Redd

I see the voltage floating from 14.4 down to 12.8 and up to 13.8 and all over in between, with no apparent reason.

I can say this has nothing to do with the loads running in the car. Day or night, idle or highway, the alternator's internal regulator is apparently looking at some thing(s) and varying what it thinks is appropriate all the time.

And it is either very clever or very dumb, because I know 12v battery power systems and how AGM batteries like to be maintained, and this marvel absolutely is either the most sophisticated, or the most confused, that I've ever seen.

Audi-US will say to ask my dealer, who will say "Huh?" which is of course appropriate since the dealer did not design the electronics....

BlackSVT

I don't know the specifics of the Q5, but I have seen both Hall effect current sensors and more intelligent battery sensing modules used on cars the past ~5 years. The goal with both approaches is to monitor actual current flow (whether charge or discharge), voltage, expected accessory load in the car, temperature, calculated charge state, etc and vary charge rate/voltage, among other things, based on that info. The push for better mpg definitely drives much of this.

As an example, if you're running steady state on the highway with light accy load, you may be running near 100% on battery to lighten the alternator load/improve mpg (i.e. you'll probably see in the 12-13V range). Other times, maybe the compressor has kicked back on or whatever, and the battery state is getting to a set "low" threshold -- the regulator kicks it up a notch or 3 and you'll see 13.5-14-5V.

So yes, it can look random, because in some sense it is relative to what you're currently doing. But...I'll bet cranking both seat warmers, Heat/AC fan to max, and rear defroster on will result in a consistent high voltage all the time

Edit: I just came across this which probably explains it much better than I did -- Link

Redd

All well and good, but batteries, especially AGM batteries, have their own preferences. An AGM battery can almost double it's ife if it is kept properly (fully) charged. It is not unusual to see them get 6-8 years when they are kept at full charge. On the other hand, the classical car alternator is designed only to bring a battery back to 90-95% charge, and more importantly, "not to overcharge the battery" during a day long highway run at full speeds.

The AGM's also have a charge acceptance rate that is 25% faster than conventional batteries, and in fact they can often charge 4x faster without any problem. But I've worked with AGMs and "lithium" charging systems, as well as conventional wet lead and gel, and never heard of a system that is always going up and down the way these cars do.

There are all sorts of charging logics. One system feeds out maximum amperage at about .2v over battery voltage, ramping it up until it reaches 14.4 then cutting out until it drops to 13.8, or a similar lower number. Some float over a wide range, others a very tight range. The trick is, will the charging logic bring the battery up to a FULL 100% charge before the vehicle is shut down, under ALL circumstances? There's no way to tell what it is doing when it ranges all over this way, but I'd have to suspect (from looking at my own battery when I get back into the car after it has been parked) that Audi's system is NOT leaving the battery fully charged all the time. Charged well enough to start the car, and to keep the car "alive" for a week or two...but leave it parked for six weeks and you'll find a dead battery. AGM's with a proper battery management system (to cut off loads) can keep charged for six months without a problem. The death in six weeks means Audi is keeping too many toys alive for too long, and any starting battery (as opposed to deep cycle) can be killed if allowed to go dead that way, as little as four to six times.
Which makes me wonder, wtf are they trying to do, and how are they trying to do it. It certainly looks like a fancy programmed regulator, not any simple logic in conventional use. Which also makes it impossible to figure out if it is working "properly" according to design, or not.
And yes, after 15 months my first battery had to be replaced "as a courtesy" after an extended unexpected period of six weeks when the car had to sit alone. The new battery only showed 12.4 volts, which is only about 75-80% capacity for a normal AGM.
Odd.