12.6 volts 100% charge
12.4 volts 75% charge
12.2 volts 50% charge
12.0 volts 25% charge
Any less and the battery is essentially dead.
Try start the car. Check the voltage.
below its desired voltage
A car cannot recharge its own battery if the battery is completely dead, because you need excitation voltage in the rotor to develop a voltage in the stator. Besides, you need voltage to run the ignition/computer system, and you need voltage to start the engine. That said, most batteries are not completely dead. If you can jump start the car, and assuming there is no electrical problem anywhere, you can run the engine and charge the battery. Problem is that most car batteries are not designed for deep discharge, and the probability that damage was done is greater than normal. Also, if the battery is very deeply discharged, you probably ought to charge it with an external charger (with the battery disconnected from the car) until its voltage is somewhat normal and its electrolyte is within limits for specific gravity.
The problem is probably the alternator. If the alternator fails to keep the battery charged the battery will keep the car running until it can no longer supply voltage to essential systems and the car will stall.
Any modern day car's ideal battery voltage is 13.8Volt. In practive however the voltage you'll find on a charged battery when testing will be in the region of 12.5V to 13V A fully charged battery will read 12.6 volts with the engine not running. With the engine running at idle speed it will read from 13.5 to 15.5 volts.
There is no fuse between the headlights and the battery. If the headlights are dead, then your problem comes down to: battery (may have some voltage, but no current) and the battery cables.
Start car. Turn on all lights, blower etc., that you can. Read voltage across battery terminals. If voltage is 13.5 or more then your alternator is okay. This assumes you have clean connections, good cables.
A fully charged car battery will read 12.6 volts, 75% charge will read 12.4 volts, 50% charge will read 12.2 volts, and a 25% charged battery will read 12.0 volts with the engine not running. With the engine running it will read 13.5 to 15.5 volts.
With the engine off check the voltage at the battery. A fully charged battery will read 12.6 volts. At 75% charge it will read 12.4 volts, 12.2 at 50% charge, and 12.0 volts at 25% charge. Now start the engine and check the voltage at the battery. It should read 13.5 to 15.5 volts if the alternator is working properly.
check the voltage regulator
get a multi meter that reads dc voltage & touch the black (negative -) wire to the negative battery post & the red (positive +) wire to the positive battery post with car running. car off is battery voltage, car on is alternator voltage.
when cranking the engine the battery voltage should not fall below 9.6volts if so replace battery