Measuring the meter

Tue Dec 06, 2011 5:35 pm   

On Tue, 6 Dec 2011 07:50:39 -0800 "Stumpy" <perilmung_at_spamnet.con> wrote
in Message id: <46OdnbHV-dFRpEPTnZ2dnUVZ_sGdnZ2d_at_earthlink.com>:


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.

Tue Dec 06, 2011 5:42 pm   

"Kaz Kylheku" <kaz_at_kylheku.com> wrote in message
news:20111205211858.963_at_kylheku.com...

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.

Tue Dec 06, 2011 6:30 pm   

"JW" wrote in message news:jvgsd7hu95dahl8gmr2hk5olqs68j8opc8_at_4ax.com...


http://www.harborfreight.com/battery-disconnect-switch-97853.html

http://www.harborfreight.com/15-watt-solar-battery-charger-44768.html

I have an old lawn tractor and a few years ago I replaced the original
lead-acid battery with a SLA battery I bought cheap at a HamFest, probably
10 years ago. I think it has a 2000 date code. About a year ago I decided to
see if it still held a charge, and it had enough to light the ignition lamp.
Then I forgot about it until a few days ago, and I was sure it must be dead,
but it still lit the lamp, after more than two years sitting outside. It's
probably too weak to turn the starter, though.

OTOH, I have a 1989 Toyota pickup truck that I use only occasionally, and
after a few months the battery has gone flat. I have been able to start it
by using a booster. In August I tried to start it, and using the booster it
started to run, but would not maintain an idle. I used the diagnostic code
to find it was a problem with the throttle position sensor, but it checked
OK with an ohmmeter. I slapped it a few times with a screwdriver and then it
was OK.

But I put the battery on charge for a couple of months, and when I went to
start the truck, it was dead. So I had to use the booster.

A year ago I had driven my car to Harrisburg, parked it for a few hours, and
when I returned the battery was totally dead. I got a jump start and it ran
for a while, but as soon as I touched the brake, the brake light load was
enough to kill the engine. The battery had been working fine up until then,
with no sign of a problem. So batteries are really difficult to evaluate.
Maybe a load test would work, but the ultimate load test is starting the
car, which I did, and six hours later it was dead.

It's really best to start a car once every week or two, and let it run long
enough to get up to temperature. And it's also good to actually drive it. My
truck's brakes froze up after a long time of sitting.

Paul

Tue Dec 06, 2011 6:37 pm   

"JW" <none_at_dev.null> wrote in message
news:jvgsd7hu95dahl8gmr2hk5olqs68j8opc8_at_4ax.com...

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.

Tue Dec 06, 2011 7:17 pm   

On Mon, 05 Dec 2011 18:39:28 -0800, Stumpy wrote:


I don't know what the capacity of a car battery is in amp-hours, other
than "lots". 10mA for a week is less than two amp-hours. Unless my head
is screwed on backwards your battery capacity is way more than that.

Why not just unplug it when you're not looking at it?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Tue Dec 06, 2011 9:09 pm   

"Stumpy"



** Errr - make that 2.7uA.

The leakage in the zener.


.... Phil

Tue Dec 06, 2011 11:26 pm   

Stumpy wrote:


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

Wed Dec 07, 2011 1:13 am   

On 2011-12-06, Stumpy <perilmung_at_spamnet.con> wrote:

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.

Wed Dec 07, 2011 1:16 am   

On 2011-12-06, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa__at_charter.net> wrote:

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.

Wed Dec 07, 2011 7:13 am   

"Kaz Kylheku" <kaz_at_kylheku.com> wrote in message
news:20111206151334.855_at_kylheku.com...

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.

Wed Dec 07, 2011 1:09 pm   

On Tue, 6 Dec 2011 09:37:30 -0800 "Stumpy" <perilmung_at_spamnet.corn> wrote
in Message id: <YN2dnT61jLVHz0PTnZ2dnUVZ_jednZ2d_at_earthlink.com>:


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.

Fri Dec 09, 2011 2:03 am   

Stumpy wrote:

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