relay isolates the N3 side during crank, and that little aux battery is what kept the head unit, climate and so on alive through the start. With it gone, that whole side goes dark every time you crank. It's unclear to me if the little aux battery is being used when the truck is off for the small parasitic draw. I had already removed the aux battery and wired everything up so I failed to measure that. My gut tells me with ignition off and when the truck 'sleeps' the aux power is flipped to the small battery to preserve the big battery for cranking.

What fixed it for me:
- 18" molded 2 AWG cable from N3 straight to battery positive. It's all under the fuse box cover and terminal guard. I did NOT fuse this, you could ALSO wire a jumper between N3 and N9 - but if you do that I'd fuse it with at least 50A and a minimum of 8ga wire. You really only need about 12" here 18" was the shortest pre-fabbed 2AWG cable I could find - if you're looking I went to walmart and bought a cable labled for switch to battery. It had molded crimped lugs on BOTH sides.

- Second sensor moved onto a brass multi-stud negative terminal, wired the same way the factory had it: one stud on the battery side, its own ground cable on the other stud. Don't put both ends on the same stud or it reads zero.

- F52 back in.

- Already had a start-stop eliminator installed (with in about 100 miles of purchase!) so take that for what it's worth.

Result: no CEL, voltage normal (13.6 settling at 13.8 with a full battery), screen and AC come right back after a restart. I logged the battery over Bluetooth overnight: 16.5 hours parked, it used 0.06Ah total, with one 2-minute wake-up at 2 AM. Sleep draw was below what the BMS can read.

One truck, so your mileage may vary. If your truck has two sensors and it's acting up after a delete, try feeding N3 before you give up on it. Let me know if you want some photos

-Z

any chance you have pictures?

zmw · Oct 5, 2026

sure - Notice both sensors still attached, and they each have their own ground correctly wired - it's important to NOT stack them they need their own lead from the negative terminal

second photo shows that I do in fact have F52 even though I have dual sensors and 3rd photo just shows the jumper from N3 to positive terminal

I tried all the options I read on here, I tried just deleting, I tried deleting and removing 52, I tried leaving the secondary sensor unplugged, etc.

One note is you COULD just run a jumper from N3 to N9 and keep it cleaner, I didn't want to do that because I wanted the wiring to be functionally identical to stock. but it would be slightly cleaner.

zmw · Oct 7, 2026

So i wanted to post an update a few days in. The battery I have has a full BMS and BT module so I can track charge/discharge/AH/etc...

So i've been watching it pretty religiously and the truck does some silly stuff - and frankly, I'm baffled why - I know the TECHNICAL reason but on an RHO it's dumb.

The truck basically turns the alternator way down (cuts the field current so it's not dragging on the engine) to save MPG - you can actually see it happen on my BMS, it will go from charging at 5-80A (depending on battery need) then switch to discharging for a few minutes, where the entire truck runs on the battery, then the alternator will produce JUST ENOUGH to let the battery sit idle (no charge or discharge, just 0A in 0A out) for a little bit, then it blasts the current up, I've seen as high as 110A into the battery. This is all controlled by the PCM off the battery sensor (IBS), and it's kinda dumb AF - doing the rough math, it MIGHT save $50-100 in fuel per YEAR with 20k miles driven.

So doing some testing

1. Unplugged both sensors (F52 was already pulled), got a CEL - but the charging went constant. The PCM falls back to just holding ~14V and the battery takes whatever that pushes, around 80A right after a startup, tapering down to about 1A once it's full (sat at 14.06V). No more CYCLING at all, it was always charging, sometimes with as little as 1A

2. Plugged back in just the MAIN sensor (keep in mind my N3 is still jumpered straight to battery +, so N3 and N9 are basically the same point) - CEL goes away after about a 20M drive and a start/stop cycle. (got out to get a drink) - but the cycling charging is back - charge for 10s-1m, discharge for a few minutes, idle for a minute, back to charge... What I noticed is on hours of very short trips, the battery trended DOWN. I lost 20% of my charge today doing a bunch of 3-5 mile trips, the draw of STARTING the truck was measurable and the BMS showed I had dropped from 80AH to about 64 over a few hours of running errands.

