CAR LED headlight - load resistor (aka "CANBUS")

[Dulus]

Hello,
short story:
I have a car light harness where i have added an 8Ohm wirewound 200W resistor parallel to the LEDs. When testing and i input 12V (from table power supply) on input of the wire harness i measure the voltage on the output of the harness (where the LEDs would be connected) is ~10V(voltage drop across the resistor ?, and as the LED is in parallel, it receives the same voltage, correct ?). When installed in the car, when i turn on the lights without starting the motor (volatage ~12V), the LEDs are ok, no problem, but when i start the car (~14V) the LEDs are flickering, sometimes only one, sometimes both , sometimes very short, sometimes for few minutes, but after a while they stop flickering. What is causing this and what should i change to end the flickering, i suspect the resistor values have to be changed, but i would like some opinions for what value.


Long story:
i am currently in the process of changing the lights in my car from halogen to LED, because the better halogens i was buying (Philips RacingVision 150% H7) were only lasting very short time. I now bought Philips X-treme Ultinon 200% H7. When i fitted them in my car they were performing very well, no flashes from oncoming traffic, good cutout etc, but were giving an error in the car, that the bulbs are burned down (obviously because the LEDs are drawing much less power than halogens). My car has sensitive "bulb failure detection".
So obviously next step is to add some resistors to "fool" the car the halogens are still in place.
I have seen some already made harnesses selling on some shops, but the cabling was thin, without any protection, and the resistors were very small, and surely would get very hot.
So i created a wire harness myself.
I bought H7 sockets, cables, cable sleeves etc., big 200W 8R wirewound resistors, glue heatshrinks etc.
I created a harness, where the resistor is parallel to the LED bulb. As showed on the diagram.

When testing and i input 12V (from table power supply) on input of the wire harness i measure the voltage on the output of the harness (where the LEDs would be connected) is ~10V (voltage drop across the resistor ?, and as the LED is in parallel, it receives the same voltage, correct ?). When installed in the car, when i turn on the lights without starting the motor (voltage ~12V), the LEDs are ok, no problem, shining bright, no flickering but when i start the car (~14V) the LEDs are flickering, sometimes only one, sometimes both , sometimes very short, sometimes for few minutes, but after a while they stop flickering.
I was thinking it might be a voltage drop on resistor, so the LEDs are receiving toow low voltage to function properly, but why they are not flashing when light are on without motor/alternator running, but when motor is running they flicker.
They were functioning ok when connected directly without the resistor harness.
What is causing this and what should i change to end the flickering, i suspect the resistor values have to be changed, but i would like some opinions for what value.

 

[duke37]

I know very little about powerful leds.
When running from a battery the supply will be steady. When the engine is started, the alternator will switch on and off as the battery is charged giving a variable supply. When the battery is fully charged, then the charge rate will be very low and the supply will be stable.

 

[Bluejets]

Calculations do not make any sense.
8R to begin with is 1.5A whereas an original H7 lamp at 55W *2 = 110W = (roughly on 12V) 9.2A so if the ECU or whatever is monitoring the system, this resistor certainly would not fit the bill.
Again 8R on 12V is 18W so do not seen why the 200W resistor.
I think you are going to have to dig deeper into why the LED's are flashing as it would certainly not be voltage drop as your current load is well below standard.
Try doing some meaningful tests rather than guessing.

At the cost of these light replacements ( some showed AU$275.00/ pair) I'd be reconsidering possible move to a pair of aux spots, possibly with the same lamp if you must but with wiring that is isolated from the current monitoring system.

One can easily trigger an aux relay from the existing system and run a separate fused supply directly from the battery, extra dash switch if required in the relay coil circuit.
Just be aware before digging into the existing circuit that earth return systems can be tricky, especially those with a constant feed line to the headlamps.

 

[majoco]

"i have added an 8Ohm wirewound 200W resistor parallel to the LEDs."

Parallel???? Why parallel??? all you're doing is draining the battery. LED's like a constant current source, not a constant voltage. A car battery is NOT constant voltage - running down the road with a fully charged battery with an alternator charging, the battery can easily get up to 16volts.

Back to the drawing board.

 

[Dulus]

Parallel???? Why parallel??? all you're doing is draining the battery. LED's like a constant current source, not a constant voltage. A car battery is NOT constant voltage - running down the road with a fully charged battery with an alternator charging, the battery can easily get up to 16volts.

Back to the drawing board.

Yes, that would make sense if i was connecting the resistor to the bare LED chips (than the resistors in series acts as a current limiters).
However these are complete LED modules with their own balasts/drivers etc (where the actual powering of the LED chips is happening), and the resistors are there actually only to what you are describing, only to drain some current, so the car system does not detect it as a faulty bulb. Its expecting some minimal current to be flowing, and the LED modules itelf are draining less than that. So i added the resistors to drain more current.
8R seems to draw enough current so the car system does not trip the failed bulb warning, although its less than what would drain the 55W 12V bulb. I guess a combination of 8R resistor draining curent+ the led models draining some additional current its enough to be inside car system limits for failure of the bulb detection.


In the end i actually found out that it looks like the ballasts themselves were the problem. because i switched them with the ballasts which are on high beam (i use same led lights on high beam and low beam, but only low beam is checked for working bulbs, so only low beam requires additional resistors), and the flickering stopped. i contacted the manufacturer, and he told me he will send additional balasts, as some of them could be affected by some quality problems. Its just that i am so unfortunate that for me both of them started flickering during short time one after another (basically during one ride). So i was expecting some "systematic" problem rather a individual component problem.
I will see if the flickering will come back after installing new ballasts.