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All the auxillary functions get powered from the 12V system, in addition to all the relays and main contactors. If the 12V lets go while driving or charging, then the relays open and will arc and damage the precious metal surfaces of the main contactors and/or weld them.
If you need to change a flat tire on a dark and rainy night, then you might want to have some reserve available for hazard lights and wipers.
The typical lead acid battery is not designed to operate with much discharge; they are best kept full all the time or will sulfate and degrade. So you need a voltage regulator to keep it charged up but not overcharged, if the DCDC doesn't have that feature then one will need to be added. A 12 Ah is probably only good for 3Ah or less before it needs to be recharged.
Failure of the 12V aux battery in an EV causes much worse problems and potential damage than in an ICE car; that is why the OEMs use full size starter batteries for the aux.
If you need to change a flat tire on a dark and rainy night, then you might want to have some reserve available for hazard lights and wipers.
The typical lead acid battery is not designed to operate with much discharge; they are best kept full all the time or will sulfate and degrade. So you need a voltage regulator to keep it charged up but not overcharged, if the DCDC doesn't have that feature then one will need to be added. A 12 Ah is probably only good for 3Ah or less before it needs to be recharged.
Failure of the 12V aux battery in an EV causes much worse problems and potential damage than in an ICE car; that is why the OEMs use full size starter batteries for the aux.
