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Old 09-01-2016, 02:40 PM   #1
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Post LEDs watt weight to alternator HP

I was going to install LEDs lights on turn signal, headlights, dash lights, etc..
LED (light emission diode) are consuming way less power than standard light bulbs which waste energy in heat. Replacing for a more efficient lights system should translate in less consumption watt, which consequently translates in less requirement from the alternator, which drains HP from the engine.
Curious to know how much?

By a conversion factor we should know that 747 watts = one horsepower. Regular automobile headlights typically run about 75 to 100 watts. So if you turn your headlights off, you save 200 watts, or less than a third of a horsepower.
The maximum theoretical efficiency of an alternator is 75% or typically 60%.

Therefore if you are taking the full load of 155 amps at 14 volts, that's 2170 Watts.

Allowing for the 75% max efficiency, you'll need to put 1627 watts of power in at the shaft to get 2170 watts of electricity out.

Taking into account the fact that the alternator is going to be far from perfect, plus drivebelt losses, I'd guesstimate 4000 watts maximum (4 kW) power from the engine to produce a full 155 amps at the alternator.

Using the conversion gives a figure of around 6 bhp to provide the full rated 155 amps.

By estimation for daily driving the engine will be giving up around 1-2 BHP to power the cars electrics.

But LEDs are cool..!

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Old 09-01-2016, 09:36 PM   #2
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Do signal lights work with led ligths? In the old days we did not use them in hot rods because they did not work because the led lights would not put enough load on the flasher to blink, I guess in a boxster the flasher is electronic and not electromechanic?
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Old 09-02-2016, 12:48 AM   #3
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If you replace Boxster turn signal lights with LEDs you have to swap to an electronic switch relay ( CF14JL-02 Flasher) which accepts 12V range 0.2A - 20A.
Cheers
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Old 09-02-2016, 08:33 AM   #4
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you also need to make sure that you got the polarity right.
on regular bulbs it doesn't matter, on LED it does.
on the boxster the external contact is the positive, and the center is ground.
opposite than most vehicles and of the shelf LED bulbs.
this is the back plate on a boxster.




and this is a standard LED bulb i got from autozone i believe.



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Old 09-02-2016, 08:49 AM   #5
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I could be wrong, but putting in a smaller output alternator would have little or no effect on engine load.

The alternator is spinning regardless of the energy it is outputting

As Meir noted, Porsche in its infinite wisdom reversed the polarity of the taillight sockets
With some cutting of the tracks and some wire jumpers, you could correct it to work with standard LED replacement bulbs

I would also be cautious about using LED H7 replacements. They do generate a lot of heat and most if not all have the light emitting chips in the wrong place for a standard halogen reflector

If you find some that work, please post a thread about your success
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Last edited by JayG; 09-02-2016 at 08:53 AM.
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Old 09-02-2016, 06:20 PM   #6
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cool, thanks for the info, I think I'll stick to my incandesent bulbs for now

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