Going camping nowadays involves taking lots of electronic equipment whether for day to day running or for fun and entertainment. Most of the time a charged lead acid battery and a power inverter would be used to ensure a smoothly organized holiday where ideally the missus and the children cheerfully use their electric and electronic gear! With rechargeable lead-acid batteries it’s invariably useful - if not essential - to determine whether the power source you’re hauling along on your travels is losing capacity and needs to be topped up. The same circuit would also come in handy when going on a car or motorbike trip as it can check the status of a 12 V (car) or a 6 V (motorcycle) battery.
Although the circuit draws so little power that it will not noticeably load the battery under test, it should not be left connected permanently. The circuit employs the familiar LM3914 (IC1) to display the voltage level. The LED readout creates a battery status readout: when the top LED lights, the battery is fully charged. When the bottom LED lights, the battery needs imminent charging! Switch S1 selects between 12 V and 6 V operation. A series diode, D1, protects the bargraph driver from reverse supply voltage. A color-coded display with individual LEDs could be used instead of the common-anode bargraph display for better indication of the state of the battery.
author: Joseph Zamnit, Elektor Electronics 2008
circuit from http://www.extremecircuits.net/2010/05/car-and-motorcycle-battery-tester.html