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From digest.v7.n1312 Sun Mar 15 18:02:45 1998
From: Pearson-Franks Family <>
Date: Sun, 15 Mar 1998 12:18:48 -0800 (PST)
Subject: <stereos> Larger Alternators


Interesting note - thanks for sharing your experience. Just a thought in return about the desire to install a larger alternator. I did the arithmetic for the devices in my son's 528e "Sound System with Wheels", since he has a raft of amps, and wanted to install a larger alternator. The logic suggests it was not necessary, but we did it anyway - went from an 80 amp alternator to a 100. So, we were illogical, but here is the logic _anyway_:

Stock alternator      80 amps
Output voltage        13 volts (rounded for ease of compute)
Wattage             1040 watts (volts x amps)

Please note that this is a _continuous_ rating, +/- the fact that your alternator output varies with rpm. Now lets talk about the 'normal' load on your alternator, presuming your battery is fully charged.

Ignition, ECU           5 amps (also varies with rpm)
Fuel pumps              6 amps (varies a little with model)
Headlights             12 amps (I run with them on, no fogs!)
Windshield wipers       6 amps (I live near Seattle!!!)
Total load             29 amps
Consumption voltage    13 volts
Power consumed        417 watts

This suggests I have 600ish watts available to run 'elective' circuits such as rear defrost, more lights, recharge the battery, or oh, yeah, run my stereo. I will make a WAG as to efficiency of the Soundstream amp at 50%, which suggests that for it to produce the rated 300 watts output that it would consume 600 watts, turning the rest of the power to heat, hence the cool <g> blue heat sink fins.

Power available       600 watts,
Power consumed      - 600 watts,
Margin                  0 watts, seems to be running at redline!

But wait! Recall that the alternator output was relatively continuous - how about the audio output? Not even close! Let's assume for the moment that you are running at 'full volume', defined as just short of clipping (the amp actually limiting the power output to the voltage swing it can manage into the speakers). Music is discontinuous, and has a peak-to-average ratio that is anywhere from 1% to 10%, with excursions higher with heavy metal, rap and some synthesizers. The result is that the average DC power draw is in the 10% range, which presents a continuous load of call it 60 watts, not 600 watts. A larger alternator is not needed, under this scenario. In fact, we never ran into a 'power budget' problem with his car with the stock alternator and battery, which stayed charged in his use scenario - and we can usually hear him coming home, even from inside the house!

With the peak-to-average nature of music in mind, a whole seperate arguement can be made for a cap, heavy wire, and an additional, stereo-only battery. So we went with a farad cap and 4 guage finestranded pure copper wire. Haven't added the gelcell battery, yet. We did not get any noteable difference with the larger alternator. We happen to be running a Soundstream 1000sx and 700sx into JL subs and DynAudio mids and highs. Looking for different subs, love the mids and highs.

I certainly hope the WOB Police note that these stereos are in our beloved BMW's! ;-) ;-) ;-) Also, corrections and additions welcome, especially on my 'typpical load' scenario.

Larry F.

>From: (Munsoor S. Malik)
>Date: Sun, 15 Mar 1998 12:05:26 +0300
>Subject: RE: subwoofer system setup in e30
>I have a 1995 E34 that has a modified stereo. The changes were as
> Kickpanel speakers: Replaced with MB Quart's.
> Door tweeters: Replaced with MB Quart's.
> Dashboard Speakers: Disconnected
> Rear Deck Speakers: JL Audio Coax

>    Subwoofer:             Sealed Box,  JL Audio 10W1 dual subwoofers.
> Crossover: MB Quart > Wiring: Completely replaced throughout the car.

>The original head unit is retained and uses line level converters to
>connect to a Soundstream 705S amplifier in the trunk. The subwoofers
>face the back seat, no sheet metal was cut to put them in. There is a
>knockout in the back seat but the sub's worked fine without removing

>The amp puts out some 300W(?) but with the subs connected it does not
>seem adequate. The only addition I would do is to add a seperate
>amplifier for the sub's. You may need to add a higher powered
>alternator/voltage requlator to drive this. I do not have enough
>electrical knowledge to know how to do this so I live with this setup.
> If somebody had knowledge of a fully compatible upgrade to a E34
>alternator I would be interested.
>Sounds fairly decent.
> Munsoor Malik

Larry Franks
Issaquah, Washington
'86 535i - Shark-infested
'86 528e - Shark-infested
'83 245 GLT (Swedish iron)

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