I purchased a 2001 Mercury Sable with the 3.0 about eight years ago with just 11,000 miles on it. It now has 87,000 and I have an electrical issue. I first discovered it during the pandemic lockdown when the car had been not driven for two weeks. Battery was completely flat. Fully charging it with an intelligent battery charger seemed to fix the issue. I left it on a battery tender while I wasn't driving it.
When I had another incident with charge too low to start the car, I tested for parasitic draws and the alternator's output. Since the draw is just 0.50A with key off and the alternator output at the battery's terminals is 14.5V, I figured it must be the battery. It's a Motorcraft purchased from the Ford dealer and they tested it as good. I also took it to a Batteries Plus store and they said the same thing.
I fully charged it out of the car and reinstalled it. It worked for a couple of days, but I could tell from the starter speed that the battery was losing charge. On its last trip out, it started, but left me stranded 5 miles away when I attempted to restart it after going into a store for less than 5 minutes.
I don't get a warning light on the cluster, the serpentine belt is a few years old (it's in good condition and has good tension), I don't hear any sound from the alternator (like bad bearings). With engine running at 2,000 RPM, the alternator puts out 14.3 to 14.5V even with headlights, fog lights, blower, map lights, & radio on.
Battery after 12 hours at rest: 12.42V
Alternator output at idle or 2,000 RPM measured at battery posts: 14.5V
Current with key off: 0.50A
AC voltage at 2,000 RPM measured at battery posts: 0 (this was something I read that would show possible rectifier problems)
Battery: Motorcraft 100 month battery, 42 months old
Alternator: it looks original
Since the alternator isn't super convenient to swap out, I'm looking for alternatives to throwing parts at it. Does anyone have any ideas on this?