Hey in my recent GEM car restoration project (Electric Car) I have come across a bunch of cool battery guru's & battery tech. In the electric golf car & electric car world battery monitoring systems are very common and much more Hi-Tech then anything I have come across in the 12v Stereo world.
And example is this Batt Six (BMS) module. They have 2 System's
#1 a Heads up 6 inch display unit. It gives you digital TFT display that shows you real time INDIVIDUAL battery voltage and strength
#2 A blue tooth unit that send's the battery info to your smart phone Android or I-phone.
Currently these unit's are set up for Battery systems running in Series, Our boats run our 12v ststem's in parallel.
Here is a look at the Smart phone display