For many people, it doesn't matter. DC is faster, and that is all that they need to know. But for the curious, this is a simplified explanation of the difference between AC and DC charging. Technical details are intentionally glossed over here.
The reason we have two types of charging is that there are two "types" of electricity, AC and DC; so we shall start by discussing them.
AC VERSUS DC
DC is the simple positive-and-negative type of electricity that you probably experimented with in 7th grade science. A key advantage is that it is easy to store in batteries. That is why portable electronics - flashlights, cell phones, laptops - use DC power; they have to store it. Plug-in vehicles are portable so they use DC batteries too (although most of them have AC motors - a complicating step we may consider another day).
AC electricity is a little more complicated because it switches back and forth, but a key advantage is that it can be transmitted economically over long distances. That is why AC power comes in through the power lines to your home, and is what is available at power outlets. Stationary appliances that use electricity directly from an outlet - lamps, refrigerators, washing machines - use AC power.
Because the electric grid provides AC, the electricity must get converted to DC when you want to charge a portable device. This conversion is done by a "rectifier". Portable electronics that recharge from wall power all have one: it is usually in a black box in the charging cord, along with some other components we will ignore. You'll notice that the more power the device uses, the larger that box is. The key to understanding AC versus DC charging is learning where the box is, and why.
CONVERSION IN PICTURES
Here is the DC charging solution for my tablet computer. It is simply a USB cable, which allows my tablet to charge from a DC USB port in a car or laptop. Both sides have DC, so no conversion is required.
Now, here is my tablet's AC charging solution. The same USB cable plugs in to a little black box that plugs in to an AC outlet - the box converts AC to DC.
Here is a simplified diagram (can you tell I didn't take art classes?) of how AC and DC charging work with a plug-in vehicle:
When you plug in to AC power - whether you plug in to a 120V or 240V outlet, or use J1772 charging equipment - your car converts the power to DC.
When you use a DC charging station - CHAdeMO and Supercharger are the varieties in active use, with CCS coming soon - the power is converted by the station, so DC goes straight in to your battery (not really, but close enough for this discussion).
Note that in both cases the power starts as AC and ends up as DC; the only qualitative difference between "AC charging" and "DC charging" is whether the conversion is done before or after it goes in to your car.
WHY USE BOTH TYPES?
Why bother with two types of charging - why not choose a single place to convert the power?
AC is more readily available at power outlets, but despite AC lines carrying vast amounts of power, outlets are limited. Dedicated DC charging stations provide more power, but being expensive to install and dedicated to plug-in charging, availability is limited.
AC PROS AND CONS
AC outlets are ubiquitous, so to make charging convenient your car should be able to plug in to them. That means every car has to be able to convert AC to DC. The conversion equipment in current plug-in cars varies; most can convert up to 3.3, 6.6 or 9.6kW of power.
For comparison a typical household outlet can continuously provide up to 1.4kW, and "high-power" 240V outlets sometimes found in garages and RV parks can provide up to 9.6kW. It is technically possible for a car to convert far more power than that, but the equipment would be bulky, heavy, expensive, and hot - and anything over 9.6kW would see infrequent use because higher-power outlets are not available.
To illustrate this point: the Tesla Model S offers a $1,500 option that allows the car to convert up to 19.2kW. Twice-as-fast charging is obviously an enormous benefit when you can use it, so some owners swear by it - but you can only get that much power if you use special hard-wired 240V charging equipment. The West Coast has a few such chargers along popular travel routes, but such equipment is hard to find, not needed for overnight charging, and still far slower than DC charging. Many owners skip this option to save money and weight.
DC PROS AND CONS
DC charging stations have special grid hookups so they can get and convert far more power. DC stations are big, expensive and have a lot of cooling - it wouldn't be practical to put that equipment in every car, even if there was a way to plug directly in to the grid.
CHAdeMO chargers vary from 25 to 60kW, and Superchargers are 90 to 120kW - almost 100 times faster than a standard 120V household outlet, and more than 10 times faster than 240V AC outlets.
At higher cost, the grid could supply even more power; but these limits are largely set to avoid harming the car batteries while charging. (Many factors determine how fast batteries can charge, but currently cars that use Superchargers have significantly larger batteries than cars that use CHAdeMO chargers. All else being equal, larger batteries can accept more power without harm).
EXAMPLE: MODULAR CHARGING
An easy way to visualize the AC/DC charging differences is to consider how Tesla handles charging for their Model S sedan. They make large quantities of boxes they call "chargers" that include a 10kW rectifier to convert AC to DC. Every car they build gets one for AC charging, and so can handle all the power than any outlet provides. Plugged in to the right outlet, this can charge a car at up to 24 mile of range per hour.
If you buy "twin chargers", you get two boxes in the car and can now handle high-power hard-wired charging equipment as well. This can charge the car at up to 50 miles of range per hour.
Tesla's DC Superchargers have a stack of 12 boxes installed at the station so the car doesn't have to do the conversion. This can charge the car at up to 300 miles of range per hour.
The boxes that do the power conversion are essentially the same; AC versus DC is largely a matter of whether the boxes are in the car or in the charging station, and how many of them there are.
Did this help you understand the difference between AC and DC charging, and why both types are important? Did you see any mistakes? (Not counting omitted details as they were left out on purpose to keep it simple - unless they are salient). Do you have additional or different points to help understand the difference? Are there related topics that you would like to know more about? Please let us know.
Posted by Chad
Images courtesy Plug In America, Aerovironment, Schneider, Microsoft Office Clipart