Though I use analog gauges (just 'cause I had a few)...

http://members.cybertrails.com/~ahwahnee/VWBattery1.JPG

... Harbor Freight has their cheapo multimeter on sale for $1.99.
Pretty easy to use direct or convert the leads to a 'cigar lighter'
plug or even hack for the needed parts.

The dingus (though the website does not show the sale price):

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899

Geo