The static battery reading if accurate, should be about 12.6 volts and should not decay too much sitting for days. The open voltage of a battery is not a good method of monitoring it but 12.6 is healthy unloaded voltage = assuming it has sat for several hours. Just after turning car off, it will be higher than the true rest voltage.

BatteryStuff Articles | Guide to Understanding Flooded, AGM, and Gel Batteries
See the chart in text.

http://www.scubaengineer.com/documen...ing_graphs.pdf

See the chart reading for REST VOLTAGE

Enjoy Woody