Is it a 12v battery?

[Amvcane]

Fun fact: once a battery reaches 12.4 volts it is at only half charge.

That is correct.... only at 10* F. At 80*F a battery with a voltage measurement of 12.4 volts will be closer to 75% charged.
If you are taking a battery voltage reading to determine state of charge you must also measure ambient temperature. With these 2 pieces of data find a battery state of charge temperature correction chart on the internet to determine % of full charge.