2007 X5 4.8l slow crank especially hot

[Ferguson]

I'm at about 97000 miles with very low usage currently, in a well maintained X5, but having a new problem.

I am just barely over 3 years on a new battery (39 months), and starting to have issues starting. I am in S. Florida so used to batteries not lasting long, but this is garage kept and a bit young to be dying.

The symptom is slow cranking, which gets worse if the car is hot. Last time out I made 3 stops over about 6 miles and the last time (after sitting maybe 20 minutes) I really did not think it was going to start.

Took it to the shop, they said the battery tested good (800+ cranking amps), they spent a long time and two computers without finding anything in specific wrong, other than my trips are too short and infrequent to give it a good charge. I know that, and always put a trickle charge (2A) on so the battery is topped off when I leave.

They suggested the starter may be failing; I am only familiar with starts that seize or just fail open, not one that gets weak, is that an expected failure mode?

Last time I parked it after a long drive I monitored voltage under the hood over 4 days with no charger, it went from 13.06 to 12.95v, so there is no unexpected drain. When the car is in a warmup mode (e.g. open the door) the voltage at the front drops to about 12.6, presumably from all the computers starting up though a larger drop than I would expect. But I have no idea what is "normal" in that case (anyone ever checked? E.g. car off, hit the start button again so radios, etc. go off, but other systems still active).

My inclination is to put in a new battery even through 3 years is pretty young even in Florida for garage kept.

But 100k miles also says maybe I do have a starter issue. But do they fail slowly by starting slowly?

Advice welcomed.

By the way, the repair shop said make an appointment to leave it, and they would do further testing (this was a courtesy check while I waited - nice folks, have used them for a while). But I wanted to do some homework first.

Thanks for any advice/experience.

Linwood