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Electric vehicle charging stations gain speed with higher voltages and currents

The widespread acceptance of electric cars still depends on infrastructure developments. These include the deployment of electric vehicle charging stations, and then, finding a station, reducing the time it takes to charge your battery.

The popularity of all-electric cars (EVs), as well as hybrid electric vehicles (HEVs) and plug-ins, has increased dramatically. Society-wise, the use of electric cars reduces the dependence on fossil fuels and their contribution to global warming. Some might say they are fun to drive. But the widespread acceptance of electric cars still depends on infrastructure developments. These include the deployment of electric vehicle charging stations, and then, finding a station, reducing the time it takes to charge.

Charging time is directly related to battery capacity — typically stated in “miles per charge.” Batteries for the Tesla Model S Sedan, for example, are rated in 200- and 300-mile increments. The all-electric Chevy Bolt, due out next year, is rated for 238 miles-per-charge.  The all-electric Volkswagen e-Golf — admittedly, a “city car” — is rated for up to 83 miles. (Of course, these ratings will vary widely, car manufacturers stipulate, according to how the car is used and maintained. Heavy use of the car’s air conditioning and multi-media console will likely reduce the mileage range of the EV.)

Perhaps the real promotional handle for the Chevy Volt — if in doubt, use gas — helped relieve drivers’ anxieties about getting stuck out-of-range of a charging station, and contributed to the Volt’s sales success. But this points to the need for a vibrant charging infrastructure. We need a network of electric vehicle charging stations, and those chargers must be readily accessible and easy to use.

The proliferation of electric cars

Electric vehicles accounted for less than 3% of global car sales in 2016, according to Goldman Sachs Research. And battery-powered vehicles will account for more than 20% of vehicle sales in 2025. Assuming 85 million light vehicles produced each year, electric car sales will jump from 2.6 million in this year to 17 million ten years from now. (See Figure 1).

 

Ohr_Fig1_EVsales_oct2016

Figure 1: Sales of plug-in electric vehicles passed the 1.5 million unit milestone in May 2016. California has lead among regions, having sold more than 200,000 EVs by May 2016. Source: Mariordo (Mario Roberto Durán Ortiz) – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=51126561.

Battery capacity and charge time

One negative influence on the use of electric cars is limited battery capacity; another is the slow and perhaps tedious battery charging process. Unlike the gasoline-powered automobile that allows you to pull into a service station, pump yourself 300 miles-worth of gasoline, and leave the station in less than 5 minutes, the all-electric car requires time and patience to refill. You can monitor the EV charging process with an app on your cell phone, but that doesn’t reduce the charging time. You may need to spend hours at a charging station.

The time will vary with different car models and battery types. Makers of the Chevy Bolt, for example, estimate it will take about one hour to charge for every 25 miles the car is to be driven (with a Level 2 charger). The battery is “nickel-rich lithium-ion, Chevy says. The battery chemistry enhances heat resistance, and a liquid cooling system helps manage the battery’s temperature when the outside climate varies. Lithium-based batteries can be incendiary under certain conditions, and their temperatures must be precisely monitored and controlled (See Figure 2). Plus, the battery pack has an energy capacity of 60 kWh – which allows you to travel an EPA-estimated 238 miles of range on a full charge.

 

Ohr_Fig2_Chevy Bolt Battery

Figure 2: Like many current electric vehicles, the lithium battery for the all-electric Chevy Volt uses multiple cells spread across the undercarriage of the car. Source: Chevrolet

 

EV battery charging levels and techniques are defined by the Society of Automotive Engineers (SAE). Regular home charging, like the kind you might perform with your first electric car in your home garage, is called “Level 1.” It runs off a 110/120 VAC house line. And it will likely take you all night to charge your vehicle.

Charging with a 240 V line (the kind you’d use for your electric clothes dryer) is called “Level 2.” The electrician-installable 240-volt charging unit offers the fastest way to recharge your battery at home. It offers more power than a 120-volt outlet. This higher-voltage system can provide up to an average of 25 miles of range per one hour of charge. You can fully replenish your battery from empty to full in about 9.5 hours.

