Yes you can use multimeter to measure battery voltage to get its state of charge using the follow table. Fully charged battery voltage is ~12.6v. Note you need to wait at least half day(24-hr is more desired) after it finishes charging to have more stable reading from multimeter. Right after you shut off the car, the battery voltage is ~13.5v, which is not real.
Depending on car model and battery, the battery should be able to start your car after it sits idle for at least 2 weeks ~ 6 months.
Battery Approximate Average Cell
Voltage State-of-charge Specific Gravity
12.66 100% 1.265
12.45 75% 1.225
12.24 50% 1.190
12.06 25% 1.155
11.89 0% 1.120
from http://www.type2.com/library/electrip/batfaq.htm
Depending on car model and battery, the battery should be able to start your car after it sits idle for at least 2 weeks ~ 6 months.
Battery Approximate Average Cell
Voltage State-of-charge Specific Gravity
12.66 100% 1.265
12.45 75% 1.225
12.24 50% 1.190
12.06 25% 1.155
11.89 0% 1.120
from http://www.type2.com/library/electrip/batfaq.htm
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