Passaable wrote:Hi - I found that using the kill switch when the engine is at high RPM (e.g. tech inspection) seems to blow both (expensive) halogen headlights. The other lights (daytime running, etc) are fine.
I had the same issue with a backup camera and video monitor that would get damaged when the kill switch was used during tech.
As others have said, the easiest thing to do is pull the fuse before tech. I have a toggle switch that I can use to disconnect all of our gauges from the battery bus so we don't damage anything during tech.
You are experiencing what is normally termed "load dump". A voltage surge is created when the battery is disconnected from the system and the current coming out of the alternator changes abruptly. Even with the resistor in place, there will be some surge, you really can't reduce it to zero. The resistor is supposed to be taking the place of the battery, but there can be a small period of time after the battery is disconnected and before the switch connecting to the resistor is closed. Even a few milliseconds is enough to create a pretty good size surge. The amplitude of the surge will vary depending on how much the current out of the alternator is changing, so a low battery (one that is being aggressively charged by the alternator) will produce a higher voltage surge than a full one (where not much current is going to the battery).
https://en.wikipedia.org/wiki/Load_dump
The load dump voltage spike can be as high as 120V, so you are drastically exceeding the voltage limit of the device, creating excess heat and current draw. Lowering the fuse rating is a crapshoot because you don't really know what amount of current will damage the lights. You would have to experiment with a few values and as you say, the bulbs are expensive. It could be that you are damaging the lights from the heat alone, so there is no value of fuse that will protect against that.
Modern cars are well equipped to handle load dump (and other electrical threats) because of all the sensitive electronics that are included in today's cars. Most use a transient suppression device to guard against damage, usually something like this: https://m.littelfuse.com/~/media/electr … de.pdf.pdf
Best bet is to just turn the lights off or pull the fuse for tech. If you are running at night and you need to pull the kill switch, you likely will have bigger problems to deal with than blown headlamps.
We Audi Be Faster
'85 Audi Coupe G(in &) T(onic)