For registration in NSW Australia every electric vehicle conversion needs a 12 Volt battery to power the ancillaries, primarily the hazard lights. But it is also pretty convenient for powering pre-charge circuitry and contactors when starting the car. In MR02EV I recently upgraded to a Drypower 12V 22Ah sealed lead-acid battery.
I chose a sealed lead-acid battery for its rugged construction and because they are designed for a trickle (or float) charge, which is most of its life in MR02EV.
The Drypower 12SB22C has 22Ah capacity which is a significant increase from my previous 15Ah auxiliary battery. In general vehicles need a pretty stiff 12V supply (minimal voltage drop under load) to keep the electronics happy especially at night, in the rain, when the wipers and lights are on. I suspect a contributing factor to the premature death of my last two DC-DC converters was the lack of a decent 12V battery to buffer the effects of short, sharp 12V loads.
I have a Degree in Electrical Engineering from the University of Technology Sydney and am employed in Broadcast Television Industry. My hobby is the (continuing) modification of MR02EV. I am also the National Treasurer of the Australian Electric Vehicle Association.