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Checking voltage on the battery is only a quick test and not that accurate. With the engine running you are not testing the battery so much as the charging system. The voltage of the battery with engine off after 30 minutes would be close to 12.6 volts. If the meter was accurate.

Get a load test on the battery for free at many fast moving parts stores.

A low battery will cause all kinds of weird things on electronics.

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Sorry I can't figure out how to download the picture I took

In a new post, look at the bottom of the post. There is a line of characters.
Close to mid point is a paper clip. Click that.
A new window will open up and you can select a picture. Or multiple pictures.
These will populate below the post you are completing as thumbnail pictures.
You can leave them as thumbnails or click full image.

Then click post reply when you are ready

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The battery cable end clamp shown is a temporary replacement. The charging system will struggle keeping the battery charged if there is enough corrosion. The exposed wire of the cable to the clamp invites corrosion to get in there and cause problems.

Replace the battery cable and get rid of that cable clamp end. Those cause so many more problems with electronics than they solve. It will not be cheap however will solve many of your battery problems.

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You have a second gen Expedition with a modular engine.
I have a second gen Navigator with a 5.4l with about 200,500 miles.

Attached are pictures of the battery and cable(s). Mostly the positive cable.
Your battery is missing the heat shield and the positive cable end cover.

This is making it more difficult for the charging system in general to do the .job and the battery specifically.
In addition, the exposed copper wires allow moisture in that creates corrosion.
The issue starts as an intermittent issue. Then progresses to more serious issues.
The electronics need a clean voltage signal in both sides. Yes as stated above the ground is important too.

A 10 second voltage test by AutoZome may show the charging system is good. And I bet it is! However the connections between the alternator and battery and battery to the rest of the vehicle need to be clean and tight. The open cable shown will not stay clean for very long.

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I am at a computer (at work) that does not have edit features, or I would copy your picture and circle the issues.

On your positive battery post is a universal battery cable clamp or end. Similar to this one.

Your's is slightly different in that there are two bolts that clamp on to the stripped back insulation on a red cable.
To that are two cables with metal ends and another cable with stripped back insulation.

This is the original design.

The cable end clamp goes over the battery positive terminal and has a stud on the end of the clamp.
Two other wires connect to it.
One that is directly to the alternator
The other supplies power to the rest of the vehicle.

The other end of the pictured battery cable goes to the starter.

The listing for this cable is shown on eBay. (linked below) Listed at $45.25, free shipping and tax depending on location. Widely available at other retailers
This would be a far better solution as it will eliminate the exposed cable.
Battery Cable Positive Standard Motor Products A544APP | eBay

Any add on power cables should have a metal lug end and be connected to the stud on the end of the cable clamp.

I would also suggest getting a battery heat shield. Not needed for the current seasons. However in the summer months keeping as much heat away from tne battery will extend the battery life.
AND a cover of some type over the positive battery cable clamp. Your current battery is sealed. If you get a vented battery, the battery will vent very small particals of acid that will create more corrosion on the battery cable clamp to battery connection. Reducing or eliminating corrosion is very important to long battery life.

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I have no parts.

How easy is it to do with all of those physical issues, I have no clue. It sounds like you are in bad shape. I would have someone else do this. Someone with just a little electrical experience. Not much.

The other end of the red cable has to be snaked down to the starter. The vehicle may need to be raised a little.

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The connector is OK, however the existing cable is not good because there is exposed wiring.
You need to eliminate that bad cable.

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