The Challenges of AC and DC Charging May Be Slowing EV Adoption
Some of the roadblocks to EV adoption lies at the core of AC and DC charging technology.
We recently discussed how one of the main problems with widespread EV adoption is the "charging deserts" in city infrastructures. While many companies are working around this shortage with solutions like pop-up charging hubs and charging robots, some of the challenges of EV adoption exist in AC and DC charging at its core.
In this article, we’ll look at AC and DC charging techniques and examine some of the technological challenges we face when designing a high-power charger.
The Charger Power Capacity
Two different technologies for recharging an EV are demonstrated in the following figure.
AC and DC charging differences. Image used courtesy of Texas Instruments
The AC charger has limited power (less than about 22 kW) and needs a longer time period to fully charge the car. The charging time can reach up to about 12 hours depending on the power level of the charger and the battery characteristics. As shown in the figure, AC charging relies on onboard circuitry to create a high DC voltage from the commonly-available AC sources.
A DC charger, on the other hand, uses off-board circuitry to generate a high DC voltage (300–700 V) that is directly applied to the vehicle’s battery management system (BMS). The power level of a DC charger can range from about 25 kW to 350 kW. This significantly reduces the charging time.
Note that the power capacity of a charger determines the rate at which energy is delivered to the battery and consequently the time it takes to charge the battery. The following table compares the charging time of AC (level 1 and 2) and DC chargers for a given battery.
Comparison of AC and DC charging times. Image used courtesy of Texas Instruments
Grid Peak Power Limitation
Feeding multiple high-power charging piles directly from the electric grid can increase the peak power delivered by the grid to an unacceptable level. An article from Analog Devices on energy storage systems that can boost EV's fast-charger infrastructure explains that when you simultaneously charge five typical EVs in 15 minutes, you can increase the peak power delivered by the grid to more than 1 MW.
Infrastructure integrating renewables, EV charging, and energy storage. Image used courtesy of Analog Devices
The grid should deliver this power for 15 minutes. With this in mind, city planners might invest in improving the grid so that it can provide high power levels. But instead of investing in the grid infrastructure itself, developers can use the power that is locally generated from renewable sources such as solar and wind. This will reduce the peak power that the grid should provide.
Unfortunately, the energy generated by renewable sources is intermittent. Hence, we need energy storage systems that act like large batteries to store the locally-generated energy and use it when charging EVs.
Managing the Generated Heat
Several power conversion systems are required to implement the charging station that is depicted above. Considering the high power levels that we are dealing with, we have to reduce losses as much as possible. For example, with a 350-kW charger, a 1% loss in efficiency is equivalent to 3.5 kW of power dissipation.
This power dissipates as heat and increases the system temperature. Without an efficient heat management mechanism, the generated heat can damage the system. That’s why we need to design the system using highly-efficient components. For example, using SiC MOSFETs rather than silicon IGBTs can considerably increase the system efficiency.
In addition, we will probably need liquid cooling to manage the heat generated by the cables, connectors, and circuitry.
The Need for a Reliable Battery Management System
When we discuss a unified infrastructure that integrates renewables, energy storage, and EV charging, we need accurate information about the battery state of charge (SOC) and state of health (SOH). This information allows us to increase the battery lifetime by about 30% by avoiding battery overcharging or overdischarging.
The importance of this lifetime improvement becomes apparent when we note that almost half of the system's overall cost is related to the battery.
Without an efficient charging infrastructure, EV uptake may be a slow-rolling process. Depending on the city structure, either home charging or public charging could become the desired charging solution. With fast chargers, we face several technical challenges such as the grid peak power limitation, heat management, and the need for a reliable battery management system.