The "Magic Number" is at 50% battery capacity which is 12.1 volts.
Voltage below that number will cause permanent reduction in battery life and recharge capacity.
Excessive depth of discharge is the number one cause of early battery failure. Using the "idiot lights", that number is just when the "G" (good) light goes out and no more. Waiting for the fair (F) light to go out will cause real damage. When the "L" (low) light goes out it is time to buy a new one.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW