Use Your EV as a Battery? Team-up Takes Aim at Bi-directional EV Charging

February 04, 2022 by Jake Hertz

As electric vehicles (EVs) carve out their place in the world, a team-up between Emporia Energy and BREK Electronics is betting on bi-directional charging. How does it work and what circuit-level challenges are there?

Amongst the many value propositions of an electric vehicle, one of the least talked about is the potential for bi-directional charging. With bi-directional technology, one could use their vehicle as a battery pack, supplying power to a home and the grid when no longer needed. 


A high-level overview of bi-directional charging for vehicle-to-home (V2H) systems.

A high-level overview of bi-directional charging for vehicle-to-home (V2H) systems. Image used courtesy of Clean Energy Reviews


Of course, there are significant technical challenges involved in developing this bi-directional technology.

In an attempt to develop new, cheaper bi-directional charging solutions, Emporia Energy has announced a collaboration with BREK Electronics earlier this week. 

In this article, we’ll look at bi-directional charging technology, its design challenges, and what Emporia and BREK hope to achieve together.


How Bi-Directional Charging Works for EVs

As mentioned, one of the emerging new value propositions for EVs is the ability to perform bi-directional charging

As implied by the name, a bi-directional charger works by allowing the vehicle battery to receive energy from the grid and allowing the vehicle battery to supply energy to the grid. 


An example power system of an EV shows the vast amount of technology packed into this system. 

An example power system of an EV shows the vast amount of technology packed into this system. Image used courtesy of Katic et al


However, a significant amount of power conversion capability must be present on the EV to achieve this functionality. 

While most electric vehicle motors today run on AC power, the lithium-ion batteries inside a car store DC power. However, AC power from the grid is generally used to charge an EV. Thus, to charge a battery, which requires DC power, the system must convert the grid’s AC to the battery’s AC.

One way to support bi-directional charging is to have an EV that can work in reverse; not only will it convert AC from the grid to charge a DC battery, but it must also take DC from the battery to supply AC to the grid.

Now that the basics of bi-directional charging for EVs are better understood, let's take a look into some of the technical challenges. 


Technical Challenges with Bi-Directional Charging

From a circuit design perspective, there are several challenges for achieving bi-directional charging

One of the main challenges is achieving galvanic isolation between AC and DC, which these charging circuits will generally employ some form of transformer. Thus the challenge becomes designing the support circuitry needed to use these transformers bi-directionally.


Bi-directional series resonant DC-DC converter diagram.

Bi-directional series resonant DC-DC converter diagram. Image used courtesy of Charged


One popular approach is to use identical, synchronous full-bridge rectifiers on both sides of the transformer side of the transformer. 

The synchronous control of each rectifier will allow for energy flow to go in both directions as desired. This topology is known as the synchronous bidirectional full-bridge, but it is more commonly referred to as a DC transformer because that is essentially what it is. 

One challenge with this topology includes pulse width modulation (PWM) control of the rectifiers since both sides can switch between serving as the input and the output. 

Additionally, the leakage inductance of the transformer and the output capacitances of the field-effect transistors (FETs) can pose a challenge to efficiency and reliability.

Despite these challenges, one team-up hopes to bring the concept of bi-directional charging to market. 


Divide and Conquer: Emporia and BREK Team-up

As mentioned this week, Emporia Energy and BREK Electronics announced a joint effort to develop new, bi-directional charging solutions for EVs.

In general, Emporia Energy is known as an energy management technology company whose products include energy-efficient smart home products, including EV chargers and home battery banks. 

On the other hand, BREK is best known for its silicon-carbide-based solar inverter platform, which is said to achieve a power density 200% greater than the competition.

According to the release, the collaboration will have BREK develop the power electronics and hardware infrastructure for the new charger, while Emporia will develop the connector strategy and smart home integration system. 

The companies claim that their new product will be “the first of its kind,” coming to market in 2023 at a price point less than $1500.

With the usefulness of bi-directional charging clear, it will be interesting to see how this technology develops and if more innovations like this will come to fruition.