Worth mentioning is that if you're going to use a voltage readout to determine when your battery is dead you should lookup the datasheet for that make and model on the internet. Different battery brands and types have different points where the manufacturer deems it "empty" and you shouldn't continue to discharge beyond, normally around 11.8V. Even deep cycle batteries can get damaged by over discharging though the point is typically around 10.5V or so.
I also use a dual volt/A display meter from ebay.
http://www.ebay.com.au/itm/20V-20ADC...item1c29453a19