BU-1004: Charging an Electric Vehicle

If you own an EV, you want to pamper the battery and charge the car at home and at the office. The power requirements to charge a mid-sized EV is similar to that of an electric stove connected to a 40A, 240VAC circuit developing up to 9.6kW. Most mid-sized EVs carry a 6.6kW on-board charger designed for a 4- to 5-hour charge. (6.6kW is derived by multiplying 220V by 30A.)

On-board chargers are limited by cost, size and thermal issues. With the availability of three-phase AC power in most European residences, on-board chargers can be made smaller than with a two-phase system. Renault offers compact on-board chargers that range from 3–43kW.

The hookup to charge an EV is called the Electric Vehicle Service Equipment (EVSE). Except for Level 1, all must be installed by an electrician if not already available. There are three categories of charging.

In the 1990s and 2000s, EV makers made a concerted effort to develop a universal charging port for EVs and this resulted in the SAE J1772, a 5-pin connector carrying AC and data. The drawback is a charge time pursuant to Level 2 that takes several hours.

EV makers agree that the future of the EV lies in fast charging. While Level 2 only gains about 40km (25 miles) per hour charge, DC Fast Charging fills the battery to 80 percent in 30 minutes. This changes the EV from a commuter car into a touring vehicle, and EV marketing has started to push the concept.

Japan was first to introduce DC Fast Charging by developing the CHAdeMO connector for the Nissan Leaf and Mitsubishi MiEV. JEVS (Japan Electric Vehicle Standard) specified the connector that includes two large DC pins with communications pins for the CAN-BUS. The CHAdeMO standard was formed by TEPCO (The Tokyo Electric Power Company), Nissan, Mitsubishi, Fuji Heavy Industries (manufacturer of Subaru vehicles) and Toyota in 2008. It charges a battery at 500VDC and 125A with up to 62.5kW charging power. CHAdeMO stands for “CHArge on the Move;” Figure 1 illustrates the plug.

Nissan and Mitsubishi lead DC fast charging and developed CHAdeMO. It fast-charges at 500VDC and 125A, developing up to 62.5kW of power.

While the CHAdeMO connector performs well, the West lobbied against it, citing “technical issues.” The reason for this may be the “not invented in my backyard” syndrome as well as a standard that favors certain brands of cars. SAE rejected CHAdeMO in favor of their version.

After much delay, the SAE International J1772 Committee released the SAE DC Fast Charging standard in 2012, a system that is also known as the Combo Charging System (CCS). The delay caused a setback in building the CHAdeMO infrastructure and some argue that the postponement was deliberate.

To keep compatibility with Level 2 charging, CCS is based on the existing J1772 connector by adding two DC pins. When charging on AC, the circular connector provides AC power and communications to govern voltage, charge rate and end-of-charge. DC Fast Charging uses the same communications protocol but adds the DC pins. Figure 2 illustrates the charging connectors for AC and DC charging with the vehicle inlet.

CCS allows Level 2 charging by connecting to the upper circular receptacle only, and Level 3 charging with a plug that includes the DC terminals.
SAE J1772 divides charging into four levels:

• AC level 1: 120VAC, 12–16A, up to 1.92kW

• AC level 2: 240VAC, 80A 19.2kW

• DC level 1: 200-500VDC, up to 80A (40kW)

• DC level 2: 200-500VDC, up to 200A (100kW)

The SAE Combo or CCS is the de facto global standard for Level 2 and 3 charging and Audi, BMW, Daimler, Ford, General Motors, Porsche and Volkswagen jointly announced their support in 2011. The Chevy Spark was the first EV to feature the SAE Combo in 2013. There is now talk to discontinue the CHAdeMO. To maintain compatibility with EVs featuring CHAdeMO, newer Nissan Leafs include an SAE J1772 port to allow Level 2 charging. Some charger manufacturers, including ABB, offer both charging plugs at their “pumps.”

Tesla Motors does not follow standards easily, and they came up with their own system. Their exclusive Supercharger fills a depleted battery to 80 percent in 40 minutes and gives a driving range of 270km. (Charging from 80–100 percent doubles the time.) While Tesla was criticized by some for introducing their Superchargers, others say that Tesla is way ahead of the game and did not want to wait for the world to get its standards right. Tesla is in discussions with Nissan and BMW to offer their Supercharger standard to these EV makers as well. They are also working on an inter-protocol charging adapter that can support the CHAdeMO and SAE J1772 systems.

Charging the Tesla S 85 on a Supercharger begins at a voltage of about 375V and 240A, consuming 90kW. As the battery fills, the voltage rises to about 390VDC and the current drops to roughly 120A. The initial 90kW into the 85kWh battery has a charge rate that is only slightly higher than 1C. After a brief moment, the C-rate falls to a comfortable 0.8C, and then goes down further, avoiding harmful battery stress that is related to ultrafast charging.

Battling three incompatible charging systems was not the plan for EV makers, but it occurred in part by not accepting available technologies and delaying their own standards. Tesla jumped ahead with their own technology and is investing heavily into building Superchargers and offering free charging; other EV makers have followed by also making charging free, at least for now. The resulting incompatibility has similarities with the railroads industry in the 1800s, when railway companies ran their trains on different track gauges. LP vs. 45 RPM, as well as Sony Beta vs. VHS are other examples of similar situation.

BMW with its SAE Combo Charging system chose 24kW rather than the more common 50kW for the DC Fast Charger. They reckon that 24 kW is cheaper, lighter and easier to install than a 50kW system. While 50kW would charge faster, the benefit is for a brief moment only before the charge acceptance degrades. Scaling down is especially apparent with the smaller i3 battery, as well as packs that cannot take the ultra-fast charge due to advanced age and other anomalies. Tests show that the 50 kW charger fills a battery to 80 percent in about 20 minutes; the 24 kW charger does it in roughly 30 minutes.

Doubling the power does not cut the charge time in half and moving up in the pyramid has diminishing returns. The main reason for powerful chargers relates to battery size. The BMW i3 carries a 22kW battery compared to the monster 85kW in the Tesla S 85. Both charging systems keep the charge C-rate at about 1C during DC fast charging to moderate battery stress levels.

DC fast charging is more complex in that it must evaluate the condition of the battery and apply a charge level that the battery can safely absorb. A cold battery must be charged slower than a warm one; the charge current must also be reduced when cells develop high internal resistance and when the balancing circuit can no longer compensate for cell mismatch. (See BU-410: Charging at High and Low Temperature)

DC Fast Charging is not designed to fill the battery completely but to allow the vehicle to reach the next charging station. Using Level 2 is the preferred routine for everyday charging.

Table 3 summarizes the charge levels and times with Levels 1, 2 and 3. The charge times may not fully agree with advertised rates as the calculations are based on charging an empty battery to fully SoC; some EV makers consider the battery charged when it reaches 80 percent. The charge time also shortens as the battery fades because there is less to fill.

* Tesla EVs come with 10kW and 20kW chargers; Renault uses 3–43kW 3-phase on-board chargers
** A 30-amp EVSE needs a 40A circuit breaker some EVs come with larger on-board chargers