Measuring the meter

[Phil Allison]

"Stumpy"

I = E/R = 12v/4,430,000ohms = 2.7mA

vs. a measured 6.8mA

** Errr - make that 2.7uA.

The leakage in the zener.


.... Phil

 

[Jamie]

Stumpy wrote:

"Kaz Kylheku" <kaz@kylheku.com> wrote in message
news:20111205211858.963@kylheku.com...

On 2011-12-06, Stumpy <perilmung@spamnet.corn> wrote:

"Kaz Kylheku" <kaz@kylheku.com> wrote in message
news:20111205165432.291@kylheku.com...

On 2011-12-06, BeeJ <nospam@spamnot.com> wrote:

How about putting a potentiometer in series with the meter +.
Adjust the potentiometer until the meter voltage reads half of what it
read without the potentiometer the remove the test circuit and measere
the potentiometer resistance. Calculate from there.

If you can read the resistance of a potentiometer, you most likely have
a
multimeter which, being multi, has modes for measuring voltage and
current.

Just put /that/ in series with the device, and flip it to current
measuring
mode to obtain the current drain directly.

I do have a multimeter but thought maybe that the multimeter itself would
change the circuit. The original meter may be "dynamic" and have varying
resistance as the needle is deflected.

For measuring current, a multimeter should have a low resistance. (We're
talking, some current sensing resistor having a small fraction of an ohm.)

Your battery meter is hopefully a voltage device with a reasonably high
resistance, against which that of the meter should be negligible.


I'm pretty much done worrying about this, but, it occurred to me that my
numbers don't add up.

I = E/R = 12v/4,430,000ohms = 2.7mA

vs. a measured 6.8mA

I can live with such a minor discrepancy. Also negligible.


if you are looking for a low battery alert trigger circuit that does not

actually use much current before it gets down to the critical state? You
could use a JFET or non-enhanced mode MosFet where the impedance is
so high that it should not even cause any drain on your system while in
monitoring state.

If you're more interested in that method, I am sure many will step up
to the plate here, if they haven't already.

Jamie

 

[Kaz Kylheku]

On 2011-12-06, Stumpy <perilmung@spamnet.con> wrote:

I'm pretty much done worrying about this, but, it occurred to me that my
numbers don't add up.

I = E/R = 12v/4,430,000ohms = 2.7mA

vs. a measured 6.8mA

I can live with such a minor discrepancy. Also negligible.

Someone upthread mentioned that there is a Zener diode in that meter so that it
measures only in excess of 9.something volts. Thus you can't measure it as as
resistor: it is not an ohmic device. The apparent resistance will change based
no what voltage your ohmmeter applies to it.

Perhaps if your meter applied 12V, it would measure a lower resistance,
due to driving the Zener diode to conduction.

The actual measured current drain is the most reliable figure, rather
than the calculated estimate.

 

[Kaz Kylheku]

On 2011-12-06, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

if you are looking for a low battery alert trigger circuit that does not
actually use much current before it gets down to the critical state?

The car is parked for weeks with nobody sitting in it, so this would need a
wireless transmitter, too! Ooooh, project!

I think once he gets an idea of how the battery behaves over N days of parking,
he will stop using the meter anyway.

Meters hooked up to the same device in the same configuration eventually tend
to stop telling you anything new and interesting.

 

[Stumpy]

"Kaz Kylheku" <kaz@kylheku.com> wrote in message
news:20111206151334.855@kylheku.com...

On 2011-12-06, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net
wrote:
if you are looking for a low battery alert trigger circuit that does not
actually use much current before it gets down to the critical state?

The car is parked for weeks with nobody sitting in it, so this would need
a
wireless transmitter, too! Ooooh, project!

I think once he gets an idea of how the battery behaves over N days of
parking,
he will stop using the meter anyway.

Meters hooked up to the same device in the same configuration eventually
tend
to stop telling you anything new and interesting.

No, I don't need another project. And the meter will just be like part of
the dashboard. I think all cars should have a meter instead of idiot light.
It will continue to inform me(or the new driver) whenever there is a
potential problem to fix with an overnight tricklecharge or even a new
battery.

 

[JW]

On Tue, 6 Dec 2011 09:37:30 -0800 "Stumpy" <perilmung@spamnet.corn> wrote
in Message id: <YN2dnT61jLVHz0PTnZ2dnUVZ_jednZ2d@earthlink.com>:

"JW" <none@dev.null> wrote in message
news:jvgsd7hu95dahl8gmr2hk5olqs68j8opc8@4ax.com...
On Tue, 6 Dec 2011 07:50:39 -0800 "Stumpy" <perilmung@spamnet.con> wrote
in Message id: <46OdnbHV-dFRpEPTnZ2dnUVZ_sGdnZ2d@earthlink.com>:


"amdx" <amdx@knologynotthis.net> wrote in message
news:f3f12$4ede0efe$18ec6dd7$19983@KNOLOGY.NET...
On 12/5/2011 10:05 PM, Stumpy wrote:
"Phil Allison"<phil_a@tpg.com.au> wrote in message
news:9k5i16FotfU1@mid.individual.net...

"Stumpy"

Oddly, with the black lead of the multimeter on the + terminal of the
original meter(and red on negative terminal) it is 4.4Mohms but with
the
leads reversed it is "open" "O.L Mohms".

** The meter has a 9.1 volt zener diode inside it - that is how it
reads
only from 9 volts and up.

The current draw is gonna be tiny and nothing to be concerned about.



... Phil


Thanks, I agree. The original equipment battery was 80AH so
80AH/.0068A=11,765 hours=490days=13.6years

I'll leave the thing in forever.

Now that you have a meter on the battery, how low do expect to let
the voltage get before you start the motor and let it run a while?

Why not just start the car once a week.

Mikek



Of course, but technically its not my car. I'm just the responsible
adult.


IIRC they do make a cutoff switch that will mount under a hood that
connects to the + terminal of the battery. Haven't seen one in ages, but
I'm almost sure you can still buy them.

A good idea, but, It's a Ford Focus - has some computer generated "tuning"
type settings so they don't recommend leaving the battery disconnected.
Would have the car relearn settings every time its reactivated. This
problem will probably go away as soon as the learning permit becomes a
license.

I guess the engineers at Ford have yet to discover flash memory. A common
theme in test equipment as well - storing cal data in battery backed ram.
In fact, there's at least one HP 70004A plug-in (Long forgot which one)
that stores its *firmware* in battery backed memory! Just toss it if the
battery dies, as it's been out of support for a long time. Sheesh.

 

[Rich Grise]

Stumpy wrote:

"Kaz Kylheku" <kaz@kylheku.com> wrote in message
On 2011-12-06, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net
wrote:
if you are looking for a low battery alert trigger circuit that does not
actually use much current before it gets down to the critical state?

The car is parked for weeks with nobody sitting in it, so this would need
a
wireless transmitter, too! Ooooh, project!

I think once he gets an idea of how the battery behaves over N days of
parking,
he will stop using the meter anyway.

Meters hooked up to the same device in the same configuration eventually
tend
to stop telling you anything new and interesting.

No, I don't need another project. And the meter will just be like part
of
the dashboard. I think all cars should have a meter instead of idiot
light. It will continue to inform me(or the new driver) whenever there is
a potential problem to fix with an overnight tricklecharge or even a new
battery.

In the olden days, when we had to walk five miles uphill both ways through
ten feet of snow to get to school where we wrote our lessons on clay
tablets, cars actually had ammeters. They didn't worry that much about the
voltage reading in those times, except for the hot-rodders. (see J C
Whitney. ;-) )

Cheers!
Rich