aerostich used to sell an led digital guage with multiple bars - I got one at aircraftspruce.com for 1/2 the price. It wasn't very accurate compared to my handheld meter.
What I do use is from RadioShack for $5 - you could hard wire it to the battery or add a BMW plug.
http://www.radioshack.com/product.as...ct%5Fid=22-112