What would make LED headlights have a faint flicker to them while the car is running, BUT not at all (no flicker) when the engine is not running?

Admittedly, this is not on an Olds, but another car of mine - I trust the depth of knowledge here more than I do other places, so asking here!
It came about as I installed the LED headlights and tested them with the car off- next day I started the car up and turned on the headlights to find the headlights flickering.

 

Sounds like voltage regulator is a little wonky. When the car isn't running, you are powering the lights from the battery. When it is running, the alternator is providing the electricity.

 

What year car is this, and does it have an old-style electro-mechanical voltage regulator or is is solid-state? The old E/M style regulators have only 3 levels of charging current provided by the alternator: Zero, Medium & Max. The regulator switches between these 3 modes as a function of the loads on the +12V system to maintain a battery voltage within its normal operating range. This results in voltage fluctuations on the +12V power lines throughout the car and can cause noticeable "flicker" in light bulb brightness. LEDs may be more sensitive than incandescent or halogen bulbs (which is just an incandescent filament in a pressurized halogen gas glass bulb). When the car is not running, no charging current is provided to the electrical system and the +12V line voltage is stable.

 

If you ran them off the headlight switch and not a relay on anything made before 2000 ish. they will flicker.
Most older vehicles, you use the headlight switch power as the relay feed/trigger. and power straight off the battery to relay.
This is a good Idea for any old vehicle that doesn't need to be 100% stock.
The voltage drop through the factory system wiring after 50 + years leave you with dimmer lights as most times you get 12 at the light with car running. with relay, you get 14.1+/-

 

 

So why do they not flicker when using exactly the same wiring and headlight switch but running on (lower) battery voltage instead of alternator-sourced voltage?

 

Good question.

The answer is that the vehicle battery acts as a buffer in the electrical system. It can do this because it has a high power density (it can deliver or absorb energy at a high rate).

The fluctuations in alternator-sourced system power that cause flicker are absent at the battery terminals because the battery has buffered them.

 

True that the battery does act as a large capacitor and smooths voltage fluctuations throughout the +12V power system, but lead-acid batteries are by no means an "ideal capacitor" (infinite capacitance and zero equivalent series resistance, or ESR). Any additional resistance in the +12V wiring between the source of the charging current when the engine is running.........as modulated by the 3-step E/M voltage regulator mentioned in my previous post.......reduces the effectiveness of the battery's capacitance to reduce +12V power system voltage ripple. The SOURCE of the (light-bulb flickering) variations in charging current, which results in voltage fluctuations, is the alternator's output as modulated by the 3-step E/M voltage regulator. My car does it, and you can see it in the volt meter, even with a brand new battery.

A solid-state regulator can modulate the alternator's output current linearly (infinite steps >>> 3 steps of the E/M regulator) as a function of +12V system voltage and result in negligible voltage ripple under constant load conditions.

 

This is why CutlassMarc suggested powering the lights directly from the battery through a relay. That's the best solution for the OP's filcker problem.

 

L.E.d. flicker all the time they are on. That is how they work. The slightly higher voltage when running or a alt diode pack failing can cause the rate of flicker to change. It is why in video's if the sampling speed or frames per second are the same as the le.d. you can see them turn on and off on and off. flickering.

 

I understand that turn signal/running light LEDs must be driven so that they can display different intensities to replicate the effect of a dual filament bulb. But you want headlights to always have full output--why would they be designed to be off part of the time?

Not saying you are wrong. Just provide further explanation.

 

I don’t know if you are referring to LED headlights specifically or LEDs in general, but LEDs do not flicker by design. With a DC source, an LED (as in a single diode) is on constantly as the current flows through the P-N junction and the recombination of electrons and holes emits photons of light.

 

Indeed, the place where the voltage on the +12V power system is most stable is at the battery terminals.

 

Normally the human eye can't see the light emitting diode turning on and off. But that is how they work. If you watch dash cam videos that are shot at 60 fpm, you will be able to see them flicker.
electrical noise also can cause this. The older Automotive Alt. isn't clean electric power, it is dirty with a lot of electrical noise . Different voltage will cause the l.e.d. to flash on and off at different rates. or he has a feed back issue as most l.e.d.'s are polarity sensitve . I'd bet if with car running he powered the l.e.d. headlight from the battery + the flickering would not be able to be seen by the eye.
I always wire them through a relay Even with sealed beams the difference in brightness is night and day.

