It's a 12v system, but I think anything in the 13.8 - 14.2 Volt range is healthy. I would check it and record your readings. If it is remaining steady over a period of time it's probably ok. If you notice a continue deterioration then you likely have another issues such as alternator or battery failure.
The battery is fairly new and was at 12.31 Vdc today and the car has net been used for over a week. After starting the alternator was putting out 13.8 V today but the "GEN" light did not go out for about 5 minutes. The "GEN" light staying on after starting is what got me started on this investigation.
The voltage measured at the battery during the start sequence was:
After sitting for 7 days : 12.31 V
With key in on position, head lights come on: 12.0 V
During start : 10.8 V
Immediately after starting: 12.0 V
It took about 15 seconds before the voltage increased to 13.8V
It stayed there even as I changed engine RPM.
"GEN" was on for about 5 minutes then went out. I don't know what the voltage at time was.