I was spending a lot of time in my truck connecting phones, looking at vehicle settings, and learning where the new features are. To ensure I did not over drain the battery I plugged a volt meter into the power outlet. Leaving the voltage meter in the power outlet eventually while driving down the road I noticed the volts starting a bit low at 12.5 going up to 13.5, then 14.5, than back to 13.5 and down to 12.5. When I stop at a light it will drop to 12.5 and remain there until I start off again. I've never seen this. It's a diesel with that large capacity battery, so it starts great every time, but its summer now, and winter is coming.

The alternator seemed suspect, but this is a covid19 truck in that went to the auction back in February where it remained until June, then sat on the lot until I bought it in Sept.
I put a maintainer on the battery over night five times, and can't seem to get volts above 12.0 to 12.4. What do you think, battery, alternator, regular?

 

My truck does the same thing.

 

But, a fully charged, good battery will read 12.7V in the "resting" state. A battery reading at 12.10V is only about 50% charged. I'd think a battery maintainer would have brought the battery up to 12.7 if the battery is good. So, I think you have a weak battery and possibly a malfunctioning alternator. I think the alternator is supposed to maintain a fairly constant voltage at the battery terminals - generally above 13.0V if the battery is charged.

- Jack

 

That's what it looks like. Hopped in the truck this morning, plugged in the voltage meter and turned on the radio. The battery started at 12.1V and climbed up to 12.2.V.

 

More details from testing today with my truck and a neighbor's 2016 F150. Here is the results.

Neighbor's 2016 5.0L F150: Started on the interstate then took side roads back, about 40 minutes.
Voltmeter went to 14.7v with alternator icon constant, and stayed there the whole time we drove.

My 2019 3.0 diesel F150: Similar route only shorter, about 20 minutes.
12.8V from alternator when accelerating or cruising along with foot on the gas.
13.5V - 14.5V from alternator/battery when coasting.
12.5V from battery when idling at a stop.

This just does not sit right with me. I have a two minute video that shows this, but not sure how to post it.
I have three issues, first the way volts jump around. Second is that I'm noting a pattern based on what the engine is doing. Third is that when I come to a stop the engine is running off of the battery for while.
One more thing I noticed. There are icons on the gage, one displays for battery, and one for alternator. Most of the time they blink back and forth between battery and alternator, with two exceptions.
During acceleration it will display solid alternator icon, and at early idle it will display solid battery icon.

Not sure if this is a technological feature or strange behavior. Neighbor thinks it is part of the auto shut off feature.

 

Ford technician today said it's fine.

 

Update: Today I bought a new Interstate battery at Costco because I just didn't trust the battery that was in there. I swapped out the battery, and I don't know for sure that it solved this issue, but as of today I can no longer recreate these voltage swings. The truck runs at a constant 14.2 volts. We will see what happens the rest of this week.

 

Green - I think you've nailed the problem - a weak battery. Good on you! I think the technician who told you things were fine didn't have a clue.

- Jack

 

I'm definitely feeling like I don't have a clue. Really thought I nailed it, but the alternator went right back to those voltage swings the next day. Something must have reset when I changed out the battery. Last night I pulled the negative terminal, for an hour, then put it back on. Then let the truck sit for 12 hours. I heard that leaving the truck untouched for 8 hours helps the PCM learn the battery, so I tried it. Today I started it and drove 21 miles with a constant 14.8 volts, then parked and shut if off. On the way back home the voltage swings were right back again. It drives down the road kicking out 12.9 volts. Take your foot off the gas and coast, and it slowly climbs to 14.8 volts. I even verified the volts with an OBD2 scanner side by side with my handy volt meter, and they both showed the same volts at all times. So far it has not been a problem, the truck starts great. New battery is the same, only charging to about 12.4 volts.

Maybe the alternator is fine, and I have OCD issues.

 

Well, I'm puzzled too. I've not seen this kind of behavior in a modern, alternator equipped vehicle with a healthy charging system and good battery. I can't imagine it being a diesel is any factor either, but I've never owned one, so maybe I just don't know enough.

- Jack

 

I believe the alternator is under PCM control these days, not a traditional voltage regulator. Maybe it's doing that to very slightly reduce the drag on the engine, part of the fuel mileage strategy?

 

Bingo!

 

Sounds reasonable - might explain why it drops when there is a load on the engine.

- Jack

 

I did see that the alternator is under PCM control. That sounds like a reasonable strategy to me also, but I would feel a whole lot better about if if we could find one other truck employing the same strategy.

 

Update: Today I drove 12 miles then did about an hour work and drove back home. On the way back home I decided to take the long route and watch the voltage the alternator was putting out. I drove about 30 miles. Volts were about 13.5 the whole trip. The volts stayed there no matter what I did. It looked quite normal to me. Maybe the PCM just needed time to learn the battery and work it out?