Once you have figured out how to squeeze an adequately sized battery of an acceptable weight into an electric vehicle (EV)—i.e., one that will handle ordinary daily commuting distances—charging time becomes the next big hurdle. However, “filling” a battery takes a lot more time than filling a gas tank (see “Can An EV In The Garage Save You Money While You Sleep?” at www.electronicdesign.com).
EV enthusiasts, though, don’t mind the time tradeoff. They generally consider the energy content of a gallon of gasoline to be about 35 kW. Less precisely, they say, that translates to about 100 miles per gallon in an EV—though that figure can’t apply equally to the mileage that Toyota hopes to get from the electric drive train that Tesla Motors is designing for the RAV4 SUV and to whatever drive train Daimler is planning for the future electric Smart car. (Tesla is designing the battery for Daimler, too.) Additionally, these are capacity issues, as the Tesla Roadster’s range is considerably greater than that of a Nissan Leaf (Fig. 1).
But back to the charging question. The catch, in charging at home, is the time required to restore a full state of charge given the limitations on voltage for a typical home service drop. A home electrical panel, the box with all the circuit breakers in it, may be able to supply 15 kW or even 20 kW beyond the family’s normal load. In that case, it might take about two hours to pump as much energy as you can squirt into your gas tank in a few seconds at the gas station. The Leaf has a 3.3-kW charger, and the Roadster can accept 240 V at 70 A (16.8 kW) from the Tesla Home Connector (see “Test Driving The Nissan Leaf” at www.engineeringtv.com).
At seven cents per kilowatt-hour, which is what I pay for off-peak electricity under the time-of-day plan I have with my grid-tied solar system, that’s $2.45, as long as the California state legislature continues to ignore the opportunity to add road-use taxes. It would go up for anyone paying a single, balanced rate per kilowatt-hour.
If you use the 100 miles/gallon figure, that’s a pretty good (as opposed to spectacular) deal. It probably puts that future RAV4 on a similar mileage cost basis with my Toyota Prius, and it changes the North American version of the Smart Car from an amusing descendent of the BMW Isetta to an appealing urban vehicle—especially since the electric version would lose the jerky automatic/manual transmission. I’ve put more than 800 miles on a rental, so I speak from some experience.
But if you’re not talking about always charging at home, with an expensive adapter that uses all the energy you can download from the power lines, the proposition gets more interesting. It’s one thing if you can leave the car attached to a charger at work for eight or more hours. It’s something else if you’re on a long trip, you’re using a public facility, and other electric vehicles are queuing up behind you. In cases like that, which is what EV enthusiasts like to envision, time-to-charge is a different proposition.
EV Battery Charging
When the battery in a conventional vehicle is discharged, say, by leaving the dome light on overnight, you can recharge it with a pretty simple apparatus from the hardware store. It’s a relatively safe operation too, because the voltage and current from the charger are limited. There is more danger from shorting the battery itself by inserting a wrench between the terminals than there is from the charger. With an EV, it’s a different story. For the U.S. auto industry, the governing document for EV charging is Society of Automotive Engineers (SAE) J1772. In Europe, the standard is IEC 61851.
(Get used to calling a charging station an EVSE, an acronym for “Electric Vehicle Supply Equipment.” The utilities and the EV community aren’t happy with the implications of the term “charger” and similar terms that might be taken to downplay the hazards associated with transferring large quantities of electrical energy.)
The J1772 standard attributes three functions to the EVSE: ac-dc rectification, voltage regulation to a level “that permits a managed charge rate based on the battery charge acceptance characteristics, and physical coupling of the charger to the vehicle at the hands of the user.” The standard also defines several “levels” of charging. Each has voltage and current levels associated with it (Table 1).
AC Level 1 assumes the charger is on board the EV and the vehicle is connected to an everyday NEMA 5-15R 15-W wall socket. AC Level 2 again assumes a single-phase ac supply, with the battery charger again in the car, but the ac voltage is a nominal 240 V and the maximum current draw can be as high as 32 A.
EVs and EVSEs configured for AC Level 3 charging may also support AC Level 2 for greater compatibility (Fig. 2). The vehicle’s low- and high-current charge port contacts are wired together to permit the onboard charger to operate from either low- or high-power sources. This is possible because the EVSE uses two separate contactors to preclude parallel current paths while charging AC Level 3 vehicles. Vehicles could also be configured for dc charging with the addition of a serial data interface and contactors between the high-power contacts and the onboard charger and battery pack.
However, dc charging assumes an external dc source. The charging electronics may be either in the vehicle or external. For dc charging, the power available can vary from power levels similar to AC Level 1 or 2 up to 600 V and 400 A, which may be able to replenish more than half of the capacity of the EV battery in as short a time as 10 minutes.
This IEC61851 standard used in Europe and China, which was derived from J1772, has similar requirements adapted for the European ac line voltages. Most differences are superficial. Where the SAE standard describes “methods” and “levels,” the IEC standard talks about “modes,” which are virtually the same.
Like J1772 Level 1, IEC61851 Mode 1 relates to household charging from single-phase 250-V (maximum) or three-phase 480-V power connections, with a maximum current of 16 A, which is a little higher than the North American limit. There are further requirements for grounding.
Mode 2 uses the same voltages as Mode 1, but doubles the maximum allowable current to 32 A (the same as Level 2 in North America). Importantly, it adds a requirement for a “control pilot function.” It also requires an integral ground-fault interrupter (GFI), which Europeans call a residual current detector (RCD). Mode 3 supports fast charging with currents up to 250 A, and Mode 4 charging uses an offboard dc charger that may supply up to 400 A.
