from: http://www.engadget.com/2011/08/30/nissan-leaf-mod-offers-more-accurate-battery-status/:

Car mods take many forms – including, even, a piece of cardboard with some switches and LED display. The car is modded in this case is the Nissan Leaf , and the mod in question (still in its temporary form here, fortunately) is simple but important. This is a homebrew device that connects to the port of the sheet of diagnostic OBD-II and shows the exact state of battery charge, as opposed to a little less specific bars that are displayed on the table leading edge. Absolutely necessary? Not exactly, but the mod seems to be catching on with a growing contingent of DIY spirit of the forum members Leaf. Follow the source link below for a few more and some more details to build your own.