Check battery voltage before starting engine, after it has been sitting over night or at least 5 hours, this is called "at rest" battery voltage
12.8v is a newer battery
12.5v is a 3/4 year old battery
12.3v is a 5/6 year old battery and time to shop for battery sale
12.2v or lower is dying battery
Car batteries work by discharging and recharging via a chemical reaction between plates inside, over use and time the chemicals and plates wear out
And plates can short out, causing battery to be self draining
If you hook up volt meter to battery and it shows, say 12.57v, leave it connected and watch voltage, if it goes to 12.56v then it might be self draining
Unhook battery from vehicle and test again, watching battery voltage over a few minutes, if it is stable, 0 change, then vehicle is draining battery even with key off.
If voltage still drops then battery has internal short, replace battery
An alternator is preset to maintain 13.5v to 14.9v
After startup voltage at the battery should be above 14volts but below 15volts, this is "recharge voltage" it should only last for a few minutes or battery will get "cooked"
After a few minutes voltage should drop to under 14volts but stay above 13.4volts, this is vehicles "running voltage" and acts as a "trickle charge" for battery, keeps it charged, but won't "cook it".