I have a 2008 Honda Fit Sport with 170k miles on it. Video of question attached.

Last year I had this issue where when defrost setting was on, the car would lose power during idle or even when driving. The lights would dim every 15 seconds, to the point of going off. The speedometer would even fall when I was driving, but it never stalled. Both the battery voltage and the alternator showed good, but the cooling fans kicking in brought the battery to 11 V, which I know is too low. I brought it to a shop and they replaced the alternator and battery for nearly $1k.

This year, around the same time, my lights are dimming again. When I check the voltage it only drops to about 13 V (lowest I saw was 12.9, but ranges 12.9 to 13.3). I can also hear the defroster skip a little when this happens. The RPMS dip, go up, and then back to normal. I've attached both videos. You can hear the fans go lower too.

I've been told by some people all this could be normal. My question is, to what degree is it normal? I got both the battery and alternator quick tested at autozone and both passed. However I'm still kind of paranoid - along with the fact that the alternator warranty expires in 8 days. I'm not sure if I'm being paranoid or not! Getting it checked at the place its warrantied under would take a couple of days, which is hard for me at the moment - so its why I'm asking here first.

Does anyone else have this issue and is it indeed normal? Does increasing the battery size to 51R help? Thank you!

 

Does the AC/defrost work well? Have you had the AC compressor checked?

While I've never turned on and observed my AC at idle, I can feel the load change when the compressor kicks on/off while driving, and that behavior has been consistent across the many small/low-powered cars (Fit, Toyota Yaris, multiple Ford Escorts, Dodge Shadow, Hyundai Accent, etc) I've driven over the years. So, a small rpm drop (as you're showing) at idle when the compressor kicks on/off wouldn't surprise me. I have never observed my headlights dimming like you show.

It seems like your AC compressor is turning on/off frequently, at least in the video where you're showing the rpm drop. I'm not sure if that is normal. If your charging system is testing okay, I would definitely get the AC system checked.

I don't think a larger battery would make any difference (assuming a functional alternator) while the car is running.

 

My heat and AC seem to work fine. I have no complaints there. It gets hot. It gets passable cold for a 16 year old vehicle. Been surviving 100+ degree summers. The headlight dimming coincides with the cooling fans turning on. I'm not really looking to get into compressor replacement and such - but one question in my mind is if its to do with the AC clutch bearing, seeing its all connected.

The headlights dimming has always been a thing from my ownership with it, but sometimes it seems worse than others - ie, last year practically turning off and back on, like i was flashing them.

I don't know if its normal, but I've witnessed a CRV cycling the air on and off exactly how mine does. You can hear the cooling fans kick in. I wasnt sure if its possibly a Honda thing?

 

So based on your comment and further inspection, I actually was able to see the light flickering is exactly when the AC clutch engages. It could be a bearing?

The fact the AC compressor is cycling could be a low coolant issue too.