[DeTomaso] Alternator test data

Sun Sep 16 12:24:26 EDT 2012

Sorry for the confusion on the consumption sheet.   I comes primarily from 2 sources.   First,  I show the A/C consumption by itself and then again combined with the internal fans.   Thus, the A/C consumption is included twice if you were to add up all the lines.   Secondly, the battery chaging is changing pretty significantly from 5A to 20A as I run the tests (and continue to start the engine).    In truth all the consumption tests include the battery charging current so they are overstated.  You can see this in the battery disconnect chart where is only takes 0.7A to run the ignition with the battery disconnected and 32A to run everything with the battery disconnected.  A real test would have isolated the battery charging out of the equation, but I figure this is still useful as its probably what most people will see in terms of average current needs. 

Thus,  actual current consumption is a pretty modest < 40A all in.  If my original alt was charging at 800 rpms,  I probably would have been fine with it and no need to upgrade. 

The voltage spike is a good point.  I measured a max of about 6V, but the good news is the duration is very short (300 microseconds) so the power is tiny.  I would be most concerned about electronic equipment (as opposed to electrical equipment).   In my test the tach and tach adaptor and MSD ignition didnt seem to mind, and these are my only electronics.    I can also imagine this spike gets bigger if the circuit breaker kicks out while the alt is pushing 75A (20A in this test), but it will probably again be very short duration.  I would guess that most auto electronics have voltage suppressor diodes on their power inputs to handle this spike given the noisy auto environment, but it could be a concern area.   I think I will put a capacitor on the ciruit to absorb the spike and take some of the noise out of the line.   When I do that I'll post the scope traces so we can see if I was successful.

Final note to anyone considering this  circuit breaker on the ammeter.  This is a 1 wire setup with a specific (fairly expensive) alternator.   All bets are off with a 3 wire setup or a different alternator (although I am expecting most 1 wire setups to be similar).

David

________________________________
 From: Tomas Gunnarsson <guson at home.se>
To: fisher95020 at yahoo.com 
Cc: detomaso at realbig.com 
Sent: Sunday, September 16, 2012 12:48 AM
Subject: Re: [DeTomaso] Alternator test data

Thanks for the data David. If I interpret your scope pic correctly your upper marker line is at 19.2 volts and the spike extends even a few volts above that level (@ 5V/div). This is quite a lot of overvoltage so some electronics might not like it. The least of my worries is the alternator itself, it's probably good for it. Your consumption sheet confuses me. Why don't the individual loads add up to 56A? It's like you have a parasitic load that offsets all the individual readings. I did a similar but not as detailed test myself over ten years ago. 

Tomas

<-----Ursprungligt Meddelande----->
From: David D Fisher [fisher95020 at yahoo.com]  
Sent: 16/9/2012 8:09:47 AM  
To: Pantera mailing list  
Subject: [DeTomaso] Alternator test data   

Ok,  I promised this would be my last entry on this subject. I ran tests today on my 1 wire alternator setup and am sharing with anyone interested.

First I wanted to know how much current each system was drawing.  I put it in a table for my future reference, and it's here if anyone has interest.

Punch line,  I pull 56Amps with everything on.  I ran this at both 800 rpm and 2000 rpm

http://www.poca.com/index.php/gallery/?g2_itemId=38007

Probably more interesting,  I ran the test of what happens if you disconnect the battery while the alternator is running.  Again run at 800 and 2000 rpm.

Punch line,  Voltage has a little spike but stays at 14.X volts and the car (all the systems in the car) don't mind having the battery circuit connected and disconnected.   NOTE: This is a one wire setup and you should run your own test on your own alternator.

http://www.poca.com/index.php/gallery/?g2_itemId=38009

Scope trace of the spike for the EE's in the group (It's just as Tomas suggested)

http://www.poca.com/index.php/gallery/?g2_itemId=38014

You might also find this interesting.   I was surprised what the factory did to connect the ammeter circuit to the alternator circuit. 

http://www.poca.com/index.php/gallery/?g2_itemId=38011

A couple of notes:  I disconnected and connected the battery when 20A was flowing in that circuit.  A fool proof test would do it at 70A, but I'm pretty satisfied. 

Given these tests, I will go ahead and put a circuit breaker on the ammeter circuit.  (I have more detail if anyone wants to email me directly) and a 4guage line from the ammeter to the fuse box.   The factory "wire merge" will get replaced with a terminal block.

It also looks like a 140A alternator was significant overkill as I'm only consuming about 60A (if the battery is charged), but it has the benefit of having enough juice for lots of stoplight to stoplight putting around town without draining the battery.  I probably could have gotten by with a 100A alt that puts out at least 40A at 800 rpm.

David
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