Perhaps, but I've never owned such a car.

Tom

E. B. White said, "Explaining a joke is like dissecting a frog. You understand it better but the frog dies in the process." So, I won't do that.

Yes agree on the wording. But as it was explained to me by the mechanics, for a certain power (P=UxI, U=tension, I=intensity) of the bulb you need to provide a nominal tension to have it work properly. In case one is "broken", apparently the other bulb receives a higher tension from the DC generator/battery that shortens its lifetime while making it light brighter.

That makes sense. Newish car, perhaps? I drive a '97 Audi and I always thought it was better not to change both at the same time - ie better if the bulb lifecycles are not synchronised. If one goes, then the other is less likely to go very soon afterwards. No fun driving in the dark without lights.

I was once driving at night, doing about 70mph on a bendy rural mainroad in Wales, when the headlight relay suddenly failed and left me driving completely blind at 70mph - no fun at all. Didn't have time to slow down. Luckily I realised what had happened and was able to whack the relay/fuse box with my free hand, which made the lights came back on. When I opened the relay up later, there was a dry solder joint which must have been there since the car was manufactured (Volvo 760). Literally an accident waiting to happen.

That's voodoo science. If that were the case, the dimensions of generator, regulator and battery would be totally wrong. The difference that really occurs is minimal.

Physical laws (I assume it's mainly Kirchhoff's, isn't it?) clearly explain why one burned-out bulb doesn't strain the one that's still working, but it does make some sense to change both all the same: In a new car, both bulbs will kick the bucket roughly after the same time, so when one goes out, it is fairly likely for the other one to die soon too.

Replacing both of them won't cost you much, since the remaining life time of the healthy one will be short anyway, but changing both saves you the hassle of another change just a few months later.

Of course, in an older car that hasn't been under this rule for many years, the bite-the-dust times (BTDT) of the bulbs may have diverged very much, but even then it costs you only a few franks to synchronize the system and avoid problems with another bulb burning out pretty soon and of course in dense fog and pouring rain.

__________________

"This is AMAZING! I have the exact amount of money Joe Biden has cost us playing golf in my sweatpants pocket!" — Kona Lowell

I think we got the same mechanic.

Sorta not really a little bit right.

You can hook a headlight to the biggest battery in the world and if that battery puts out 12volts it will shine the same as it would on 8 little flashlight batteries (1.5v x 8).

Where he's kinda right is that if there is an extreme load on the battery that will affect other things working off of it. Like starting you car- most now disconnect the headlights when in start mode but you've probably noticed the lights dimming when starting sometime in the old days. When you finish running the starter the lights are bright again. But one headlamp more or less is not going to affect the brightness (or life) of the other.

Then they are idiots!

The voltage (aka tension) of the battery/generator system is the same regardless of how many lights are on.

Tom

*Paging Dodgyken* (or somebody with automotive skills and knowledge)

I was a mechanic in a former Stateside life.





Edit (MC will merge me again so I further the banter here)

I was a bicycle mechanic as well.

Fixed that for you...

And I as well when I was at university (it paid a lot better than working at McDonalds).

(and judging by the two BMW boxer engines in pieces in my office at work, I still am)

Tom

I am just gona wear this when I drive

Damn I am so upset the audi guy bullshited me with his explanation and I did not react. There will be some ass-kicking around at the next service and battery torture to check if he's brighter.

Now wait a minute, the car that had the bulb issues was a 97 Volvo....I believe your Audi is about a 97 as well. The bulbs stopped going off when both light bulbs were replaced...I believe there has to be a logical explanation to that.

As I alluded, there is a bit more than just no change at all on the remaining bulb, as claimed by other posters. After all, the battery has an internal resistance which must be factored in when calculating the circuitry. The voltage change on the surviving bulb causes a slight increase in brightness, but nothing breathtaking. It's just noticeable in a so called A - B test, i.e. directly switching one bulb on and off while watching the other one. There are other factors that affect the voltage just as much. Actually, while the battery is being charged, the voltage is significantly higher, namely up to 14.4 Volt.

__________________

"This is AMAZING! I have the exact amount of money Joe Biden has cost us playing golf in my sweatpants pocket!" — Kona Lowell

let me know when you're going to be driving thru Mies would ya ? ( I wanna be off the road ... and sidewalks )

Ahhhh! Don't encourage them CG!

You're right of course, but like you said there are other influences that have a greater effect than a bulb being out.

Maybe I can distract them with a Temperature vs. Germs = Common Cold thread .

It has all to to with the internal characteristic and resistance of your alternator and wiring. Known as Ri in the electrical community. Every real life voltage source such as battery, generator, or nuclear power plant will behave the same.

The more load (a.k.a light bulbs, heaters, etc) you connect to a real voltage source the more the output voltage will drop. The "bigger" your source is the less the effect. So a big battery will behave differently then a small battery. Bigger sources have a lower Ri.

You can see this effect when you switch on your light and the switch on for ex. the heater. The light will get dimmer. You can also see the difference of a 'bigger' voltage source, to it with the engine off (battery = small source) and while running (alternator=big source).

The lifetime of a lamp is proportional to V^-16 (Voltage to the power of -16). You can see the effect of a small Voltage change on the life time here: http://www.ushio.com/support/LampLifecalculator.htm (use 120V instead of 12V as it can not handle fractions.)

So for the sake of your lamps, always let the heating system run on full power.

In fact, to be on the safe side, it's best to run all electrical accessories at max all the time. Always have windscreen-wipers on max speed, the stereo and the GPS turned up to full volume...