Measuring the meter

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

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.

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.

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.

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