How bidirectional EV charging maximises battery utility

This article originally appeared in the March'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

When power can move both ways, an EV becomes more than just four wheels that move people around. It’s an energy source in a smart grid that can help with demand shifting, power a residence during an outage, or act as a mobile charging unit for a commercial fleet.

Bidirectional charging is still in its infancy, but the technology is available to equip both the charging stations and the EVs themselves to support smarter power distribution in cities as well as enable a variety of industrial charging applications in agriculture, construction, and logistics.

A two-way street

While there’s been a lot of talk about the potential of vehicle-to-grid (V2G) charging, today’s charging infrastructure and EVs are typically set up to go one way: from the grid or another source to the vehicle. In a typical battery charging scenario, an AC/DC converter applies power factor correction (PFC) and converts the AC grid voltage into a high voltage DC output that supplies the input of an isolated DC/DC converter. The DC/DC converter provides galvanic isolation for safety and converts the fixed DC input voltage into a constant current (CC) or constant voltage (CV) output that charges the battery with the help of the battery management system (BMS).

Bidirectional charging is a two-way street. Rather than just drawing power from the grid, an EV can send it back. There are two general ways to get AC power back out of the battery.

• Separate DC/DC converter and DC-AC inverter
• Bidirectional power conversion topologies

The most straightforward option is to use an inverter in parallel with the charger to convert the batteries DC voltage to an AC grid voltage, usually with the help of a DC/ DC stage. This method takes up space, adds weight, and increases cost, but is simpler to design and control.

To avoid the added space, weight, and cost a true bidirectional charger uses bidirectional switching topologies with complex digital controls to allow each power conversion stage to transfer power in either direction.

AC/DC conversion and PFC

Not all power converter topologies are capable of bidirectional power transfer. One obvious reason is the use of bridge rectifiers in the AC/DC stage, which only pass current in one direction. These are often used for lower power chargers and followed by a boost converter with PFC, due to cost and simplicity. However, they are inefficient and don’t work well in parallel, which makes thermal management difficult at high power levels.

So-called bridgeless topologies remove the bridge rectifier to improve efficiency. These converters are more complex, as simple diodes are replaced with active switches, and some topologies are capable of bidirectional power flow. Perhaps the simplest example is the bridgeless totem-pole PFC topology which essentially replaces the bridge rectifier diodes with active bidirectional switches.

In the AC/DC direction this topology resembles a boost converter and converts the AC input to a DC output, while shaping the input current to match the input voltage for PFC. In the DC-AC direction the DC voltage is chopped up and the pulse-width is modulated and filtered into an AC voltage.

Other bidirectional topologies exist and are selected based on cost, power density, efficiency, and complexity. Efficiency is important to minimise waste and maximise battery utilisation and life.

Bidirectional DC/DC conversion

The DC/DC stage is conceptually simpler as it’s performing DC/DC conversion no matter which way power is flowing. Like the AC/DC converter, the rectifiers must be replaced with active switches to enable bidirectional power flow. You must also be able to actively control the switches on both input and output sides, which is complicated by the presence of isolation. The common topology for this stage is known as the dual active bridge (DAB) converter. This converter contains an active full bridge on each side of the isolation barrier. Resonant topologies are often used to minimise switching losses, reduce size, and maximise efficiency. The typical unidirectional LLC (inductor-inductor-capacitor) converter becomes a CLLC (capacitor-inductor-inductor-capacitor) DAB converter for bidirectional applications.

V2X: vehicle to everything

Regardless of whether the charger incorporates bidirectional topologies or a separate bulky inverter, there are many benefits to being able to export power from an EV battery. The benefits cover a range of applications, many of which have received a variation of the V2X naming. These include:

• V2G – Vehicle-to-grid
• V2B – Vehicle-to-building
• V2H – Vehicle-to-home
• V2E – Vehicle-to-equipment
• V2V – Vehicle-to-vehicle

The most obvious benefit of bidirectional charging is that EVs could be used in a V2G model to power the electrical grid. By design, most electrical grids do not have substantial built-in storage. Generated power is consumed immediately or wasted, which leads to complex estimates of demand to supply enough energy without excess waste. It is also slow to react, which can lead to voltage sags and brownouts when large load changes occur. When it comes to renewable energy generation, such as wind or solar, energy production is dependent on environmental conditions and may fluctuate drastically. Without energy storage, excess energy will be wasted, and non-renewable sources will be needed to supplement production when renewable energy production is low.

By utilising EV batteries for storage, electrical grids can maximise the power generated by renewables by storing the excess energy for use when output is below demand. Because the battery is closer to the load, it can also help supply load transient current to help maintain grid stability. Batteries could essentially act as capacitors, supplying peak power needs locally, which can reduce stress on distribution lines and transformers and reduce voltage fluctuation.

Emergencies are another compelling case for EVs to supply power back to the grid. In the event of a power outage, city vehicles could keep essential buildings (V2B) and (V2E) equipment running, while a high-rise tower could leverage EVs connected to charging stations in its parking garage to power the building – essentially an alternative uninterruptible power supply (UPS). Even a single-family home could potentially rely on an EV to get them through a blackout through vehicle-to-home (V2H).

Use cases

Beyond the obvious residential and consumer applications for bidirectional charging there are many use cases in a variety of industries and environments, including commercial fleets, construction, and agriculture. Not only could an EV charge another car, but it could also be used to charge other vehicles such as e-bikes and commercial fleets.

Bidirectional charging capabilities would enable a shipping and logistics firm with a fleet of trucks to use its EVs to meet its own energy needs by creating a self-sustaining ecosystem. By establishing its own microgrid or nano grid systems, a business could become more energy independent. In combination with renewable energy systems such as solar panels, it would be possible for a business to become less dependent on the grid and make its buildings self-reliant.

Not all V2G or V2V scenarios need to take place in urban areas. Rural communities with farms can benefit from bidirectional charging. All-electric tractors have been available for several years now, and more recently, V2V charging tender trucks. Rather than the tractor being charged by a standalone charger in a fixed place, tractors and other farm vehicles can be charged in the field, much like a fuel truck that’s driven to where it’s needed rather than bringing the tractors back to a gas pump.

Bel delivers bidirectional power solutions

Given all the benefits and use cases, there’s clearly a need for bidirectional power transfer capabilities in EV’s and EV supply equipment. Bel has developed a broad portfolio of EV power solutions that enable bidirectional power flow including inverters, chargers, and bidirectional combo units. These products are suitable for a range of vehicles, from medium to heavy duty, on and off road, and can help maximise the utility of the battery.

Bel’s bidirectional chargers are designed to address the many emerging use cases where there’s a compelling reason to draw energy from a vehicle to redirect it back to the grid, other vehicles, or equipment in the field, and ultimately make businesses more efficient and productive. Bel’s product and design team is also available to create custom solutions to fit a variety of needs and deployment use cases.