I actually think this is a symptom now made WORSE by the lithium battery. An AGM rests around 12.8V when full. From what I can tell, when the PCM drops the alternator target to save fuel it's somewhere around where an AGM sits, so an AGM mostly just stops charging or gives up a few amps. My lithium rests around 13.3V (it only reads 14V when the alternator is holding it there), so when the target drops it's half a volt ABOVE the alternator and takes over the whole truck. When the target goes back up, lithium takes current way easier than an AGM, hence the 110A. Same PCM logic, but with lithium it's literally turning the spigot full on and full off instead of nudging it. The sensor is also set up for an AGM, so I'm pretty sure it sees 13.3V resting and thinks the battery is full.

I'm going to check the modules with my T8 and see if there is anything that can adjust the battery type or possibly the target voltage

zmw · Oct 8, 2026

more updates - so no where can I find a way to adjust target voltage or change IBS settings - either those things are decoded for 2026s or they dont exist as variables - what's odd is other FCA vehicles DO have the ability to set the battery type - if it exists somewhere I can't find it.

Not gonna lie, I may just end up going back to an AGM and just get the biggest AGM I can fit in there, the lithium is H8, so was hopeful it'd work.

I'm going to try some inline diodes first and see if I can trick the sensors into reading a lower voltage

zmw · Oct 20, 2026

okay well updates aren't great, but here it is.

I've tried to fool this IBS system, combined with a lithium battery - I've tried to use some diodes to drop sensor voltage so that it indicates the battery needs charging - just some background, Lithium batteries tend to sit at about 14v as they are built in 4x3.5v packages, so resting voltage is 13.8-14.1v - according to the config in the BCM the resting voltage shoudl be 12.7, and below 12.7 is when it kicks on the alternator to charge the battery up - it uses the IBS sensors to determine this.

Unplugging the main IBS sensor throws a CEL but the truck goes into "default charge mode" where the alternator is always charging and when the batteries are full the current drops to approximately 0-1a which is how a normal regulated alternator works.

when fooling the IBS Sensor, resting side shows 13.7v - post diode leg shows appriximately 12.5 so I'm pulling 1.2v off the sensor wire, i can SEE that in the BCM - with the truck off the sensor reads 12.5 while running it reads about 12.8 so that should fall right into where the IBS and charging system is expecting to be.

End result... absolute catastrophy - dropping the voltage on one IBS sensor I see trailer connection errors, I see AUTO braking errors, all kinds of things - it worked fine for a day and then just tripped out, end result alternator started sending 15+v - it's almost as if it detects a battery failure

Not sure what to do next - using a high quality lithium on this truck just may not be an option. I'm going to try to reduce the dropped voltage down to maybe .5v or .8v but my hope is dwindling...

TO BE CLEAR - this would be a non issue if I just bought an H8 AGM - the H8 Lithium with the slightly higher riding voltage is really what's causing my problems - I still think a single AGM H8 is a better system than the 2 battery setup.

zmw · Oct 26, 2026

okay one final update, think I finally have this thing beat.

Latest change

1. Moved ground lug OFF of the secondary sensor to the main sensor - so now only the ONE sensor has current going through it

That's it - that solved the entire problem. Both sensors still plugged in, both sensors still on the main negative terminal of the battery but only the sensor for the MAIN battery has a ground lug attached to it now, and it has both of them.

The truck no longer is confused, it's properly tracking Ah use in and out and tends to FILL 30% more than it uses so it's kept the lithium at 92-100% for the past 6 days.

