The electrical system is split into two main parts:
The 'high voltage' system is derived from the battery and this could be anything from about 100V to 800V. There are a few manufacturers working with 800V systems though (Hyundai and Kia) due to the safety issues. The main reason for using higher voltages is the ability to use more powerful motors.
We are using a 12V battery in our MX-5e and the OSSEV 1 because it is needed for the following items even when the EV is switched off:
It is possible to have a dc-dc converter connected to the main battery all the time but, this is not very energy efficient and you often want to disconnect the high-voltage circuits for safety reasons. Having a 12V battery makes it much easier to test things in the MX-5e as we build it, without the main battery being required at all.