I found a couple of sites for a neat plug in voltage meter. I haven't found these at walk in stores but they are available online. You can check individual batteries via the battery switch selection. They tell current voltage and status of charge on the battery circuit
Radio Shack http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F004%5F014%5F003%5F000&product%5Fid=980%2D0706 (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F004%5F014%5F003%5F000&product%5Fid=980%2D0706)
http://www.campingworld.com/browse/skus/index.cfm?deptID=3&subOf=61,55&skunum=19983 (http://www.campingworld.com/browse/skus/index.cfm?deptID=3&subOf=61,55&skunum=19983)
Barb K : 12+ yrs ago (before I had a battery monitor) I installed two female bannana plugs on the main pannel at the Nav station. Just plug in a multimeter (on Volts) and you have a voltage reading depending on which battery the selector switch is on. I made a small chart with approx. %state-of-charge for 14 voltage readings.
You need a multi meter on the boat anyway, so why go out and buy something else? Interested in how I wired it and a copy of the chart let me know. ronphylhill@erols.com
Ron, Apache #788