A new hardware project: Crazy Volts. Drew and I are working on a fuel gauge for his Crazy Cart. It's based on the ATmega328P + Adafruit's OLED display, and a custom PCB to hold it all together.
The power switch for the cart has 3 leads: ground, battery, and switched to controller. The controller lead is switched and provides the voltage sense, so we just wired the input + power leads together. The display running in the pic above shows our calibration test, and the display itself hasn't been permanently mounted yet.
The design is general enough to be adapted to anything with batteries. In software we'll probably only try to estimate the power characteristics of SLA, but it's just software....
2 layer board of 1.55x2.05 inches (39.47x52.15 mm).
We also considered using a general-purpose ATmega328 breakout (the bduino), or a teensy 3.1 + adapter board to wire up a regulator + OLED display.