Naturally, the interconnect arrangement is a key element of the standard, given that it involves connectors that can carry lethal ac voltages and currents. The standard has a number of safety requirements, but meeting them has been left to manufacturers. Apparently, at this stage, adapted cables will deal with the mechanical incompatibilities. The pin functions are standardized.
One of the main functions of the standard is to define an interface that the car owner can safely use, where safety implies both protection from electric shock and protection of the charging electronics and the traction battery.
With respect to the actual connection between the EVSE and the vehicle, the standard offers functional and safety requirements but declines to prescribe a single physical configuration. In fact, for simple, slow ac charging it considers an ordinary wall plug (with safety ground) adequate. For high current levels and dc charging, it describes the pinout functions, contact dimensions, and safety characteristics of a nine-pin interface (Table 2).
For the heavy current paths, that’s straightforward, with connections for each phase plus the neutral return. The most interesting feature is the “pilot control” function, which keeps high voltages off the “hot” pins until the EVSE is mated to the vehicle connector and can tell the vehicle how much current the EVSE can supply in certain cases. There are also three serial data lines and a data ground.
AC Level 3 charging refers to EVs with on-board charging systems that can accept currents higher than 48 A. The J1772 conductive coupler allows for two sets of current-carrying conductors. Contacts 1 and 2 are for ac charging at line currents of 6 to 48 A. Contacts 3 and 4 are for ac charging at current levels up to 400 A, driven by a 208/240-V ac supply.
Most of the functions in Table 2 are obvious. The exception is the control pilot circuit on pin 6. This circuit provides the handshake function between the EV and the EVSE when dc charging is to occur. As an aid to understanding its ac charging methods, J1772 includes an appendix that describes how manufacturers have implemented the required control pilot functionality. It provides a generic illustration of the overall approach. In operation, the charging station handshake works like this:
• Prior to connection to the vehicle, the EVSE generates a static positive 12 V relative to ground, waiting for connection to the EV.
• When the EV is connected, and assuming that S2 is open, a 2.74-kΩ sense resistor on the vehicle pulls down that +12-V line in the EVSE to +9 V. The EVSE senses this and starts to generate a 1-kHz square wave that toggles between +9 and –12 V. A diode in the vehicle clamps this at –12 V. The diode clamping is a safety feature intended to allow the EVSE to distinguish between a vehicle and some random resistance accidentally bridging the charging line.
• The vehicle’s electronics have to know the battery’s state of charge. If the vehicle requires ac energy transfer, it closes switch S2, which typically switches a 1.3-kΩ resistor in parallel with the 2.74-kΩ resistor. (There are exceptions. See below.)
• The addition of the parallel 1.3 kΩ reduces the total resistance to 882 Ω and pulls down the positive peak of the square wave to +6 V. The EVSE interprets this as a request for ac power and closes the contactor.
• However, to accommodate batteries that emit hazardous gasses during charging, requiring the EVSE to turn on an exhaust fan if it is located in an enclosed area, the vehicle must switch in a 270-Ω resistor, pulling down the positive peak of the square wave to +3 V, telling the EVSE to switch on the fan.
• When the vehicle detects that its battery is topped up, it opens S2. The positive square-wave peak rises to +9 V and the EVSE opens the contactor, removing power. The +9-V, –12-V square wave continues until the cable is disconnected, when it reverts to the continuous +12-V state.
Under J1772, After a connection is made, the EVSE provides information about the maximum available continuous current capacity to the EV by modulating the pulse width between 100 and 800 μs (Fig. 3). The relation is linear: 100 μs corresponds to 6 A, and 800 μs corresponds to 48 A. A pulse width of 900 μs would mean that the EVSE has its own dc charger, in which case the EVSE and the EV would engage in a more complex data exchange on those pins in the connector designated 7 through 9. The data link specified in J1772 is based on SAE J1850, SAE J2178, and SAE J2293.
How fast an EV can be recharged ultimately comes down to how much voltage and current the cabling and connectors can handle. An SAE J1772-2009 connector can supply up to 16.8 kW (240 V, 70 A), and the VDE-AR-E 2623-2-2 connector in Europe can supply up to 43.5 kW at 400 V, 63 A, via a three-phase grid connection (Fig. 4 a and b).
That said, another limitation of J1772 ac charging methods relates to the losses associated with rectifying ac to dc within any EV that integrates its own charger. As an alternative, beyond a certain level, say above 240 V and 75 A, having the EVSE supply dc is preferable because, costwise, it’s easier to justify an expensive high-efficiency rectifier in an EVSE that will be more or less continuously generating revenue over any 24-hour period than it is to justify one in every vehicle. It’s also easier to transfer the heat to the environment with large fans or water cooling in the EVSE than it would be in the close confines in the car.
To that end, the now very well known Tokyo Electric Power Company has a patented fast-charging technology that uses dc voltage levels up to 500 V and currents up to 125 A (Fig. 4c). Under TEPCO’s CHAdeMO protocol, the vehicle exchanges battery parameters with the EVSE.
With its 24-kWh battery pack, the Nissan Leaf takes approximately eight hours to recharge using the 3.3-kW charger Nissan will install in the owner’s garage. But with a CHAdeMO DC Fast Charge station delivering 62.5 kW (500 V dc, 125 A), the time to recharge the Leaf to 80% capacity would be about 30 minutes.