suspect you're looking for something a little fancier than this,....I find it very handy.
I keep mine pugged in all the time....interesting to watch the voltage changes under various conditions.
hard to beat at £4.
Digital 12V/24V LED Car Cigarette Socket Voltmeter: Amazon.co.uk: Electronics
should check against a known acurate volt meter.... mine reads 0.2V under the true voltage.