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bazz54

Voltage supplied to Headlights

Question

As many will know, the light output of a headlight goes up sharply as the feed voltage increases in the range 12-14 volts. Considering that even in a new car that there is always some voltage lost between the battery and the bulb, I was wondering what it might be in an older car, as it probably gets worse with age of the car. I did an experiment over the weekend measuring the volts drop in my old Rover 600 (because a gorilla can access the headlights on that car, unlike the Gen7!). I was quite surprised to find there was (near as dammit) 1V being lost.

 

Does anyone have any feel for how typical this may be?

 

I've heard stories in the past about people wiring in new supply circuits to their headlights, using the shortest possible runs of comfortably rated cable, operating from a separate relay. Could be worth doing, as opposed to buying the brightest, and most short-lived bulbs?

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1V wouldn’t be an acceptable amount of voltdrop, that’ll give you a lack of performance in the bulb. 

 

You can can do a quick simple volt drop test in less than 30seconds on a dipped beam bulb. 4 measurements all taken with load on the circuit. 

 

How did you measure the circuit? 

 

A quick volt drop you need to put the dipped beam on and measure from the back of the consumer carefully. 

 

V1: Measure battery + to -

V2: measure battery + to dipped beam + 

V3: measure dipped beam + to - 

V4: measure dipped beam - to - 

 

add V2, V3 and V4 together and you’ll end up with a number close to V1 if you have a good circuit. 

 

Diffcult to explain without doing it in front of you. Handy thing to know how to do. 

Edited by Cleario

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As above, you need to measure drop in the Earth return as well as the positive feed. Ideally measure the voltage across the bulb at the bulb, and the voltage across the battery - or better still on the alternator as this is the source of all electrical power when the engine is running. Measure from the large output terminal on the back to alternator body.

 

The voltage drops come from cable resistance (which doesn't usually deteriorate with age), resistance from connections (which is usually the worst deterioration with corrosion - particularly the Earths as these go through the steel body) and resistance from the fuse(s) and switch or relay contacts.

 

If you do run new cabling, use well over-rated cable and components (apart from fuse) as these will have a lower resistance. Minimise the number of connectors used and use good quality gold plated ones. Make sure the fuse is at the battery end of the cabling, and ensure the wire doesn't run over any sharp edges, hot items, or near moving parts.

 

For a given resistance, power is proportional to voltage squared, This is why a small drop makes a big difference.

 

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We don't have any arguments here; the real point of the thread is to ask the question; how typical is a 1 volt drop on a 20 year old car? Many Gen7's are heading towards 20, and of course, the earlier Celicas will be well past it.

 

Before I measured it, I was maybe expecting 0.1-0.2V, or 0.5V very tops, but never dreamed it would be 1V. This is in a car which starts first time, every time. The electrics in modern cars have become so complex. The first car I owned had just 4 fuses and no relays in it, but modern cars have more like 40 fuses and a bunch of relays. All of the numerous connections are made by crimping cables rather than soldering and as the metals tarnish, it seems inevitable that they will deteriorate.

 

I've now acquired all the bits and pieces I need to wire in an entirely new circuit for the headlights going straight from the main fuse box direct to the headlights, cable rated at 16A and all connectors to be soldered. It will be an easy job to do on the Rover, where access is really good. I got enough materials to do the same on the Gen7 if it needs it, but access to the headlights will make that a much more difficult job if I did want/need to do it. Excluding the cost of the relay, which I already had, the spend was less than £20 (for the 2 cars)

 

Re the idea of a 15V convertor, to supply the current required, I guess that would be quite pricey? I'll settle for full battery voltage.

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Yes vehicle electrics have become more complex but not a problem on this circuit. 

 

Before you go throwing parts at the rover have a quick measure of the drop on the power and ground cables. As Chris said you may have a connector between relay and the bulb, a relay contact with some kind of build up or an earth that’s corroded.

 

My 7 is 14 years old, I’ll have a quick poke around  see what’s occurring in the week. 

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Be careful with solder joints that they are not subjected to any vibration as soldering makes wire brittle. You can get adhesive lined shrink sleeving specifically designed for solder splices which goes pretty rigid once shrunk to keep the stress away from the joint. This will, of course, increase the stress in the connector itself.

 

As a general rule, connections from a wire to a fixed item should br crimped. Wire splices or items which are in the loom are better soldered. This assumes good quality crimps.

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