The DC/DC converter effectively replaces the alternator on an Internal Combustion Engine (ICE) car. Instead of taking energy from the rotation of the ICE motor to charge the 12V battery, it pulls power from the main High Voltage (HV) battery pack and converts it down to 12V. The 12V systems (headlights, stereo, seat heaters, etc.) use a lot of power and would quickly drain the on-board 12V battery if it were not charged while driving.
The stock RX8 has a 100A alternator. Multiply this by 13V (a more typical operating/charging voltage) and you get 1,300 watts of power. So whichever DC/DC converter we choose, it must have at least 1,300 watts of 12V output power. This wattage may vary for your particular vehicle conversion, but this is a pretty universal alternator wattage for passenger cars.
Some DC/DC converters also have the ability to boost convert. In other words, it can work backwards to take power from the 12V battery and boost it up to about 300V to charge the HV pack. There really isn't a strong need for this feature, but in OEM applications it could be used to 'un-brick' a HV pack that has fallen too low in charge to safely wake up. For personal EV conversions though, I plan to just throw this in the book of future features to implement if I can find a need someday.
(Delphi DC/DC Converter)
I chose the Delphi 2.2kW DC/DC Converter. It's rated at 2,200 watts (175A at 12.5V) and this should be plenty to supplement the 12V systems of the car.
The Delphi module was used in production on the Coda EV for brief production run, then Coda went out of business. This leads to pretty much the only downside of this Delphi DC/DC Converter... a special connector that appears to be impossible to buy now. Follow along in the RX8 blog if you're curious how I swapped out the connector for another one that is more readily available. CAN specifications are also available in the RX8 Conversion Build Blog.
Other Notable Options:
DO NOT BUY a Chinese DC/DC Converter! OEM solutions should pretty much be your exclusive choice here. I've heard nothing but horror stories of these cheap Chinese DC/DC converters failing and leaving a well built, professional, expensive conversion stranded on the side of the road.
The three most common OEM DC/DC converters are the Chevy Volt, Nissan Leaf, and Delphi DC/DC I chose. The Volt DC/DC Converter is called the Auxiliary Power Module (APM), but it serves the same purpose. The CAN specs for the Delphi and Volt converters are pretty well know and available because of the awesome work at EVTV. If you're not super programming savy, they've developed CAN controllers for both of these two units.
(Chevy Volt DC/DC Converter)
The Nissan Leaf Converter on the other hand has gotten little support from the DIY community and I would consider it an inferior choice compared to the others because it also contains a HV junction box and the HV charger. This drives up the size, weight, and complexity of this solution.
(Nissan Leaf DC/DC Converter)