RomRaider

So, I've read some places that Subaru starts and stops alternator from charging when there isn't load or something. But I don't think this is how it's supposed to work:
Normal charge is around 14,2 volts, but then once every few secs it drops it as low as 13,2 V for a second, picks up again to 14,8 V and back to normal charge for few secs. Engine idle changes at the same time: 800rpm -> 860rpm -> 750rpm and back to 800.
I wouldn't mind this otherwise, but it does this on idle and when driving, and you can see (and hear on idle) this even happening: lights get dimmer every time voltage drops, and it's really visible especially on dark, when you use high beams and it's really disturbing. Also, cars driving behind can see this happening because tail lights...
The dimming isn't as big as "high beam/low beam", but more like having cheap light bulb vs. High quality light bulb

Car is Subaru Outback 2011 2.0D

I've bought this car 2 months back, so don't know much about it yet, but I changed alternator for it due to alternator pulley/clutch being stuck (making tensioner squeak all times) and couldn't get pulley itself off from alternator. Seems like it being seized broke tensioner too, (still squeaks while the alternator gets back on/voltage gets to 14,8) so that needs to be changed yet, when I find the cash to buy it, propably next month. But I don't think thats the problem, but the squeaking being symptom of the problem.

Any knowledge about this or how to start diagnosing it?

Fix the squeaking belt first, make sure all tensioners and idle pulley are OK and spinning freely. No point in diagnosing much further as long as the belt slips every time there's some significant load (like A/C compressor clutch engaging or turning the steering wheel).

Thanks for the quick answer, I will change the tensioner when I am able, but I assure you, it's not the belt that is slipping, but the tensioner thats making the noise. I've had same tensioner problem on other cars too, which was caused by same issue: worn out tensioner due to alternator clutch not working for too long.

That aside, when I changed alternator, I checked the other pulleys and they seemed fine. I think this problem lays somewhere deeper, as it happens even when all electronic are off or on, idle or high rpm, AC compressor does nothing to affect it, it cuts the charging for a second and then cranks it back up again.
Idk if you understood my statement right, but when the voltage dropped, RPM went higher. And when voltage picked back up to normal, RPM lowers and it makes the sound fraction of a second (well, logic has the rpm lowers due to getting more load to engine, and rpm gains due to load getting off, load probably being alternator turning off and back on, but question is why does it do it so often, especially when it affects lights brightness in the car)

But nevertheless, I will follow your tip and change the tensioner, to get one more problem out of the way, since it needs to be changed anyways (who knows, I actually might be wrong and it's fixed with tensioner alone) and get back here, either for more diagnostic help or to say that was the heart or the problem

Was this issue the reason (or one of the reasons) for replacing the alternator? Or did it start with the new alternator?

Tbh, I'm not sure if it the dimming problem was with old alternator. I've owned the car like 2 months for now, so I didn't really drive it much, if at all, in the dark. Also, since my car has xenons, it doesn't affect them, only the 12 V electronics.

Only reason I changed it was the fact that it's pulley/clutch was broken (pulley rotated both directions/couldn't spin freely either direction) making tensioner vibrating like crazy and making the noise. Also the pulley was so stuck I couldn't get it off even with 3 meter long braker bar, so my option was just change the whole unit. Changing it made it better, since it was squeaking all the time, now only when the voltage is fluctuating. And I also threw away the old one... Well, it's in recycling bin but I'm not home for couple next weeks, so chances are it's gone by then.

Ok, as I imagined, tensioner was not the cause of this problem, changed it today, and problem is still there. Squeaking stopped fully, since tensioner was faulty, but voltage jumping and lights dimming is still there.
Any help how to start troubleshooting this? Could it be bad battery? Any info on how that alternator works, as in when or why it stops/starts charging, is it via voltage treshold?