i was lucky to meet another great guy (rbracy10406), who has experience with this, and sent this valuable data:
"The part number for the regulator will depend on whether you have the Bosch or Valeo alternator. The brushes in the pictures I had were worn to the point to where they were starting to fail. (i think he was referring to his alternator).
In my experience, they fail about every 50Kish on these cars.
Ive heard of some people replacing these just as a PM but I never went that far."

This picture is of a worn brush and a new 1 on a similar valeo alternator
https://www.myturbodiesel.com/d2/ima...on-brushes.jpg

 

1 final thought: there are several ways to replace the brushes, depending on your facilities.
1-remove the alternator and inspect the brushes. if they are worn out, order new 1s and wait until arrive-then you have to unsolder the old 1s and solder the new and install them and the alternator.
2-remove the alternator and inspect the brushes. if they are worn out, order a new voltage regulator (that comes with new brushes) and when arrive, install it and the alternator.
3-take your car to a service that you trust-if something like that exists-and they will replace the whole alternator at much higher cost.
4-and of course you can bring your car to a chrysler shop and get robbed.
(i dont have a garage, and the xf is my only car. also, the good thing is that the alternator, although behaving erratic, still charges regularly so the battery is fully charged, so i could wait. )

edit: please go to the last post for final conclusions.

 

eventually managed to remove the alt. it was a valeo. installed the new 1 and seems to be running fine. no light on . volts 14 with engine running, as before.
update: i was able to disassemble the old 1. found an instructional that explains how to overhaul it in great detail. even if the brushes were short, still had some life, so it was not the cause.
on the other hand, the area of the shaft where the brushes are in contact is blackened, so that makes a false contact (the instructional tells that has to be cleaned). i think this is the cause of erratic behavior as was my case. there are brushes and regulators available that can help fix the alternator without the need to replace it, so it depends of the situation of each 1.
here is the instructional:
( when you do this, at 8:03 you see that he slides the brush about 1/3 of its length into the receptacle, so about 2/3 are left to wear.)
so, considering the advice that i got from an expert, i think checking (or replacing) the alt at 50,000 miles (80,000kilometers) should be the way to prevent failures.
mine failed at 66,000 miles (110,000 kilometers), but i bought it about 2 years ago, with 60,000 miles (100,000) kilometers, so couldnt tell if it is original.
up to you folks.

 

from what i have learned, am reaching the following conclusions:
1-if an alternator fails without previous warning, most likely was because the brushes worn out completely;
2-if begins to give erratic "battery off" messages, is that the shaft where the brushes rub has blackened out and there are false contacts.
in any case, means that the alternator has to be removed, replaced, or disassembled and brushes checked for life left, and shaft for blackening.
if brushes still have life, can be used (ideally you have the spares so replace them now).
also, clean the shaft.
if anybody has better solutions and ideas about this, please post.
this is just my approach based on what happened to me.
a link for brushes: brushes are about 14mm; need to have at least 4mm running into the cavity, the rest (about 10mm) is the life available. the measure you find tells the % it still has.
edit: but wait... please read the next post. by now i have learned some things that are worth considering....

 

at the end of the day...having replaced the alt and all running fine, i had all the time in the world to review the situation (and it only cost me cdn$143).
i also ordered replacing brushes. and a voltage regulator that looked identical( $17 including shipping!), just to get the small cover for the brushes that broke when removing it.
armed with that, i desoldered the brushes and replaced them with the new, soldered in place and reinstalled, and fitted the small cover. now the old alt is as good as new.
(seems that the wear of the brushes is what kills the alternator. otherwise are built to last forever).
but i learned that this is a job too delicate. that, if in need to replace the brushes is better to replace the voltage regulator (that can be found for about $30).
in my case, the brushes still had about half-life so there was no need to replace them. so it was not the cause of the failure. it was that the surfaces where the brushes touch and make contact were blackened so the contact was erratic. now that i know, the only thing that was needed was to rub them with sandpaper until clean, assemble the voltage regulator and fit the alt in place and go!
and in my case was the small cover that broke that needed the replacement voltage regulator just for it.
so my advice is that,
1-if you have the time and place to remove the alt and check the brushes, do it. this may need only to use sandpaper to clean the surfaces where the brushes touch.
2-or if the brushes are worn out, replace just the voltage regulator.
3-or if you are 1 of the lucky to have an honest mechanic, see with him to do this-if he has not already suggested it.