Headlight voltage

[Guildbass]

Over the years, one remedy for mediocre headlight performance has been to use relays to maximise the voltage available to the headlamps. Instead of using a power line which is several metres long and which consists of relatively narrow cabling going through several connections and switches, the relays connect the headlights more directly to the battery.
Incandescent bulbs lose significantly more output than one would expect from a small voltage drop as there is (I think) an inverse square law thing happening.... Something like a 25% loss in light output for a 1.2v drop from the feed.
Has anyone experimented with using a DC to DC converter for the lights?
You can buy a 9v- 12v input to 13.8 v output 10a convertor for less than £20. They are simple 4 wire 'in line' units.
I haven't measured any voltages but it seems to me that having a solid 13.8v going into the headlight switch could be a way to ensure the headlights are sure to get the full voltage they are designed to have.
Thoughts?

[alanworland]

Sounds a great idea - all those 0.2 ohms quickly add up.

Alan

[Sleeper]

LED headlights

Main beam

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Dip beam

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prior to final adjustment , less amps,more light..


John ;-)

[geoberni]

Sounds like a lot of faffing around for negligable increase in output.

I've checked my voltages this afternoon, as I presently (I didn't want to say 'currently' ) have the headlights removed to change the bowls.

At the battery I measured 12.7v, at the headlamp connector with the light switch on, I read 12.3v, so 0.4v drop on the wiring and the Switch. Not bad at all.
But that's on the battery only, not the Dynamo running, which kicks out a lot more...

You can buy a 9v- 12v input to 13.8 v output 10a convertor for less than £20. They are simple 4 wire 'in line' units.

With engine running and dynamo charging, the battery terminal voltage was 13.8v and at the headlamp connector was 13.1v, so 0.7v drop, which is what I would expect with the increase in voltage applied.

So what exactly is the point of getting a DC - DC converter ?
If it's up circuit of the headlamp switch, it's essentially the same as the Dynamo output?
If you're still running with 12v incandescent lamps, then doing anything that will increase the 13.1v which I measured up to 13.8v at the lamp, is just going to burn the lamps out sooner...for what, an increase in brilliance that you probably won't be able to perceive

[Guildbass]

Sounds like a lot of faffing around for negligable increase in output.

I've checked my voltages this afternoon, as I presently (I didn't want to say 'currently' ) have the headlights removed to change the bowls.

At the battery I measured 12.7v, at the headlamp connector with the light switch on, I read 12.3v, so 0.4v drop on the wiring and the Switch. Not bad at all.
But that's on the battery only, not the Dynamo running, which kicks out a lot more...

You can buy a 9v- 12v input to 13.8 v output 10a convertor for less than £20. They are simple 4 wire 'in line' units.

With engine running and dynamo charging, the battery terminal voltage was 13.8v and at the headlamp connector was 13.1v, so 0.7v drop, which is what I would expect with the increase in voltage applied.

So what exactly is the point of getting a DC - DC converter ?
If it's up circuit of the headlamp switch, it's essentially the same as the Dynamo output?
If you're still running with 12v incandescent lamps, then doing anything that will increase the 13.1v which I measured up to 13.8v at the lamp, is just going to burn the lamps out sooner...for what, an increase in brilliance that you probably won't be able to perceive

Were those voltages measured at the headlamps with the lamps in circuit or was it measuring available voltage 'off load' ?
Looking at data sheets it seems 13.2v is the optimum voltage across the bulb so if your figures are 'on load' then those values are pretty much perfect!
There's a 20% drop in luminosity between 12v and 13.2v according to the same data sheets.
The pachyderm in the room, as you point out, is that the Vdrop is between the upstream of the lighting switch and the lamp so resistive losses would not change... looks like the H4 plus LED lamps route remains the only realistic option.
Many thanks for taking the time to get those figures! Much appreciated.

[geoberni]

Were those voltages measured at the headlamps with the lamps in circuit or was it measuring available voltage 'off load' ?
Looking at data sheets it seems 13.2v is the optimum voltage across the bulb so if your figures are 'on load' then those values are pretty much perfect!

Sorry, I'm not following you with the 'on load' and the 'off load'....

Yes, the voltages at the headlamp connector were without a lamp fitted, because if the lamp is fitted then you're just going to measure the volts drop across the lamp, mere millivolts .
I've got LED headlamps anyway, so wouldn't be relevant to your quest...

[Guildbass]

'On load' with a 60w bulb pulling 5 amps or so will drop the voltage available to the lamp depending on the resistance of and within the power line. Even an LED headlamp bulb will typically pull 2 or 3 amps.
The whole point of the mods that use relays was to have more substantial cabling (lower resistance) from battery to lamp, increasing the available voltage at the bulb back to the optimum design voltage thus increasing light output to what was claimed.
Like I said, it doesn't really matter as all the resistance responsible for voltage drop is in the line responsible for the voltage drop under load is within and after the switching and realistically you could only fit a 13.8v source between the battery and the switch, where there is still going to be decent voltage levels anyway

[geoberni]

'On load' with a 60w bulb pulling 5 amps or so will drop the voltage available to the lamp depending on the resistance of and within the power line.

With the engine running and dynamo outputting, it will compensate for the current draw, within the design parameters of the dynamo...
The battery along might show a small terminal voltage drop, if an accurate enough meter, because that's a characteristic of a battery.

[Guildbass]

'On load' with a 60w bulb pulling 5 amps or so will drop the voltage available to the lamp depending on the resistance of and within the power line.

With the engine running and dynamo outputting, it will compensate for the current draw, within the design parameters of the dynamo...
The battery along might show a small terminal voltage drop, if an accurate enough meter, because that's a characteristic of a battery.

The voltage drop is developed between battery and lamps due to inadequacies in cabling. Yes. the battery is at maintained at 13.8v but the issue is whether that 13.8 can be maintained at the lamp. In a no loss set up, you'd have 13.8 at the lamp when the lamp is in circuit but the very existence of those relay conversion modifications we've all done indicates there are significant losses due to the stock cabling.

[geoberni]

The voltage drop is developed between battery and lamps due to inadequacies in cabling. Yes. the battery is at maintained at 13.8v but the issue is whether that 13.8 can be maintained at the lamp. In a no loss set up, you'd have 13.8 at the lamp when the lamp is in circuit but the very existence of those relay conversion modifications we've all done indicates there are significant losses due to the stock cabling.

Any circuit is a trade off between current carrying ability and the losses.
As I said, my car gave a 0.4v difference, considering that's 2 switches (Lights and Dipper) and around a doz or more terminations that's good, so I would dispute your assertions of ' there are significant losses due to the stock cabling'.
It's far more likely that losses are due to old age and insufficient maintenance; my car had a new loom about 25 years ago, but there's still cars driving around with 60-70 year old wiring.

I also think you're in a minority in claiming 'relay conversion modifications we've all done'. It's common when fitting Spotlights, but that's a different scenario to modifying the headlight wiring.