Here is the voltmeter that I use - the Lascar EM1200. http://www.lascarelectronics.com/tem...datalogger=116

What is so nice about this unit is how it mounts using only one threaded (hollow) stud through which all the wires run. Drill one hole and you are set. Great stuff.

There is also an analog version. http://www.lascarelectronics.com/tem...datalogger=111

Simpler still is the single multi-color LED from http://www.gliderguider.net/batttery_monitor.htm.

Enjoy! - John