Battery life, 12 volt

Three to five years is a good life for the OEM 12v. If you've had yours for six you should consider replacing it before it starts giving weird results.

 

It's a deep discharge battery on "float", so 13.8 is about perfect, though it would be nice if they had added temperature compensation. Pearl runs at 13.8V, but if the battery is discharged a bit she will run at 13.9 for a while. Your battery may be not long for this world if you see 13.9 all the time. This may indicate it's running on the edge of full discharge (capacity low due to age).

There are several "types" of liquid electrolyte lead-acid batteries. They modify the electrolyte chemistry a bit for each one. This also slightly changes the float voltage.
Off the top of my head,
1. "Starter" batteries. What you find in most gas guzzlers (normal cars).
2. Deep discharge batteries. What you find in solar systems, alarm systems, emergency lighting, and electronics systems. If the systems move a lot they will use gel cell types or even absorbed glass mat.
3. Calcium grid batteries. Can be found in heavy duty systems mentioned in #2.
4. Absorbed glass mat batteries (AGM). Used where the battery may be tipped over. More expensive.
5. Gel cell batteries. Used where the battery may be tipped over often. Not good if fully discharged, as you can't dislodge the sulfate with charging, so they tend to die if left discharged for very long.

The float voltage -can- be run from about 13.6 to 14.2 depending on the type and how long you want to leave between refilling the water. Prius' battery isn't designed to be refilled with water, so they run it a bit low so it will last longer.

 

This question was covered just recently on another thread, did you search?