Weird Charging Issue

[tumbleweedsbns]

Strange Electrical Issue.

BLUF. My Alternator isn’t charging my battery to 14V. I’m getting a constant 12.6V. But when I decelerate it jumps to 14V like normal. When I finally stop or accelerate, the voltage drops back to 12.6V.

2019 JL Rubicon. Running a dual battery system from Genesis Offroad. I recently changed out the batteries and they are brand new. Cold snap earlier this year zapped them. I bought the Jeep new and made a fair number of mods after the warranty ran out. Everything electrically has been fine, except for a short in USB and license plate light.

Everything was running well. Just completed a 12 hour long drive the day prior. Heading up a HWY to a trail, I hit a huge pothole I couldn’t avoid. The impact rattled our teeth. I didn’t notice anything immediately, but when I got to the trail, I noticed the alternator wasn’t putting out 14.1 – 14.2 volts like normal. I was getting a steady 12.5 to 12.7V. I put the battery on a charger when I got back from the trail and ensured I was topped off before making the 12hour drive home. After the trail it was at 75% but I did have to run my headlights because I returned after dark. After the 12-hr drive during the day, the battery was 82% charged.

On the way home I noticed something peculiar. I’m getting a consistent 12.6V at idle, at highway speed, higher RPMs while in park/neutral. But when I decelerate, either braking or coasting to a stop, the voltage jumps to 14.1, 14.2V like normal. As soon as I coast to, or brake to a stop the voltage drops back down to 12.6. If I need to accelerate after coasting to or braking to a stop, the voltage also drops. Why would momentum change how many volts I’m getting from the alternator.

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[Reinen]

It is a smart alternator and it is designed to avoid charging during acceleration unless absolutely necessary. So this behavior doesn't necessarily mean something is wrong. The dash voltage display is also notoriously deceptive. If you're concerned about your battery voltage, checking the battery directly with a voltmeter will be much more accurate.

 

[Bob Burd]

It is a smart alternator and it is designed to avoid charging during acceleration unless absolutely necessary. So this behavior doesn't necessarily mean something is wrong. The dash voltage display is also notoriously deceptive. If you're concerned about your battery voltage, checking the battery directly with a voltmeter will be much more accurate.

I agree that this may be perfectly normal behavior, your Jeep believes the batteries are fully charged.
But I disagree that the dash voltage display is notoriously deceptive. A voltage reading is very simple. A separate voltmeter will not add accuracy.

 

[Reinen]

I agree that this may be perfectly normal behavior, your Jeep believes the batteries are fully charged.
But I disagree that the dash voltage display is notoriously deceptive. A voltage reading is very simple. A separate voltmeter will not add accuracy.

That would be logically correct. But have you ever compared the two?

 

[THAW]

But I disagree that the dash voltage display is notoriously deceptive. A voltage reading is very simple. A separate voltmeter will not add accuracy.

A voltage reading at a battery will differ from a voltage reading at a device (e.g. ECU) that's drawing significant current over modestly-sized wiring.

In my experience, dash voltage and IBS voltage generally differ by about a quarter Volt. There are even small discrepancies between the "Battery Voltage" displays on the EVIC and Off-Road Pages.

 

[Bob Burd]

There are few things drawing significant current, certainly not the ECU or any of the electronic modules. If the grounds are good, the voltages are solid.
And the EVIC and off-road pages both get the same voltage info from the CANBUS, so they can't be different.

 

[THAW]

There are few things drawing significant current, certainly not the ECU or any of the electronic modules. If the grounds are good, the voltages are solid.
And the EVIC and off-road pages both get the same voltage info from the CANBUS, so they can't be different.

The "significance" of the current as it relates to voltage drop at a device is proportional to wire length/diameter.

The (small) differences in voltage between the EVIC and Off-Road Pages could be due to polling frequency, but I can assure you JLs have more than one voltmeter, so both displays receiving voltage information over the CAN bus doesn't guarantee the same source voltage. It could be one uses the PCM and one uses the BCM.

In any case, voltage on the dash is significantly different from IBS voltage.

 

[Bob Burd]

That would be logically correct. But have you ever compared the two?

I've chased multiple electrical issues over the years and have spent much time measuring battery voltage and current with multimeters. So, yes.

