What does it mean when the battery gauge is near the high voltage red line? When we drive our car the bat gauge is in the normal 11-15 volts range but after a little while of driving its slowly creeps up toward the 18 volts end which is in the red area.
Inside the charging circuit of every battery charging device are devices called rectifiers. These devices are used to help turn DC into AC — cars use alternators for their electricity, not generators — with the assistance of bridge diode rectifiers and capacitor and the like.
Normally, the charging circuits work so well on a vehicle and for so long that no one gives them a second thought. However, in your case, it’s quite evident that one or more of the protection rectifier circuits has gone.
As you note, it happens slowly as you are driving along which makes me think that the circuit has gone into a soft error mode where it takes time for heat or something else to cause the fault to occur.
The good news is that it’s fixable. You need a new alternator or you can pick up a used one at a local scrapyard to save some money. You may enjoy doing it this way, however, I am of a different mindset. I like the first option — dropping in a new alternator — the cost is rather minimal for all the things it does — $840 — and since it will likely provide you with many miles of good service, the price of the part becomes more minimal over time.