On Aug 23, 1:26*pm, "Dave Plowman (News)"
wrote:
There are some gotchas with these meters. Not all can be used to measure
the voltage of their own power supply - like a car battery.

Course they can - you drop the supply voltage with a big linear reg to
5v or 3.3v as needed and you divide the measured voltage down to about
half of this. They'll thus work quite happily, as you're not trying to
measure a voltage anywhere near the supply rail voltage, and you've a
stable voltage to drive the meter circuit. If the battery voltage
drops too low to drive the meter chip (on the far side of a linear
that's expecting to be halving it), then your battery is flat and
you've probably noticed this by now.

Otherwise just rewire your car with a commercial DC-DC module inside
the dash and have cleaned-up 12V & 5V supply rails available to feed
anything you hang in there. My MG's dashboard even has a crankcase
accelerometer to tell me to back off from breaking the crankshaft -
output is a brown-backlit LCD text display which just looks like a bit
of art deco bakelite when the power is off.