 

[THAW]

@tumbleweedsbns, JScan is the most convenient way to deduce what's happening with the alternator and CR[an]K battery. It's possible to monitor battery current to see if it's being charged or discharged (for the voltage and temperature), and observe target/sensed alternator voltage plus duty cycle.

 

[roaniecowpony]

Some similarities between the OP's condition and my JL's condition on a road trip last week. Wondering if the IBS is going bad. I had similar voltage displayed and verified battery voltage with a multimeter at 12.4v

 

[roaniecowpony]

Continuing my saga of basically the same issue as the OP's. I changed the IBS Saturday, drove from the coast to Big Bear ~3 hrs. Gauge voltage started at 13.7 with the new IBS and along the 3 hr drive continued to reduce to ~12.8v. The next day, I noted that it was only indicating 12.6v. After about 5 hrs of use on trails, still indicating 12.6v, I headed home for the ~2.5 hr drive. Upon getting home, batteries checked at 12.4v with my multimeter. The voltage indications were as described by the OP; 12.6v during acceleration and cruise, up to 14.x during deceleration.

Today, I replaced the unlocked PCM with my baseline spare PCM. The removed unlocked PCM, is of unknown service history, was unlocked and cloned to my baseline PCM, which was installed about a month or so ago, and has a HP Tuners modified radiator fan schedule. My baseline PCM was purchased new 2 years ago and programmed to my VIN by WranglerFix for replacement with my new engine 2 years/~15,000 miles ago.

I drove it for ~45 mins, after installing the baseline PCM today. It showed 14.2v immediately. But, this could be due to reset of the IBS. I made a point of making several full stop/restarts while on the drive to run errands. The dash gauge showed a decay of voltage to 14.1v. Upon returning home, I put a multimeter on the batteries and they were 12.9v. I'll continue with some more drives over the next few days, to see if the voltage continues to work properly. I'm hoping that unlocked PCM is the culprit and I can simply get another and get it unlocked and cloned and modify the radiator fan schedule.

Note: I have a Genesis dual battery system with the aux eliminated by way of Genesis instructions. I've had a Genesis gen 2 for at least 3 or 4 years, and recently revised it to Gen 3 and replaced both Odyssey batteries with Fullthrottle batteries. Fullthrottle literature indicates that 12.4v is about 60% SOC.

 

[azjl#3]

It is a smart alternator and it is designed to avoid charging during acceleration unless absolutely necessary. So this behavior doesn't necessarily mean something is wrong. The dash voltage display is also notoriously deceptive. If you're concerned about your battery voltage, checking the battery directly with a voltmeter will be much more accurate.

Also you have dual huge batteries so the battery computer is thinking no worries, float charge at 12.6

After you start after letting it sit all night, does it go to a higher voltage?

 

[THAW]

Also you have dual huge batteries so the battery computer is thinking no worries, float charge at 12.6

In the Genesis Offroad system the IBS will register current for only one of the two batteries, and that battery actually has about 20% less capacity than the factory IBS-monitored battery.

 

[roaniecowpony]

So, in reading through my factory service manual, the IBS is stated a high quality type that measures voltage, current, and temperature. This signal goes to the BCM on LIN buss and from the BCM to the PCM by CAN-C, where the PCM has an Electronic Voltage Regulator (EVR) that utilizes PWM to modulate the output of the alternator by controlling the field energy. Before the BCM sends data to the PCM, it does all the calculations on high and low charge parameters. So, the logic is in the BCM and the PCM executes by way of the EVR. But, the PCM also has a logic that it applies to the charging function, by considering vehicle acceleration, deceleration, cooling (fan) and a host of other power related requirements that need to be met.

So, while my issue might be the PCM, it is possible it's the BCM. I'll know in a day or two of driving it.

Note: the IBS resets whenever power is (re-)connected and the learning process starts. The manual states that one start and 4 hours of quiet (all systems off, no disturbances) is required for the relearn.

 

[THAW]

I'm hoping that unlocked PCM is the culprit and I can simply get another and get it unlocked and cloned and modify the radiator fan schedule.

Interesting theory/experiment. I've suspected for some time there are substantial differences in voltage/charging regulation between engines and/or model years, mainly because - unlike what others report - the running voltage in my '23 2.0L never drops below low 13 Volts (excluding blips while the power steering is under heavy load at less than 5 MPH).

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