Dear Rick:
I just love the condescending tone of a teacher!!
We are talking here about current flow. And voltage drop. They are related.
In all but super conductors, when current flows there is a corresponding
voltage drop. This is what the discussion is about. The light bulb is
analogous to the meter test because they both present a resistance to the
(in this case parasitic) load. In the case of the meter (remember we were
talking about a d'arsonval movement here) will also show some movement even
though the meter is a higher resistance (but still not in the tens or
hundreds of megohms like modern meters are). So yes, in this case even a
VOLT meter can be used to measure current.
The original answer (by Paul?) did also discuss using the light bulb and I
also agree that since this is a lower resistance series element (on the
order of a few ohms) it will be a better test. I have conducted the DVM test
I mentioned on many cars also and found it to be helpful. You said yourself
that the last digit of your meter responded in your testing. This is the
one-tenth volt sensitivity I talked about. Since your test was in new cars
with new batteries, I would expect the voltage to be very "stiff", and not
move around much. My own MGBGT with 4 year old batteries drops 20mv when the
cables are connected, with a radio and ignition load (after the fuel pump
stops). But will start the engine quite quickly, so I know the batteries are
still good. So maybe not the absolute best test, but made more quickly that
the light bulb test because there are no disconnections necessary.
As far as your self discharge stats, that's cool with me. There are lots of
variables with vented lead acid batteries such as age, manufacturer, time
stored before selling, temperature of operation, charging method and so
forth. So many variables even the texts on batteries don't all agree. Your
numbers are 20 percent / (16 weeks * 7 days ) = 0.18 % per day, which I
suspect is for a fairly new battery, in moderate temperature. For an older
battery it will be closer to my number of 0.5% per day. For a really old
battery or one in hot climates, it can be much worse. I have read reports of
1% per day self discharge rates for certain lead chemistries.
I have spent many hours in the U. of Wash engineering library studying
batteries, and I would be glad to get you references (off the list of
course) if you would like. I also have designed electronics equipment for
the past 20 years. Even designed battery chargers for the past 8 years,
including one battery charger product that has sold over 100K copies since
1995. (Yes they work for liquid, Gel, AGM and Nicads).
Now...back to the discussion about electric MGAs...
Can I convert my MGBGT and join the 90 days around the world bunch??
How many cats can I take??
Tim Economu <smiling>
Mona '69 MGBGT
----- Original Message -----
From: <REwald9535@aol.com>
To: <economu@whidbey.com>; <mgs@autox.team.net>
Sent: Tuesday, May 02, 2000 10:35 PM
Subject: Re: Fw: Alternator Woes
> Tim,
>
> <<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
> short.>>
>
> 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)
>
> Note
> that
> << 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.
> Rick
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