Is 13.9 battery voltage bad?
The ideal car battery voltage with the engine running is between 13.7 and 14.7V. With the engine off, you should get a reading of 12.6 volts. If the battery isn't fully charged, it will diminish to 12.4V at 75%, 12V when it's only operating at 25%, and down to 11.9V when it's completely discharged.
Is 13.5 V enough to charge car battery?
To charge the car engine must be running. To explain further, the open circuit voltage of 12.9 volts is not enough to move the chemistry even in a fully discharged battery. You need to be above 12.9 to start the chemistry moving at all, and up at 13.5V to charge at a significant rate.
Is 14.8 volts too high for battery?
What voltage is too high for an alternator? For standard passenger vehicle 12v systems, anything avove 14.8v is worrisome. Anything below 13.6v will not adequately charge the battery.
Is 15 volts too high for battery?
If it is really 15 volts it could cause a problem with some batteries. Regular lead acid won't care much but the AGMs need lower voltage and can be damaged with high (> 14.8 ) voltage.