I've noticed this before but today it was really noticeable on my way to work. I drive on a route that goes from speeds up to 55 mph to stoplights. What happens is when I arrive at a stoplight (my headlights are on) and my car begins to idle, the idle sounded like it dropped down a bit and my headlights would go dim. Then the idle immediately picks back up and they go back to normal. Then...30 seconds later or so it would drop back down and my headlights would dim for a second before going back up. What causes this and do I have to worry about it? I always assumed that the headlights ran off the battery but then I thought that maybe they rely on the alternator and since the engine idle drops, the alternator in turn drops and maybe that is what is causing them to go dim briefly.

2010 Fit sport

 

I've also heard it could be due to the fact that I am running my defroster / heat in my car.

 

Mine does that as well... 2014 canadian Fit

 

Common on fits since they have a tiny battery and a weak charging system, every fit I've been around dies this when idling with the ac or defrost on.

 

All the above.


The battery is there to start the engine. The alternator supports the electrical load once the engine is running.


The alternator is not efficient at idle RPM, charging rate drops so you can see the lights dim as voltage drops a bit. More obvious with a large electric load like front and rear defrosters on.

The AC is operating whenever you run the defrost, and it uses a LOT of electricity and engine power to run the compressor and fans, so you CAN see lights dim whenever it cycles the compressor on and off. (30 seconds comment)

Then there's Hondas ELD system, that turns the alternator down when it's not needed as much, then waits until there's an electrical load increase before turning the alternator up.


Sounds normal to me.
HTH

 

Excellent...thanks for the info.

 

Normal,,, and yet stupid... LOL

The ELD system arg.. The bump in mine triggers a bad cat error code..

 

Like others have said, this is normal. When the compressor kicks in with the defroster on, it uses more power and so the engine responds by kicking idle up a little bit. You'll see the same when you put your window down or up (just pulling on the window control to go up - even when closed - makes my headlights dim for a second. One solution is to do what many with aftermarket stereo systems do. Add a capacitor to your electrical system. when the compressor kicks in, the capacitor will make up the difference in power. Many who have subwoofers in the trunk have one because of the power needed for Bass. A Side-effect/benefit is that when the stereo is off or not hitting a hard note, the capacitor feeds that extra power back into the system as needed. Starting a car in the dead of winter is easier because of the power "dump" that occurs from the capacitor. I went 7.5 years on my stock integra battery because I had one installed. 20 below zero on a 6-7 year old battery and never once did she not start.

~SB

 

I have that issue as well. Hopefully it's not a problem with the alternator. OEM, even refurbished ones, are not cheap.

 

Mine does the same thing, '12 Sport. Even if I'm driving at above idle I see my headlights dim here and there when the AC compressor kicks in.

 

accord did the same thing too. Annoying but totally normal.

 

Yep, common. First noticed this on a 97 Corolla. Nothing to worry about.

 

Indeed, it's normal. At low engine speeds, the alternator can't provide as much current as everything electrical together requires, so the voltage dips down to the battery voltage (13.x volts down to 12.x volts). The ELD throws another monkey wrench into things, but the general idea is still similar.

If the lights dim a whole lot (and the heater fan slows down a lot, and the wipers go at half speed), then there is a real problem such as a weak battery or a bad battery connection. I had a car once where a battery cable clamp had stripped out and the battery cable negative connection would occasionally slip off the battery post enough to not make contact. The lights would dim a whole lot when the engine idled, and the car of course went completely and totally dead when the engine was shut off...until I banged the battery cable back in place securely. I was a poor college student at the time, so this was obviously preferable to spending actual money for a replacement battery post clamp.