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#16 stef123

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Posted 07 February 2013 - 06:16 PM

heres a question, this morning on the motorway going along at 65 bout 2900rpm i put the heated front screen on and noticed the headlights dip, surely if the engine is running at 2900rpm therefore turning the alternator, surely there would be enough power being supplied for the lights to be on and the front screen on?


Any heated screens i have come across are fed from 2x 30 or 40 amp fuses (cant remember which), so even with good revs its still alot of power for the alternator to supply. the headlights alone without any other lights will be drawing around 9 amps

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#17 alexp999

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Posted 07 February 2013 - 06:20 PM

Don't forget as well, the alternator puts all its power through the battery first to provide a stable voltage and everything is still sharing that power through the battery.

#18 stef123

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Posted 07 February 2013 - 06:27 PM

Don't forget as well, the alternator puts all its power through the battery first to provide a stable voltage and everything is still sharing that power through the battery.


the fuse-boxes are fed from the same point where the alternator connects to the battery so the battery only really supplies more power where the alternator cant provide it.
voltage stabilisation is done at the regulator

just my 2p's worth

#19 alexp999

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Posted 07 February 2013 - 06:30 PM

Ah ok, I thought the alternator passed the power across the battery at higher voltage then the battery provides a stable 12V.

Otherwise the entire electrical system would be running higher than 12V?

#20 stef123

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Posted 07 February 2013 - 06:35 PM

Ah ok, I thought the alternator passed the power across the battery at higher voltage then the battery provides a stable 12V.

Otherwise the entire electrical system would be running higher than 12V?


correct, the alternator produces an a/c voltage, which is then turned to dc (by a rectifier) and at a stable voltage by the regulator (the black bit on the end of the alternator)

so yes the electrical system does run at a bit more that 12v

#21 Stoney871

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Posted 07 February 2013 - 06:41 PM

The system usually puts out about 14v and as stated the rectifier makes sure that the voltage is stable then the GEM makes sure the right amperage gets to the right parts.

#22 Nathan_M

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Posted 07 February 2013 - 08:16 PM

If its the same as my MK6 Fiesta you get 4 minutes on the front screen, although it doesn't seem to affect my motor as it sits at a high idle when its cold and this seems to counteract the power draw from the heater in the Front and rear windows, if you have the interior fan on 4 as well you will notice that it sounds more like position 2 when you have it all on, i would have thought that even at idle the alternator would have enough output to power the engine and all internal electronics by design they must expect people to use everything at once.

#23 vince_13

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Posted 07 February 2013 - 08:28 PM

correct, the alternator produces an a/c voltage, which is then turned to dc (by a rectifier)


didn't know that, i was under the impression they output dc, is it rectified at the alternator then?

i thought the same Nathan, i suppose though if the alternators job is to keep the supply of average 12V constant and stable, when the heater is on it will draw on the battery (like any other device) therefore leaving the alternator to catch up, (if that makes sense)

#24 stef123

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Posted 07 February 2013 - 09:40 PM

didn't know that, i was under the impression they output dc, is it rectified at the alternator then?

i thought the same Nathan, i suppose though if the alternators job is to keep the supply of average 12V constant and stable, when the heater is on it will draw on the battery (like any other device) therefore leaving the alternator to catch up, (if that makes sense)


the alternator on its on will produce ac (no rectifier/regulator), its then rectified to dc and then regulated to 14.4v (the actual voltage is determined by the ecu)
the regulator and rectifier are all part of the alternator

the alternator cannot cope with all electrical demand, and more so as the alternator heats up so the battery is what supplies the power required - hence why you get dimming lights and the fan slows down (the voltage has dropped because the alternator cannot supply anymore juice)

#25 vince_13

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Posted 08 February 2013 - 06:10 PM

thanks stef123 learnt something new :)

#26 whitelightning

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Posted 19 February 2013 - 03:50 PM

As long as the engine is running the alternator charges and runs the electrics as a 12v system. It not a dynamo...the lights don't dim when you slow down ( i hope)! Or get brighter when you speed up..........The front screen takes a huge whack of power as do headlights, we have to look after these old alternators especially when you hook up two amps and a sub and 4 6x9 speakers!!

#27 vince_13

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Posted 19 February 2013 - 05:16 PM

we have to look after these old alternators especially when you hook up two amps and a sub and 4 6x9 speakers!!


haha dont i know it, those days are long gone :P played havoc with my old fiesta alternators, 2 alternators and 3 batteries in 6 months is not good

#28 Lenny

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Posted 19 February 2013 - 07:48 PM

As long as the engine is running the alternator charges and runs the electrics as a 12v system. It not a dynamo...the lights don't dim when you slow down ( i hope)! Or get brighter when you speed up..........The front screen takes a huge whack of power as do headlights, we have to look after these old alternators especially when you hook up two amps and a sub and 4 6x9 speakers!!

If you have an amp fitted in your motor you may like to purchase a pivot mega razin voltage stabiliser kit.
Its a series of capaciters which supplys more instant power resulting in consistant voltage supply to your electrics, with no strain or draw being put on your altinator.

Beware of cheap ebay imitations they don't work, I've bought two replicas on ebay 100% crap.
Buy only from the offical pivot website.

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