For people familiar with conventional engines, looking under the hood of a fully electric vehicle (EV) is a mystifying experience. The main difference will, of course, be the lack of an internal combustion engine (ICE). Instead, they are likely to find the electric traction inverter, which is often the same size and mounted in a way that resembles a conventional engine. Other systems will look even less familiar, but there is a good chance they will be able to identify one component that has changed little - the 12 V battery.
In a non-EV vehicle, the 12 V system is required to power the starter motor, which provides the initial rotation of the ICE to start the four-stroke combustion cycle. Given that an EV has no need for a starter motor, it may be surprising to discover the presence of a 12 V battery. However, the majority of an EV's electrical systems still operate from 12 V. With no ICE or alternator present, the 12 V system must be completely powered from the high voltage, traction battery.
This presents an interesting design requirement. The traction inverter system is likely to operate at a DC voltage in the region of 800 V. This high DC voltage is converted to AC in order to drive the traction motors. However, the traction battery in an EV is not simply multiple 12 V batteries connected in series to create 800 V; it is a sealed unit. The addition of this high voltage system, and its role in the vehicle, means the 12 V system is now commonly referred to as the auxiliary system. It powers everything that is auxiliary to the traction system (including the traction control system).
The main high voltage battery is now responsible for providing power to the 12 V auxiliary system in order to keep the battery charged. For safety reasons, it needs to do this in a way that