The vary bare minimum charge rate for standard lead-acid batteries is 0.2V over the nominal 2.10 volts per cell at 25°C (80F). 0.3V rate is desirable, and 0.4V high for warm weather but acceptable for cold.

That would make 13.8 volts a minimum warm charging voltage but insufficient for cold weather. 14.4 V is nominal and 15.0 V near the high end for all year setting. Higher intial voltage, such as 15.5 when cold, using a temperature compensated regulator help warm the battery faster. To the battery, the difference in surface charge after 14.2 volt versus 13.9 volts is significant.