Chevy’s Bolt EV offers DC Fast Charging capability (where available), which provides an estimated 90 miles of range with about 30 minutes of charge time. DC Fast Charging stations, sometimes called “Level 3,” though not with consistency, are available for public use (at city parking garages, for example). These stations offer the highest power, but their proliferation is a function of public financing .

There is also a growing network of Tesla “Superchargers” designed to work with Tesla Sedans and Roadsters (sometimes called “Level 4”). Roughly 850 Superchargers were deployed across the U.S. in 2015 and 1221 this year. But the Tesla lacks the SAE J1772 charging plug and is not readily compatible with other electric vehicles. EV Solutions, AeroVironment’s online catalog for Electric Vehicle Charging Products and Services, publishes a time-to-charge guide for a large list of electric car models (see Figure 3). Note: the charging station which is an external appliance is different than the car charger, which resides inside the car near the battery.

 

Ohr_Fig3_Time To Charge Guide

Figure 3: A Time to Charge Guide provides you with an estimated time to fully charge your Battery Electric Vehicle or Plug-in Hybrid Vehicle. For 240 V EVSE RS charging, your EV must have a 6.6k W on-board charger. Source: AreoVironment https://www.evsolutions.com/?gclid=COvr3b7J4s8CFQ9Efgod8jgL5A

 

More electric charging stations need to proliferate to support electric car use. This dilemma may resemble the classic chicken-egg (which comes first) debate. The existing suppliers — companies like Siemens, Bosch or Schneider Electric — are used to industrial infrastructure investments and their sometimes glacial rates of return. Charging stations are a more enticing investment for those with network and cloud capabilities (and billing software) to which the hardware will attach.

The existing wireless charging infrastructure

ChargePoint, for example, sees its car charger network as a platform for its home chargers as well as its commercial and industrial products. The company is reporting an installed base for roughly 30,500 commercial charging spots — and a 75% share of the charging station market. In addition to the public charging stations installed by business or government agencies, the company’s “ChargePoint Home” product offers charging currents of 32 A or 16 A at 240 V. Average charging charges run for $0.49 to $0.79 per kWh. (Your office electric bill, in contrast, is typically $0.10 per kWh.

Plug Share will map charger availability for your cell phone or navigation system, on the road, even tell you about special conditions (whether you need to drive through a narrow alley, for example, or whether there’s a dog with a proprietary attitude waiting for you). https://www.plugshare.com/

For charging in Northern Europe, the Charge & Drive network offers over 500 chargers, including more than 200 quick chargers, through Finland, Norway, and Sweden. Membership in the charger network is by mobile payment subscription (your charging bill is delivered on your smart phone). The charger network uses all leading smart chargers. https://www.chargedrive.com/increased-mobility/

Optimizing Level 2 and Level 3 chargers

In the U.S., most users will be familiar with Level 1 charging (120 VAC at 15-to-20 A). Level 2 (240 VAC at 40 A) will allow for faster charging times compared to the Level 1, but will require a larger power source for currents and voltages. For engineers, Texas Instruments has published a series of charging station reference designs: https://e2e.ti.com/blogs_/b/smartgrid/archive/2016/08/29/electric-vehicle-charging-stations-are-getting-smarter-and-charging-faster.  The Level 1 and 2 charging system consists of an AC/DC converter (with power factor correction), which generates a DC battery voltage from the AC line. (A simplified schematic of a Level 1 or Level 2 in shown in Figure 4a. Note the proliferation of power MOSFET transistors and diode, and the central controller.)

Ohr_Fig4a_TI Level 2 Charger

Figure 4a: Charging and Charging Infrastructure Systems use single-phase AC Levels 1 and 2.  Source: Texas Instruments

 

At the heart of the inverter is a real-time microcontroller, programmed to perform the power management functions, including AC/DC conversion with PFC and DC/DC to create the necessary charge profile for the battery.

