10.14.2013 - by Mary Kathryn Campbell
AC versus DC charging – what is the difference

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.

DC charging cord

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.

AC converter to DC cable

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:

AC and DC charging paths

 

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.

Max power from various sources

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.

FEEDBACK

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

13 comments on “AC versus DC charging – what is the difference”
  1. You cover the differences between AC slow charging and DC fast charging – but it seems like a US-centric view: in Europe we have AC fast charging (e.g. on my Renault Zoe), typically at 22kW or 43kW, and that seems to have been ignored.

    Trevor
    MyRenaultZoe.com

    1. Chad says:

      Hi Trevor, thanks for reading and commenting. Plug In America is indeed a US-centric organization. We would like to change that; but for now our volunteers are kept plenty busy focusing on the US. 43kW AC charging sounds awesome! Do you have to add an option to the Zoe to be able to convert that much power? Do you know any details like how much it costs, or weighs? Thanks.

      1. Guillaume PORCHER says:

        @Chad : actually, the Renault ZOE has a charger within the car that convert AC to DC current up to 43 kW. Therefore, one cannot speak about AC charging.
        The “cameleon” charger is the name Renault gave to this charger. The basic idea is to offer electric car some chances even out of big cities, tanks to cheaper charging station (AC).

        1. Som says:

          Thanks for article.

          Hi I can see 2 input for any charging IC eg: max1555EZK+T.
          DC and USB.

          Could you please explain me the difference in terms of charging.
          What should be input for charger.

          One more question :
          I am using 3100mAh and MAX1555 is not suitable for this battery.
          Could you please suggest me correct IC for my Li-Ion of 3.7V and 3100mAh capacity.(It’s typical samsung Note 2 battery)

          Thanks,
          Som

  2. Raymond Ramirez says:

    I am an EE and I have been working and experimenting with AC and DC for over 50 years. The Volt motor can run with AC, but the only method that exists to store alternating current is in a flywheel generator. So for now, all electrical energy storage for consumer use is in a DC format, using mostly a chemical battery (there are electrostatic batteries but are not efficient).

    As for DC charging, the most common method is to supply a higher voltage than what the cell chemistry uses. In a lead-acid automobile battery which has six 2.2 volt cells in series, the alternator applies more than 2.4 volts per cell, or about 14.4 volts for the entire battery (you can see this by measuring across the battery with a DC voltmeter while the engine is running). Lithium-ion cell chargers (such as USB types)used in mobile devices, such as smartphones and tablets, apply up to 4.2 volts for the cell. The cell voltage average is 3.7 but actually operates between 3.4 volts (discharged) and 4.0 volts (fully charged.

    There is another item that hardly anyone notices, but the actual charging method in use is ADC (alternating direct current). True AC alternates between a positive and a negative current flow (like a pendulum) and crosses zero volts twice per cycle or Hertz (home current cycles sixty times per second or at 60 Hz), while direct current only flows in one direction and is constant. ADC is a current flow that alternates between a positive maximum and zero volts.

    Why is it better? Because it allows the ions in a chemical battery to move when the voltage is at the maximum, then allows them to organize against the electrodes when the voltage is zero. This is like trying to get people to move into a conference room. You apply some pressure to move them, but release the pressure so they can move more freely and get in line across the rows. A constant pressure will move them but they will not fit well, create some resistance and friction, and cause some to trip and fall on the floor. Ions are atoms with missing electrons and are just as hard to move!

    Proof? Get a multimeter that can measure both AC and DC, then (using the plug-in USB charger as an example) measure the AC voltage first on a 20 VAC scale. You will see up to 7 VAC! (remember that USB is based on 5 VDC) Now if you could use calculus for a half wave signal and use 7 as a peak voltage, the average or RMS (Root-mean-square) is about 4.1 volts, which is the average DC measured at the same time, and about the correct DC voltage that will charge a lithium-Ion cell.

    Now you see why you need an engineer to explain , design, and manufacture there charger devices! Just remember that DC storage devices use a charger with some AC in it.

    1. Chad says:

      Hi Raymond, thanks for all the extra details. Especially about ADC – that was news to me.

    2. Sethi says:

      Hello Raymond, Chad. Thanks for sharing this valuable knowledge, it really helps. I know I’m a bit too late for this conversation, was looking for information on AC and DC mobile device charging and which is better of the two.

      So Raymond, I want to know if I understood you correctly. You mean that in case of lithium ion batteries, the AC (which you say is actually ADC) charging option will give you the most closer value to the correct specified voltage ? Is this true for all storage batteries say like an I-pod/MP3 player battery ?

      And by this do you also mean that the rectifier in the charging device does not completely transform the AC to DC ??

      Looking forward to learning something from you on this..

    3. People at work are using 120v phone chargers and plugging in to 87 volts DC and charging their phones/mobile. What’s the charging voltage and is it DC or AC!!
      Lithium-ion cell chargers (such as USB types) used in such as smartphones and tablets mobile devices, apply up to 4.2 volts for the cell. The cell voltage average is 3.7 but actually operates between 3.4 volts (discharged) and 4.0 volts (fully charged. There is another item that hardly anyone notices, but the actual charging method in use is ADC full meaning alternating direct current.

  3. robert christ says:

    If time isn’t an issue why couldn’t I trickle charge something like a chevy volt or tesla right from a 1kw array? I’m guessing you can’t do this for one reason or another. Instead you need a larger array and a battery bank and an inverter which would not make it cost effective.

    1. Chad Schwitters says:

      You are correct that a solar panel, which outputs DC power, could theoretically charge an EV’s DC battery – if you had a way to connect the two.

      But you are also correct that in practice this is too complicated. The typical connection path would be through the AC charging circuit, so you would have to convert the panel output to AC, create a circuit that speaks the correct protocol to make the car accept the current, make sure you have enough panels to provide the proper voltage level, etc.

  4. LJ says:

    People at work are using 120v phone chargers and plugging in to 74 volts DC and charging their phones. What’s the charging voltage and is it AC or DC..

  5. Leslie says:

    I read this article to better understand the difference between AC/DC power as I troubleshoot some laptop problems I’m having. It was very easy to understand and I have a much clearer understanding of my laptop issue just because I now understand the terminology being used. I know it’s basic info, but those of us not so technically inclined have to start somewhere…
    Thanks!

  6. sifiso says:

    what happens to a phone when charged with AC

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