Spot lights power question

Wow, I've heard everything now......
Don't give advice unless you know the facts...Everything Eric has said in this post is wrong.....Absolutely wrong.....

Batteries are not charged through a resistance in order to lower current to the battery...It does not matter if you take the power from the Alternator or the Battery...You may acheive a .25 volt increase to the lights as there is a small voltage drop from the alternator to the battery..

The regulator on the Alternator makes sure that the Voltage remains stable from the Alternator to the Battery and the rest of the Electrical system..What mosty people don't understand is that the battery also acts as a filter to filter out any unwanted voltage spikes and AC Ripple from the Alternator...

I could go on and on..And I don't mean to offend anyone it's just that everyone seems to be an Expert....People must be careful when offering advice and make sure it's the right advice....

Oh, And why have I put my two cents worth in? It's because I do know what I'm taalking about...

Damian.
Sorry to be a pain but I do have a bit of an idea as I have spent many years designing and testing prototypes, if you were to untape a loom you will find what I say is true. by running a higher voltage and then running all the loads through a small resistance in the form of the loom you can improve the performance of the lights by limiting the current when the globe is cold, a cold globe has a very low resistance which can cause damage to the filament if the current is not limited.
You may remember the bad old days when globes had to be replaced at regular intervals, these days a globe can last the life of the vehicle.
The other reason the concept of wiring was changed was to comply with the design codes, it is a requirement that a vehicle does not catch fire when involved in an accident, if a vehicle is wired with heavy cables directly to the battery in an accident you will have hundreds of amps flowing when the body is deformed. Just think of the hundreds of accidents that occur every day in Aus and very few people are burnt, If you were to talk to insurance companies you would lean that bad wiring is a major reason that these companies do not like modified vehicles. It is a good thing for the community that there are some people who do understand the fine details of wiring and try to protect others from there stupidity.
Eric.

Sorry Eric.. Being an Auto Electrician with over 22 years Experience and I am Highly Qualified at that..I have to disagree with you, 100%..I'm not trying to upset you and I know I can come across arrogant and that's not my intention...

Everything you say in your first post and your second post is absolutely wrong..and could be dangerous..

As an example, If what you say was true about wiring designed not to set the car on fire, How do you explain the Cable from the Battery to the Starter motor..A very heavy cable..The wire from the Alternator is 6mm or more..
According to you, this would not apply to "Code".
It's actually up to the deisgners and engineers to make sure that wiring does it's job..
I'm not sure by what you mean about "Code". If there were "Very Strict Codes" to wiring, every vehicle on the road would almost certainly fail..
How many Audio Systems are installed by unqualified personell and not installed correctly..?
To top it all off, the wiring in Automotive Applications just does it's job and that's that..I'm not even sure that Automotive Wiring has anything to do with the Roadworthy Certificate..Not that I know much about RWC's..
All Wiring that supplies power to anywhere in the vehicle should be adequately fused..That's what stops fires in the event of an accident or inadvertant shorts to ground..

Secondly, Any incandescent Bulb no matter what vehicle, Is the same as it was 40 or more years ago..I defy you or anyone that can tell me that Incandescent Bulbs that are used every time a vehicle is used, eg: Tail Lights, Headlights and indicators, Last the lifetime of the vehicle..They don't..Bulbs that are used rarely, eg: Reverse Lights etc. May last a long time...Dash illumination is run through resistance (For Dimming Purposes), so they do seem to last a long time, Maybe even the life of the vehicle..

Thirdly, I would rather have a heavy cable short to ground than a thin cable...If you know a little of what you're talking about, you'd know that a thin cable, say 3mm or 4mm shorted to ground has a much higher chance of setting something alight. Try shorting a 2ft length of battery cable and do the same with a 2ft piece of 3 or 4mm. Then post your results..

Fourthly, There should be no resistance whatsoever to any device on the vehicle, especially Light Bulbs..The only thing that has deliberate resistence is something that requires a voltage drop. Eg: Some Ignition Coils, Fan Motors (to control the motor speed), Some Fuel injectors and Adjustable Resistance to Dash Illumination Bulbs..

I'm going to repeat myself again... There Should Be No Resistance and No Voltage Drop, Whatsoever, In Any Wiring Loom Unless It's Deliberate..The Only Deliberate Resistance In A Wiring Loom, That I know Of, Was In The Early Holdens, To Drop The Voltage To The Ignition Coil...
Headlights, Especially, Should Have No Resistance In The Wiring To Them At All..If You Add Higher Wattage Bulbs To The Headlights, Then Make Sure That You Fit A Relay To Supply Power To Them...The Wiring in a vehicle to orignal wattage bulbs is only adequate for the original wattage bulbs unless the manufacturer says otherwise..

Really, When it's all said and done, Automotive Wiring takes Common Sense..If you know what you're doing and you're confident you know what you're doing, then go ahead, but, Do Not Take Advice From People Who Don't KNow What They're Talking About...If you are not sure, Take it to someone who is Qualified and has a good reputation.....Otherwise you're asking for trouble...

Sorry Eric, I'm Afraid I Have To Say That You Got Quite A Lot To Learn When It Comes To Electricity..I Don't Mean that Nastily, Either..But, You Have Been Severely Misinformed Somewhere..

Damian.
You believe what you want to believe. All I offer is the truth about modern vehicles not the wiring of vehicles 25 years ago. You must ask yourself why do all modern vehicles even the $100k+ jobs have very thin wires to all the lights. Eric.

Some People just don't get it..??

