Assuming the 12 volt battery is lead-acid, there MUST be a regulator controlling the output of the DC-DC down-convertor. There are systems that control the charging rate of the battery via sensing the battery's thermal level but I think not here. But even those must have a method of tapering the charging voltage down when/if the battery's thermal level begins to rise above a safe level.

If you were to apply a SOLID 14.4 volt source to a lead-acid battery with only a 10% charge you would soon have boiling electrolyte and shortly thereafter a very high probability of an explosion.

In short, the lead-acid battery's charge level will ALWAYS dictate the 12 volt buss' actual voltage level.