 

Yes, the LED driver electronics (if used) switches the power to the LED on/off at a very high frequency using a technique known as PWM (pulse-width modulation). The duty cycle of this PWM signal (% time ON) determines the apparent brightness of the LED as viewed by the human eye, which struggles to see high frequency changes in brightness due to persistence of vision. However, if the LED assembly (the "bulb" that may contain multiple LED elements and power conditioning circuitry) contains no electronics and the LED (and its appropriately sized series current limiting resistor) is connected across a constant DC voltage source, the LED remains illuminated continuously (although may still vary in apparent brightness due to voltage ripple on the power source.

ALL LED elements are polarity sensitive, but they may be contained in a "bulb" assembly along with full wave rectification that changes AC (alternating / alternate polarities) to DC.

Barely related trivia: Guess what an LED (element) does when it is cooled to LN2 temperatures (~77 kelvin)? I was tasked to find out during development of an application that required a calibration light source for cryo-cooled optical IR detector. I used a test setup that provided a constant current to the LED and I measured the Vf (forward voltage drop) as I dipped the test article into a dewar full of LN2. It got REALLY bright and the Vf dropped a little, indicating that its efficiency (power in vs. brightness out) increased dramatically (similar to how IR detectors behave when cooled).

Is my LED nerdiness showing?

 

Wow you guys wrote up a storm while I was not looking! Interesting thoughts and ideas!!

The car in question is a 2007 Ford Mustang 4.0L V6 - I am new to the Ford world, but have heard they have alternator issues.

I'm still reading over the above, just skimmed through so far!

 

Yes, but I understood your explanation much better than

 

Sometimes the solution is easy. Is a vibration while the engine is running causing the bulb/housing to vibrate, and you're interpreting that as a flicker?

 

You all are absolutely missing my point. The OP said that the lights do NOT flicker when he turns them on with the engine not running, so that's 12.5V through the existing headlight switch and wiring. When the engine is running, they flicker on 13.8 volts THROUGH THE SAME EXISTING WIRING. How will adding the complexity of relays (for LED lights that likely draw less current than the incandescents) fix this. The cause that makes sense is that the voltage regulator or diodes in the alternator are adding an AC ripple to the voltage to the lights. Relays won't change this.

 

Correct, if you assume that relays send current to the light from the same source as the OEM wiring.

The "yeah-but" in this, is the ability of the battery to smooth voltage ripple. The battery quickly absorbs spikes and supplements sags, similar to what a large capacitor would do. If the relay source is a direct connection to battery terminals, the voltage ripple is minimal.

Voltage anywhere in the wiring system away from battery terminals contains ripple.

Ripple is the greatest at the alternator terminal. The farther from the alternator and closer to the battery you check, the less ripple is apparent. At the battery, the ripple is at a minimum. The ripple is proportional to the remaining resistance in the wire's connection to the battery divided by the total resistance between the alternator and battery.

 

Okay, so given all the thoughts here I actually got the car in a spot where I could do some testing.
I *think* it is a bad battery- or a battery on its way out.

I never gave that a thought given it starts just fine, and the whole thing with headlight flicker while running, but not while just being on battery- threw me off.
Also my tell-tale sign of bad battery is if it doesn't make it through storing the car over winter (I detach the negative cable)- and this one started the car right up after winter.

I have a battery tester I tested with- which stated "charge and test again". Before I started the test, I noticed the battery was in the lower 12's for volts. When the car is running, the voltage at the battery is at the upper end of where things should be (14.6/14.7V). I guess makes some sense if the battery isn't holding a charge / is low and its trying to bring the battery up some.

From the above too, I guess it could make sense if the battery is not 'smoothing' the power out when alternator is running and if battery is not in good health? I noticed a 2019 sticker on the battery (I bought the car last year) so I'm not even going to bother charging up that, will buy a new one and see.

 

The battery will always be 12.5-ish volts. The alternator puts out at least 13.8. Do you have a battery load tester? Voltage is not what you need to be checking.

 

I don't think he has a battery "load" issue, it is electrical noise/ pulsing. Same reason when everyone in the 80's installed an aftermarket high powered radio you got the pulsing hum till you added a noise compressor to the + feed and many times to the ground also if the radio didn't come with one. The old style Alts put out almost no power at idle and pulsed ,as rpm went up so did the speed of the pulsing hum. with the lights it can't be seen by that point. Heck many of the alt output at full song are 68 amps or less.

 

So what I said back in post #2 and again in post #19.