I don't care if you believe me or not. I'm not interested in aruing with you.. All I care about is that your advice is wrong and may cause someone's vehicle to burn to the ground.
That's the problem with unqualified advice. At least I'm Qualifed.

If you can find facts with your comments then by all means share them with me..But, until then......

Sayonara....End of discussion....

Hi Eric, I’ve been working with automotive electronics for around 30 years and dealing with auto electricians for that time. Damian007’s 22 years experience as an auto electrician is not, in my books, any form of qualification for having the knowledge required to know what is needed in that trade. Over the years I have found that many auto electricians are no more than experts at unplugging and replacing, few have the real expertise to be able to work outside the basics.

Having said that, Damian007 is obviously one of those few. His post is not only right on the money, he also has the rare ability to of being able to put it in terms that most people can understand and I bet that comes from having to explain to customers in the most simplest of terms, what is needed to be done to their vehicles to resolve a given problem.

Damian’s info above is correct and I can assure you the $100k + vehicles DO run thicker wire for things like headlights. I have a new Rangie and even though all the headlight globes are only 55w, I could simply change them to a much higher wattage globes and not have to upgrade any wiring.

You should go and look at the wiring in any vehicle that has been professionally set up for off road or rally use and you will see you definitely have things back to front.

Cheers

Sorry drivesafe

A globe is a resistor.

I = V/R

Assume for a moment we are talking a 100 w globe, and that V = 12 & R = 1.4 ohms.

I therefore is going to equal 12/1.4 = 8.57 Amps.

Lets up the voltage.

V = 14 and R is still going to be 1.4 ohms.

14/1.4 = 10 Amps

"the higher voltage means a lower current through the globe so always fit thick cable for driving lights and try to keep the wire run from the battery through the fuse and relay to the lights as short and direct as possible, this all helps to keep the light as bright as possible. "

If we were trying to achieve lower current we could use thinner wires.

Higher voltage means higher current which means more power radiated in the form of light.

The reason you use thicker cable and short runs is to get lower resistance between the supply and the globe, therefore less voltage lost as drop across supply cables. Then the maximum voltage is applied to the lamp filament. This filament resistance is a fixed value (there will be a some change in value as it heats and cools but it's not really relevant in the context of this conversation.) If you apply more voltage to it you get more current and more light.

Dave

This doesn't seem to ring quite right Dave. A 100w globe is say rated at 100w at a nominal voltage, say 12v. So your resistance is constant, yes, but its the only constant. At 14v the power would increase. as P=IV then power would be like 10 amps (as you calc'd) at 14v = 140w output.

As you say, with the larger wire, the same thing happens. Resistance of a thicker wire is less. That means less voltage drop over the wire for the same current and more volts at the light = more power output. Thinner wires wouldnt necessarily limit the current, they would increase the current as the load voltage would be less = higher current.

Drivesafe, the higher voltage doesn't mean lower current thru the globe, it means same current and higher output, as only the resistance is constant. I can't see how this would make the globe last longer either, as theres a greater electro-mechanical stress on the globe at higher voltages.

The above info is disclaimed if I have cocked up so early Sunday morning.

Oh Bonz. Now my head hurts. Could it be the beer?

I did say you would get more power radiated in the form of light and you could calculate the amount quite easily.

The globe would only be 100 watt at a specific voltage. 12v = 100w. Power and current would increase with supply voltage.

I wasn't suggesting thinner wires would limit the current, but that they would reduce the supplied voltage to the lamp due to losses in the rest of the circuit.

That's why the likes of ARB sell exorbitantly expensive looms to "upgrade" your OEM headlights.

The bit where you said, "That means less voltage drop over the wire for the same current and more volts at the light = more power output.
" is exactly what I was saying.

But, "Thinner wires wouldnt necessarily limit the current, they would increase the current as the load voltage would be less = higher current.

......the higher voltage doesn't mean lower current thru the globe, it means same current and higher output, as only the resistance is constant." isn't quite right.

Lets consider for a moment that the resistance doesn't change in the filament, and that the voltage at the filament has decreased due to inadequate wiring. Lets assume the wire from the supply was utter bleep e and dropped 2 volts off the supply to the load. The current in the lamp must decrease as the voltage drops over a fixed resistance - the filament.
I = V/R
= 10 volts /1.4 ohms (filament)
= 7.1 Amps

P = IV
= 7.1 Amps x 10 volts
= 71 Watts

I'm not a metalurgist and I can't remember the physics of it but I would reckon that at a lower wattage you are extending filament life. You've also got lousy lights.

Dave

Yeah but once it's hot and running the resistance and current are fairly constant.

The calculations get far more complex if you take into acount thermal effect, and my recall is that that is really only relevant during warm up and cool down of the filament.

Correct me please if I'm wrong.

Dave

Okay, I'll put my 2 cents worth here, (Although I said to myself I wouldn't get involved in this debate)..

Geocacher is right as are all us Geocachers..

An Incandescent Bulb could be described as a resistor..As the filiment in a bulb heats up, The resistance actually goes a little higher... So, when you switch your headlights on, the current draw is higher at first as the Filliment is cold and as they heat up they draw less current.
Theoretically, a 120 watt bulb should really draw 10 amps when they're on but they don't..12 into 120 = 10. You'll find that the bulb would draw anywhere between 8-10 amps and that all deppends on the resistance of the filliment and the Voltage being supplied..

Someone once made a device that kept the headlight filiment warm by allowing a small amount of current through them so that when you turned the headlights on they didn't blow due to thermal shock..It was also claimed to help the bulbs last longer..Which it probably would have..

Was it worth the hassle? I doubt it..