Measuring the meter

Stumpy used his keyboard to write :

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.

Check the meter with an ohmmeter.

John

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.

I'll put a multimeter in during daylight hours.

Shows 4.43Meg ohms! But that is with the needle at zero/9v

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".

I don't think that a multimeter so profoundly cheap as to make the measurement invalid exists.

Is the multimeter a light bulb, with a little card that equates filament color with current? No? Then it's good enough.

--
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

I did a test with an old battery in the garage. 6.8mA and the multimeter didn't seem to effect the reading on the original meter.

So I guess that answers my question. Now I just need to decide if that is too high a value to leave it connected to a battery that may only get charged up once a week.

"Stumpy"

** 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.

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.

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.

So be responsible and start the car. BTW the battery can get quite low in starting capacity and still read nearly 12 volts.

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.

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.

I have an old lawn tractor and a few years ago I replaced the original=20 lead-acid battery with a SLA battery I bought cheap at a HamFest, = probably=20

10 years ago. I think it has a 2000 date code. About a year ago I = decided to=20 see if it still held a charge, and it had enough to light the ignition = lamp.=20 Then I forgot about it until a few days ago, and I was sure it must be = dead,=20 but it still lit the lamp, after more than two years sitting outside. = It's=20 probably too weak to turn the starter, though.

OTOH, I have a 1989 Toyota pickup truck that I use only occasionally, = and=20 after a few months the battery has gone flat. I have been able to start = it=20 by using a booster. In August I tried to start it, and using the booster = it=20 started to run, but would not maintain an idle. I used the diagnostic = code=20 to find it was a problem with the throttle position sensor, but it = checked=20 OK with an ohmmeter. I slapped it a few times with a screwdriver and = then it=20 was OK.

But I put the battery on charge for a couple of months, and when I went = to=20 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=20 when I returned the battery was totally dead. I got a jump start and it = ran=20 for a while, but as soon as I touched the brake, the brake light load = was=20 enough to kill the engine. The battery had been working fine up until = then,=20 with no sign of a problem. So batteries are really difficult to = evaluate.=20 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=20 enough to get up to temperature. And it's also good to actually drive = it. My=20 truck's brakes froze up after a long time of sitting.

Paul=20

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 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