The Story ...
Time ago I had a problem with my over 10y old car; the lights seemed during the night driving a bit dimmed, than later too shiny & later I heard also during a journey a bit strange noise coming from its engine department after some high engine RPM driving. One day after coming home when I opened the engine cover, I found strange thing happening: the electrolithic fluid (acid) in the car accumulator was emmiting strange noises & a car battery was wet & hot & damn, the acid was leaking down. A mess, I had to clean with water the leaked acid everywhere, because I did not want it to "eat" my car (rusting everywhere inside engine department later started fast a bit anyway!) .

When I checked with a multimeter the battery, seemed that some battery cell shorted & others got too much voltage charging. Since it was still under warranty, I wanted to replace it & for a meantime my friend a mechanic, borrowed me his spare one, but when later at the service place a personal checked my battery, they found it Ok. Strange, now seemed Ok, so it must be something wrong with the cars electric generator (alternator) , probably charging regulator "gone south" I thought.

I went to my friends friend owning a same model car & he allowed me (believe it or not) to take that regulator from his newer than mine car alternator to put it onto mine one just for a test drive for a day or so (friends to allow you to do something like that messing with their car are IMHO very rare these days ), to be sure, if mine is bad not to waste the money if something else is wrong. The problem with charging the car battery more than max allowed 15V started time to time again, so than I knew that the real problem was a partially damaged alternator itself with some occasional shorting inside it time to time. So it was time to replace it, but the new one costs and I did not wanna throw much money for that stuff for mine so old car, so I thought to fix it temporary somehow to normally use my car & later than, when my friend a mechanic has time for me, get a good used alternator (cheaply) from some car junk yard to finaly properly repair my car.

The basic idea was to limit the current thru the alternators voltage regulator current source by some resistance to supply its rotor with less current & so producing less electomagnetic field inside & in consequence, limiting current/voltage supplied to the battery. For an "active resistor" I used a 21W/12V light bulb normally used for back brake lights mounted on a support from some old broken car fog backLight & wired serially in-between + pole of battery & alternator voltage regulator. It worked & schematics & pics looks like this ...

car alternator Mod  
bulb inserted bulb lit on
bulb inserted bulb lit on
( For more detailed pics & some night light special effects gallery, go to the next site later ... )
So how does that light bulb stuff work ?

When the engine its pushed to a bit more RPMs, the regulator switches charging battery on & simultaneously the bulb gets electricity & starts to lit a bit (right two pics). When rising engine RPMs, the alternator sucks more bias current charging the battery more and to higher voltage & so also the bulb gets more current & voltage & lights more, but at the same time the wire inside it gets hotter & lights brighter and so has major own resistance than & so starts limiting the bias (charging current) to the alternator so that it does not overcharge the car battery to much. It worked quite well with normal driving and at least parking lights switched on & shorter journeys for a month, but then I finally decided to change the whole generator to play safe long term (got cheaply refurbished one), because if accidentally something would go wrong [another similar my Car Mod story] , my car could cauhgt fire & that I did not wanna risk, since I remember my experience a lot of years ago with even older Citroen Diane6 - "tend on wheels" vehicle whose alternator almost caught fire because of failed power rectifier diode inside it & I remember blowing on it later & pulling wires off it after parking, when I realized that time that my "car" was "smoking" ... But this time the frends mechanic labour fee was null, since in a meantime I have fixed him his problematic computer.

I also did not wanna explain to some police officer, if I am being stopped at night, what the hell that light is coming under from my car ... ... yes the faster I was driving, that light was more illuminating the road under my car ! Well it was not such a good effect like those blue neon lights some guys put under their car, but anyway! See the bigger size pics Gallery on next site ...