After start up, battery should read above 14volts but under 15volts, this is "system voltage", if not alternator is not ON
This could be a blown fuse or alternator failure
After 10min of engine running(and still idling, no restart) battery voltage should read 13.5v to 13.9v, under 14v, if alternator runs above 14volts too long it will ruin the battery, same as if its not working at all
A failing battery can also cause system voltage to stay above 14volts, see below
A Battery is ONLY used to start the engine, the alternator powers the WHOLE VEHICLE once engine is running
The 14/15volt is a quick recharge in case engine stalls and you need to restart it(use battery again)
After a few minutes battery is recharged and system voltage drops under 14v, but stays above 13.4v to KEEP battery charged for next start up
New battery voltage, key off, should be 12.8v
3 year old battery 12.5v
5/6 year old battery 12.3v and time to shop for battery sales
12.2v or under is a dying battery, drained, and will not "hold a charge" very long, this causes alternator to run above 14volts, longterm, it is trying to recharge an "un-rechargeable" battery