Electric vehicles (EVs) may be dramatically less mechanically complex than their traditional internal combustion counterparts, but that's where the simplicity ends. The battle that began with Thomas Edison and Nicola Tesla over direct vs. alternating current continues to this day, the battlefield has just shifted to the EV powertrain. Electrochemical batteries can't store alternating current, the electrons will only flow directly in or out.

That means an interface is required between the battery and the highly efficient AC traction motors/generators as well as the electrical grid used for charging. That interface handles the transformation between AC and DC.

Despite the fact that the pretty much the whole world uses alternating current, countries can't even reach consensus on voltages, currents and frequencies for electric power. As a result, automakers design chargers to be installed on-board EVs to regulate the power flow to the battery to prevent over-charging or heating. Think of the grid as the water main coming into your home. The vehicle charger behaves like a combination flow-restrictor and anti-scald device on your shower head or bath tub faucet. Read on after the jump for more on the hidden complexities of EVs.

[Source: Plugin Cars]

American homes typically get power at up to 240 volts and 200 amps and the normal three-prong outlets in the house supply it at 120 volts and 15 amps per circuit. Those standard outlets are what is known as a level 1 supply. The so-called level 2 charging stations can provide 240 volts and up to 60 amps but most homes are only wired for 30 amp, 240 volt circuits for devices like dryers and ovens. Most home level 2 stations are likely to be sitting on these 30 amp circuits. A 60 A circuit can push 14.4 kilowatts of power while a 30 A circuit does about 6.5 kW after accounting for losses in transformers. That reduced output means more than double the charging time.

However, the real bottleneck in these first EVs will be the on-board chargers that convert the AC to DC for the battery. The Nissan Leaf and Chevrolet Volt are equipped with 3.3 kW chargers which will double the charge time again. In the case of the Volt, with its smaller 16 kilowatt-hour battery (of which only 8 kWh is used), it should be able to charge in 3-4 hours on a 30 A level 2 station. The Leaf, on the other hand, uses about 90 percent of it 24 kWh capacity and will need 10-12 hours to fully charge.

In the case of the Volt, General Motors likely opted for the lower-power, lower-cost charger because the smaller battery could get by with it and slower charging will help avoid overcharging that can slash battery life. Nissan hand probably chose the 3.3 kW charger because it's a closer match for the 200 V/15 A circuits used in Japan.

Finally, because of its range extender and smaller battery, the Volt doesn't have support for DC fast charging (sometimes called, somewhat erroneously, level 3 charging). The Leaf will have optional support for 400 V DC charging, but since no AC-DC conversion is needed, this big-pipe technique simply bypasses the on-board charger and the energy goes straight to the battery.

Share This Photo X