Industry Article

A Look at Next-gen Bi-directional EV Charging

January 27, 2023 by Frank Vondenhoff

Electric vehicle (EV) batteries often store energy for propulsion and charge using uni-directional setups. There are situations, however, where it may be desirable to redirect energy from the car back to the grid or other equipment. These situations call for bi-directional EV charging instead of uni-directional. Learn more about bi-directional charging and the future of EVs.

Electric vehicle (EV) batteries often store energy for propulsion and charge using uni-directional setups like that in Figure 1 below. There are situations, however, where it may be desirable to redirect energy from the car back to the grid or other equipment. These situations call for bi-directional EV charging instead of uni-directional.


Figure 1. EV charging does not have to go one way: Bi-directional chargers support power transfer applications, such as V2V, V2G, and V2E. Image used courtesy of Pixabay


There is an increasing demand for EVs that support bi-directional chargers, which places the burden of finding effective charger solutions for the next generation of EVs on engineers. 


What Is a Bi-directional Charger?

Bi-directional chargers utilize active rectifiers that can invert battery voltage back into AC so the energy is exported elsewhere. These chargers are designed differently than traditional uni-directional chargers and provide several benefits based on the application. Their usefulness is expanding as new products for bi-directional charging come onto the market.


How Bi-directional Converters Work

Bi-directional chargers supply AC/DC in one direction for charging and DC/AC (inverter) in the other direction. The typical bi-directional charger involves two power stages: an AC/DC power factor correction (PFC) stage and a DC/DC stage. Both stages must be bi-directional or additional converters will be required.

For example, dual active bridge topologies enable bi-directional charging in the DC/DC stage– either side of the converter may be controlled as an inverter or rectifier to transfer power across isolation. In such cases, transistor switches replace diodes that typically would rectify output.

Active PFC topologies are rapidly becoming the most common type used in charging applications, primarily due to advancements in semiconductor technology. For EV applications, these are the most common topologies:

  • 2-level
  • 3-level NPC (Neutral Point Clamped)
  • 3-level Vienna, 3-level TNPC (T-type Neutral Point Clamped)
  • 3-level ANPC (Active Neutral Point Clamped)

Of the topologies discussed, only the 3-level Vienna does not support bi-directional conversion. In the case of 2- and 3-level topologies supporting bi-direction, the PFC stages use transistor switches instead of diodes so the stage can serve as either an AC/DC converter or a DC/AC inverter.


Uses, Benefits of Bi-directional EV Charging 

Bi-directional charging offers many benefits that can enhance the design and usefulness of EV chargers, making new designs from engineers more attractive to end users. The best way to examine the benefits is by reviewing the most common V2X applications.

Vehicle-to-grid (V2G) refers to sending electric power from the batteries in an EV car back to the grid, such as the one shown in Figure 2. V2G requires a DC/AC converter (typically embedded in the EV charger). As EV infrastructure expands and the EV market continues to grow, EVs sitting in parking lots can be used to stabilize electric grids during the day and charge during off-peak hours. 


Figure 2. As power grids age, they may not be capable of providing a stable, sufficient power supply–an issue that can be solved through V2G. Image used courtesy of Pixabay


According to the United States Department of Energy, such approaches can enhance grid stability and support peak usage shaving and smoothing. In addition, end users could financially benefit from selling unused electricity back to the grid.

Vehicle-to-home (V2H) means electric power from the vehicle is provided back to a house (as opposed to the power grid). This makes the EV batteries a backup power source, which can benefit those living where the power grid may be unstable. Experts estimate a fully charged EV can support the average home for several days. V2H also serves as an attractive approach to renewable energy storage and a means of cost shifting.

Similar to V2H, vehicle-to-building (V2B) charging can serve as an additional backup source of power to a building, enhancing the building’s power stability. Notice the emphasis on power as a backup source: The capacity to support a building is significantly greater than most homes. 

No one wants to be stranded by the side of the road, and EV owners may be especially wary because so much EV technology is new and unfamiliar. However, using bi-directional chargers makes vehicle-to-vehicle (V2V) use possible for charging batteries in other EVs. In addition, the concept of V2V leads to mobile chargers that allow EVs to charge their batteries when access to the grid is limited.


Figure 3. Not all power equipment is used in a shop or factory: Sometimes, equipment needs power for on-site tasks, an area where V2E excels. Image used courtesy of Pixabay


Finally, vehicle-to-equipment (V2E) means bi-directional charging can power worksite tools and equipment, serving the same function as a gas generator but powered by batteries. V2E serves as a sustainable energy solution compared to fossil fuels. V2E also provides power to tools that depend on electricity or charged batteries, such as the grinder shown in Figure 3 above.


Bi-directional EV Charger Options for Engineers

There are several bi-directional EV charger options on the market. For example, consider the BEL BCV200-700-8 or BCV200-350-8, solutions from the BEL BCV200 Series Power Conversion System.  This is a combo unit system with three subsystems: 15 kW bidirectional inverter/charger, 4 kW down converter (12 V), and 1 kW down converter (24 V). These multiple subsystems can power a wide range of ac and dc applications from the energy stored in an EV battery, with a single charger. According to its datasheet, the bidirectional inverter/charger system powers up to 15 kW in either direction at an efficiency of 92%. And for charging, the BCV200-700-8 and BCV200-350-8 can connect directly to an EVSE charging station or the public grid. The inverter charger functionality works independently of the DC/DC converters. There is also an option to connect a solar panel which can provide 4 kW to charge the HV batteries.


Figure 4. The BCL25-700-8 onboard battery charger is an example of the bi-directional charger options available to engineers. Image used courtesy of Bel


Another example is the BCL25-700-8, a 22kW / 25kW bi-directional onboard charger shown in Figure 4. The output voltage covers HV and EV batteries ranging from 240 to 800 VDC, providing a constant output current of 60 A. It can export 22 kW to 25 kW to three-phase AC equipment and creates its own grid when not running from the engine but from the batteries.


Bel Solutions for Bi-directional EV Charging

The future of EVs includes bi-directional EV charging capabilities that support V2X, including V2G and V2H. For engineers looking for efficient, effective bi-directional charging solutions, Bel has a range of eMobility bi-directional chargers and charging products that can make a difference and future-proof EV designs.