For four and a half years I've been operating without any battery instrumentation. Professionally I work in the field of battery charge estimation, algorithms that decide whether to light 1 LED or 5 LEDs. My cousin recently gave me a ride in his Tesla Model S and pointed out the irony to my lack of battery charge gauging. I simply make sure I don't drive more than 40~50 miles on a charge.
When in a pinch I have read the voltage of a single cell with a voltmeter to get an idea, when fully relaxed these voltages are directly related to state of charge.
I've run out of charge only 2 times so far. One time because I forgot to charge the night before. And the other time is still a bit of a mystery. I thought it was perhaps due to running the air conditioning over lunch break but that would not account for how much charge I lacked to get home. Both times I simply called AAA and was done.
There is a communication port on my battery management system and I've used it for diagnosis using a laptop and to log data using an iPhone. The iPhone was really too small to be practical and was abandoned. My family recently upgraded from an old iPad to a Kindle Fire and I've adopted the iPad with cracked screen to up my game.
As a security feature it hides nicely under the passenger seat. With this I can monitor and log voltages, temperatures, and current. The app I'm using is called Get Console and is basically a HyperTerminal for iPhones. I hope to do some more data collection and efficiency calcs soon!