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Re: Fw: Alternator Woes

To: <>, <>
Subject: Re: Fw: Alternator Woes
Date: Tue May 02 22:35:43 2000

<<This is a very valid
 and low cost way to measure leakage. From here you just disconnect loads
 until the leakage drops to zero or near-zero and viola! you have found the

I've been quiet through this exchange, but I can't stand it any longer.  
Voltage is electrical pressure NOT current flow.  current flow makes batteries 
go dead not voltage (pressure).  Think if you had a leak from your water pipes, 
I doubt that you would call your plumber and say that you had a leak that had 
already leaked 50 PSI.  More likely you would tell him that the leak was so bad 
a five gallon bucket had already filled up (current) 
ANY acessory that gets current with the key off will show a "12V" draw, clock, 
radio whatever.
A much better and more practical way to find a current draw is to use a 12 test 
light between the battery cable and the battery post, if the light lights the 
draw is large enough to cause headaches, if the light does not light don't 
sweat the small stuff, unless of course the car is being laid up for a VERY 
long time, in which case I would disconnect the cable and either put a trickle 
charge on every 2 months or so or check the open circuit voltage on the 
batteries every so often and rechange when the stae of charge gets below 50% 
(about 12.2 Volts)

 <<Another way to do the leakage test, is with a 3 1/2 digit DVM. Connect
 across battery and pull fuses and/or disconnect suspect loads until the
 battery voltage pops up at least 1 digit (10 mV or more). Even milliamp
 loads will bring the battery voltage down a couple tenths of volt.>>

Next thing to do if the clock or the radio memory is causing you battery to 
drop it's voltage a tenth or two, like you say, is to buy a new battery!  I 
just went out to the shop and checked this on two cars (a 99 and a 2000) both 
of which have many items that are fed with the key off.  On the one car there 
was no difference between battery cable connected and off, the other had a 
1/100 of a volt difference (which I attribute to the rounding off of the meter 
as my Fluke only reads two digits to the right of the decimal)

<< we are not worried about microamps (millionths of an amp). The battery SELF
 discharge rate alone (with no load connected) is on the order of 1/2 % per
 day with flooded lead acid batteries. >>

Actually it's way less than that when the batteries are properly stored.  
According to a chart I got from some electrical engineers ater 16 weeks the 
battery is about 20 percent discharged...

These are common misconceptions that I see in technicians that come to class, 
you should see them try to fix a "12V" draw in a modern car that has stay alive 
power to several computers, clock, radio, alarm..... you get the idea.


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