Battery was screaming for help, but I didn't know how to listen.

[Zapper]

I figured out a symptom of a dying battery that may help others.

These trucks of course have a "smart" alternator. Mine was generally sitting at 14.1-14.4 volts all the time. Not very smart, but at least it was charging. I only saw it drop down significanly during a very long road trip (8h with basically no stopping).

My "toy" car (an old Mercedes) needed a battery so I moved my Jeep main battery into a comfy retirement in that car, and bought a new one ($$$) for the Jeep. I decided to disable to aux battery at the same time because my wallet was feeling a bit too empty to shell out another $100 for the aux.

Just did a four hour drive and after 30 minutes or so, the smart alternator dropped from 14.4 volts to 12.6-12.7 like its supposed to.

Also, not 100% sure but I think this made a significant gain on my gas mileage. My mpg's have been getting worse since I bought the truck- but then of course that makes sense since it started as a bone stock Sport S, and now its been lifted, has bigger tires, a winch and winch bumper, and other airflow resistors. But on the way home after the battery change my mileage went up from what I've been averaging (15mpg) to closer to 20. Will need to do a proper roadtrip but it seems promising.

My theory is that the aux battery was not taking a charge properly and was converting serveral horsepower into heat.

So if your truck is constantly at 14+ volts even after driving a while, you might want to replace your battery(s).

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[ShadowsPapa]

I figured out a symptom of a dying battery that may help others.

These trucks of course have a "smart" alternator. Mine was generally sitting at 14.1-14.4 volts all the time. Not very smart, but at least it was charging. I only saw it drop down significanly during a very long road trip (8h with basically no stopping).

My "toy" car (an old Mercedes) needed a battery so I moved my Jeep main battery into a comfy retirement in that car, and bought a new one ($$$) for the Jeep. I decided to disable to aux battery at the same time because my wallet was feeling a bit too empty to shell out another $100 for the aux.

Just did a four hour drive and after 30 minutes or so, the smart alternator dropped from 14.4 volts to 12.6-12.7 like its supposed to.

Also, not 100% sure but I think this made a significant gain on my gas mileage. My mpg's have been getting worse since I bought the truck- but then of course that makes sense since it started as a bone stock Sport S, and now its been lifted, has bigger tires, a winch and winch bumper, and other airflow resistors. But on the way home after the battery change my mileage went up from what I've been averaging (15mpg) to closer to 20. Will need to do a proper roadtrip but it seems promising.

My theory is that the aux battery was not taking a charge properly and was converting serveral horsepower into heat.

So if your truck is constantly at 14+ volts even after driving a while, you might want to replace your battery(s).

To clarify some things -

It's not common to see voltages drop clear down to 12.6-12.7 unless the battery is getting hot. So I'd be careful with the "as it's supposed to be".

It can take hours for it to drop out of 14 volt range. If the vehicle isn't driven often, you can go hours at 14 volts. So that's not necessarily a bad thing. Don't assume you have a bad battery just because it stays in the 14 volt range! It takes a LOT of driving to see numbers under that unless you drive long drives daily.

Horsepower - you'll only be losing a couple HP unless you have a real heck of a load. Typically it's more like 1 hp for about 25 amps (so you have to count the AC, lighting, electronics and so on.

Mine went 2 days of 15-30 minute drives at 14.6 volts. Then the next day it started running ~13.1 and 14.2 when letting off.
Depending on the history is has, it may still stick in the 13 volt area, depending on what's running and so on.