Steve, have a look at the RS site, http://rswww.com and do a search on
2212845.

This is a 3.5 digit LED voltage meter and claims 3mA rms, there is more info
on the site.

I am thinking of getting these myself and I suppose could always be linked
to the battery via a relay switched through the ignition circuit to prevent
battery drain when the motor is not running.

Cheers, Brian