Last Updated: 10/03/09
EVs need 12 volt DC power to operate standard automotive accessories, such as lights, horn, radio, fans, and such. There are several ways to provide this 12v power.
You can use a 12v battery and the original alternator, belt driven off the electric motor. It works the same as it does in a normal car, i.e. it's not particularly elegant or efficient.
Some budget EVs eliminate the alternator, and just use a large deep-cycle 12v battery that gets recharged when the traction battery is recharged. It works, but makes for weak headlights and poor accessory performance unless the 12v wiring is improved to eliminate the typical 1-2 volt drops between battery and loads.
Another less-than-satisfactory approach is to tap the traction batteries to get 12v. This tends to unbalance the batteries, and creates safety problems unless the traction pack is itself a low voltage (24-48 volts).
Most modern EVs thus use a DC to DC converter. This is an electronic power supply that takes high voltage DC power from the car's traction battery pack, and provides an isolated 12 volt output to power standard accessories. They are small, light, silent, and have no moving parts. The DC to DC converter is usually set to provide a solid 14 volt output so lights and accessories work the same as they would in a normal car with the alternator charging the battery. The most common DC/DC converters used in conversions are made by Todd, Sevcon, Curtis, and Vicor.
Special thanks to Lee Hart for the assist on this page.
Suitable DC/DC converters for electric conversions are available from most EV parts suppliers. Follow this link for a current list.