Although the power-hungry components (Motor, heating and in some cases the A/C) of electric cars are supplied with 400V, electric cars still have a 12V system. This is also active, when the car is switched off. It is responsible for restarting the 400V network when the car is switched on. For safety reasons, the 400V network is only active when the vehicle is driving or charging.
The 12V is also suppling all on-board computers and comfort systems with power. Radio, navigation system, all lights, seat heating and everything else also depends on the 12V. In order to supply them with electricity, there is a so-called DCDC converter. Its task is to convert the 400V from the large battery to supply the 12V and charge the 12 V battery. The DCDC converter is only active when the car is charging and when the car is turned on. In some cars it can also switch on, when the 12 V battery is getting close to being so discharged, that soon it won't be able to turn on the car anymore. This may happen, when the car isn't being used for multiple weeks or months.