So the fix right now

1. remove both batteries
2. Install H8
3. put BOTH sensors on the negative terminal
4. Put BOTH ground cables on the PRIMARY SENSOR's lug

No CEL's no weird charging patterns no over voltage, according to my scanner and logging on the battery BMS the battery receives slightly more than it puts out over the course of long and short drives, after starting the truck spends approximately 60-90 seconds charging the battery back from the initial cranking effort, and then cycles on and off as it needs to.

Prior to this the battery would trend down, and I finally figured that out by logging the data stream, it was only seeing amps one direction on each sensor. i.e. the aux sensor say all the INPUT and the primary sensor saw all the OUTPUT so the ECU never properly realized it wasn't charging the battery - and that was exacerbated by the battery being a lithium which continued to show 13.2+v all the way down into the 20% range.

Because the aux lug no longer has a ground cable attached it never sees ANY consumption and the primary IBS sensor is properly tracking in/out and the system is triggering the alternator properly to fill up.

There is no way in the 2026 RHO that I can determine to either 1. tell it to only use ONE battery 2. Change the battery chemistry, 3. Change the charging profile. In my scanner I can see the battery chemistry is set to lead acid, I can see the "100% SOC" value is set to 12.7V - which would indicate that those settings COULD be changed, but even with the SCM unlocked and my snapon scanner on a current autoauth subscription those settings don't appear to be able to be changed - it is possible a DEALER could do it, but it's not clear to me.

next step for me will be the ground the secondary sensor somewhere else to clean up under the hood a little bit - the sensor has to be grounded and has to be plugged in. It does NOT have to be on the battery terminal.

gram10 · Oct 26, 2026

zmw said:
okay one final update, think I finally have this thing beat.

Latest change

1. Moved ground lug OFF of the secondary sensor to the main sensor - so now only the ONE sensor has current going through it

That's it - that solved the entire problem. Both sensors still plugged in, both sensors still on the main negative terminal of the battery but only the sensor for the MAIN battery has a ground lug attached to it now, and it has both of them.

The truck no longer is confused, it's properly tracking Ah use in and out and tends to FILL 30% more than it uses so it's kept the lithium at 92-100% for the past 6 days.

So the fix right now

1. remove both batteries
2. Install H8
3. put BOTH sensors on the negative terminal
4. Put BOTH ground cables on the PRIMARY SENSOR's lug

No CEL's no weird charging patterns no over voltage, according to my scanner and logging on the battery BMS the battery receives slightly more than it puts out over the course of long and short drives, after starting the truck spends approximately 60-90 seconds charging the battery back from the initial cranking effort, and then cycles on and off as it needs to.

Prior to this the battery would trend down, and I finally figured that out by logging the data stream, it was only seeing amps one direction on each sensor. i.e. the aux sensor say all the INPUT and the primary sensor saw all the OUTPUT so the ECU never properly realized it wasn't charging the battery - and that was exacerbated by the battery being a lithium which continued to show 13.2+v all the way down into the 20% range.

Because the aux lug no longer has a ground cable attached it never sees ANY consumption and the primary IBS sensor is properly tracking in/out and the system is triggering the alternator properly to fill up.

There is no way in the 2026 RHO that I can determine to either 1. tell it to only use ONE battery 2. Change the battery chemistry, 3. Change the charging profile. In my scanner I can see the battery chemistry is set to lead acid, I can see the "100% SOC" value is set to 12.7V - which would indicate that those settings COULD be changed, but even with the SCM unlocked and my snapon scanner on a current autoauth subscription those settings don't appear to be able to be changed - it is possible a DEALER could do it, but it's not clear to me.

next step for me will be the ground the secondary sensor somewhere else to clean up under the hood a little bit - the sensor has to be grounded and has to be plugged in. It does NOT have to be on the battery terminal.

pics?

gram10 · 2026-11-05T08:49:22-0600

Thanks to @zmw i followed his lead and converted to a single h8 battery as well. I have some things I'm going to change and clean up, but long story short, it has been working for 2 days now with no issues whatsoever.