A fully charged 12 volt automotive battery should have 12.68 volts.
12.6 volts when fully charged.
12.6 volts when fully charged.
Virtually all automotive batteries nowadays are nominal " 12V " batteries, but will measure up to 13.6V when fully charged.
12 volt batteries if fully charged should show 14.2 volts
There are two measures, cell voltage and specific gravity of the electrolyte. Cell voltage is the simplest to measure. For most automotive, flooded cell (having liquid electrolyte) batteries, 12.6 volts is considered full charge, although it is typical to have 13 volts or a bit over. The specific gravity of a fully charged battery, measured with a hydrometer, is 1.265.
Not fully, but these days come more than half charged, yes.
No
Batteries used in UPS, emergency power supplies and as a power source for instruments, commonly of the VRLA or deep discharge tubular types employ float charging techniques to keep the batteries constantly topped up. Float or maintenance charging method applies a continuous voltage fractionally higher than the fully charged voltage of the battery so that the battery never overcharges but remains in a fully charged state all the time.
12v
The two batteries can be wired in series BUT unless they are identical batteries in an identical state of charge they should be checked regularly to see if one of them is fully charged (the voltage has risen to 14.5 volts).
12.7 volts
12.8 - 12.9 volts is a fully charged battery