In the public charging area (like city parking garages), Level 3 can reduce charging time and make it more practical for users on-the-go. Level 3 charging uses an AC/DC converter to generate a DC voltage from the AC line. This incoming power needs to undergo power factor correction (PFC) to meet regional regulatory standards.

The inverter will similarly use a real-time microcontroller. It is programmed to perform the control loops for all necessary power management functions, including AC/DC with PFC and DC/DC to create the necessary charge profile for the battery. Developed for motor control applications, parts like the TI C2000 controller have advanced peripherals such as a high-precision pulse width modulator (PWM) and ADCs. Level 3 chargers may include complex displays with online billing/reporting applications (see Figure 4b).

 

Ohr_Fig4b_TI Level 3 Charger

Figure 4b: Level 3 EV chargers operate from 3-phase AC lines, and use additional control loops.  Source: Texas Instruments

 

Later this year, Texas Instruments plans to introduce a Level 3 EV direct-current charger reference design. The charging station will be scalable to 600 V and 400 A. This will cut charging time down to only 20-30 minutes, TI’s promotional literature says — enough time to stop at a Wi-Fi-enabled restaurant that has a charging station and charge the vehicle during lunch.

More intelligence in the charging station

While the charger designs make use of many power transistors and drivers, the devices have many sources of supply and their pricing is subject to the laws of supply and demand. In other words: a commodity. This is the fate of many otherwise useful and popular semiconductor components. Almost everyone could readily see the growth of solid state lighting (for example, white LEDs) as they replace incandescent bulbs in homes and T1 fluorescent tubes in offices and factories. But their revenue growth curve will likely flatten before the end of the decade. One reason is that bulb replacement (if the promoted 50,000-hour life cycle for LED bulbs is to be believed) is a one-time activity. Another reason, relevant to power transmission, is that the real value of the circuit is in its communication intelligence.

The connections between an EV’s battery and electrical grid required a fair amount of monitoring and control — especially as electric car users wrestle the tradeoffs between fast charging times and longer driving range (a function of battery capacity).

Increasingly, microcontroller intelligence is required for the charging station to “negotiate” with the vehicle. The charging system must know how much power is available at the station, and what safety parameters need to be observed to ensure the charging process is safe — especially for lithium based batteries. Thus, communications intelligence (in addition to current sensors and monitors) increases the capabilities of the EV battery charger.

Texas Instruments’ contribution to the process can be visualized on a number of levels. First, microcontroller products like TI’s C2000 or its MSP430 could speed charging, but keep the process within designated limits. The associated reference design uses TI’s “SimpleLink” Wi-Fi wireless microcontrollers which allow design engineers to create stations that intelligently detect available charging stations and activate charge at non-peak times. TI claims this capability is a first for charging station, allowing electric vehicle owners to remotely monitor and control the charging of their vehicles from just about anywhere, using a smartphone or tablet.

A Wi-Fi enabled EV charger reference design intended to create unique remote monitoring and charge control opportunities for home and public charging stations. The reference design will support other applications such as point-of-sail payment, authentication, and home automation.

In addition to special Wi-Fi capabilities, TI offers specialized sensors which can stabilize the interaction between AC and DC components during a high-voltage transfer. One example is a Fluxgate sensor design that measures DC, AC, and pulsed currents. It allows for Galvanically isolated current measurements up to 55 A, with a sensitivity of 41.4 mV-per-Ampere (see Figure 5).

 

Ohr_Fig5_TIfluxgateSensor_oct2016

Figure 5: Monitoring power levels accurately is essential for all types of energy transfer applications. The requirement is to monitor currents precisely in high-voltage applications. Texas Instruments’ fluxgate sensor measures DC, AC and pulsed currents over a ±15A range. Source: Texas Instruments.

 

 

By Stephan Ohr, Consultant, Semiconductor Industry Analyst

 

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