The worldwide push to address decarbonization goals focuses on replacing internal combustion engine (ICE) vehicles with electric vehicles (EVs). The movement has created both opportunities and challenges [1]. Some of the key opportunities include vehicle cost reduction due to fewer parts and lower maintenance cost from the elimination of items like fuel pumps, coolants, engine oils, hydraulic liquids, spark plugs, and moving parts along the drive train [2]. However, a key challenge in the adoption of EVs is the insufficient availability of accessible, dependable, interoperable, efficient, and affordable battery charging facilities [3]. Research has found that aside from government subsidies, positive experiences with EV chargers can encourage their adoption [4]. Analysts predict that up to 42 million EVs will be on U.S. roads by 2030, generating the need for up to 35 million charging ports [5]. Consequently, the U.S. bipartisan infrastructure bill signed into law in 2021 allocated USD 5 billion to help states build an EV charging infrastructure.

Batteries are the most expensive items on EVs, so they dominate both the vehicle price and repair costs [2]. Therefore, it is critically important that charging a battery does not degrade its life cycle [6]. Consumers also expect that charging an EV at a dedicated facility should take no longer than refilling the gas tank of an ICE vehicle [7]. However, the current state of the art exhibits a tradeoff in charging speed and battery life degradation [8]. Therefore, the industry must conduct more research to find alternative solutions.

2. Charger Market

Studies have consistently found that aside from government subsidies, a positive experience with EV chargers can increase the propensity to purchase EVs [4]. A recent study posited that the lack of charger availability and their unreliable operation has been impeding their adoption ^[9][10]. This suggests that companies are still trying to address certain underlying challenges in EV charger design for reliable operation and widespread deployment. The scholarly academic literature does not yet provide sufficient insights into the underlying causes for poor charger reliability and availability. Therefore, a few researchers adopted the strategy of analyzing patents to uncover insights into the underlying challenges that companies are still addressing. Choi (2018) suggested that patenting activity can be a robust indicator of technology development trends [1]. Phirouzabadi et al. (2020) also supported the notion that patent bibliometrics data can inform applied R&D activities in a knowledge domain such as the evolution of vehicle powertrain technologies ^[10][11]. The enormous size and unstructured format of patent databases complicates their analysis to distill knowledge about the specific objectives of inventions in a knowledge domain. Only a few articles, consequently, analyzed patent databases to discover trends in challenges that companies are addressing. For instance, Yuan and Wu (2020) analyzed battery development trends for EVs and found that the key technologies focused on solving problems related to battery heating, battery cooling, and charging methods ^[11][12]. Ma et al. (2022) analyzed patents about EV development from 1970 to 2016 and found that topics related to safely and quickly charging a battery and contactless charging are becoming a research frontier [3]. Yuan and Li (2021) found that companies filed 93.94% of priority patent applications at the Japan Patent Office (JPO), the China National Intellectual Property Administration (CNIPA), the U.S. Patent and Trademark Office (USPTO), the German Patent and Trademark Office (GPTO), and the Korean Intellectual Property Office (KIPO) but less than 6% in France, the U.K., or other countries ^[12][13]. Analysis of the scholarly academic literature found that studies reviewed trends in EV infrastructure development, circuit design, fast charging, and wireless charging. The next subsections summarize some of the key findings.

3. Charger Infrastructure

Bommana et al. (2023) comprehensively reviewed EV charger topologies and characteristics ^[13][14]; and Acharige et al. (2023) separately reviewed standards, architectures, and converter configurations [6]. Table 1 summarizes the information gleaned from both articles. Chargers are available in four levels based on the duration required to charge the battery with 20 to 50 kWh of energy. Chargers are either on-board or external to the vehicle. The Society of Automotive Engineers (SAE) published several standards that define various aspects of EV chargers such as their general physical, electrical, communication protocol, and performance requirements. Faustino et al. (2023) presented a methodology to increase the utilization of chargers per station by defining and allocating charging zones ^[14][15]. Khamis et al. (2023) proposed a charging strategy that utilized demand-side management to allocate power in the EV charger network ^[15][16]. Johnson et al. (2022) surveyed publicly disclosed EV charger vulnerabilities to cyber-attacks and suggested the vendors must incorporate continuous processes to harden their deployed infrastructure and conduct regular vulnerability assessments ^[16][17]. Al Attar et al. (2023) reviewed switching control strategies to enable bidirectional power transfer between vehicles and the electric power grid. They classified control strategies as either linear or non-linear ^[17][18]. Each control strategy presented had its advantages and limitations in terms of performance, size, and cost.

4. Charger Circuit Design

Vishnuram et al. (2023) conducted a comprehensive review of EV power converter topologies and found that there are many types of implementations, each with advantages and disadvantages in performance, size, and cost ^[18][19]. In a similar review, Ali et al. (2023) classified power electronic converter (PEC) topologies into the four possible quadrants of AC and DC converters: DC-DC, DC-AC, AC-DC, and AC-AC [8]. They identified ongoing challenges as power conversion losses, bulkiness, and electromagnetic interference. Thanakam and Kumsuwan (2023) designed a phase-locked loop (PLL) control method to enhance the quality of bidirectional power transfer in EV chargers ^[19][20]. Berrehil El Kattel et al. (2023) reviewed the implementation of battery charger structures and found that they classify into either on-board or off-board embodiments that implement either unidirectional or bidirectional power transfer by incorporating either isolated or non-isolated AC-to-DC conversion stages ^[20][21]. Karneddi and Ronanki (2023) presented the design of a charger that can be reconfigured to charge multiple types of battery packs requiring different voltage levels ^[21][22]. Gupta et al. (2023) presented the design of an on-board charger that can charge multiple EVs using multiple outputs ^[22][23]. Na et al. (2019) reviewed and classified the topologies of on-board EV chargers into three groups based on the components integrated with the vehicle traction motor system ^[23][24]. On-board chargers must normally communicate information to the charging source, such as the current charge state and end-of-charge time preference ^[24][25]. The study proposed an adaptive voltage-feedback controller for an on-board EV charger that obviates the need for real-time communications with the power source.

5. Fast Charging

Polat et al. (2023) noted that consumer expectations for the speed of recharging an EV to be the same as that for refilling an ICE vehicle tank will increase the demand for fast chargers [7]. Deploying more fast chargers, however, increases the load burden on the electric grid during times of peak demand. Therefore, researchers recently proposed that fast charging incorporate a battery energy storage system (BESS) with controls for cooling to even out the load burden. Pradhan et al. (2023) conducted a comprehensive review to identify the system level and use case-related challenges in transitioning on-board chargers to recent fast charging standards ^[25][26].

6. Wireless Charging

Song et al. (2023) found that although wireless charging for mobile devices and wearable equipment is widespread, the technology is still a developing trend for EV applications, and interoperability requirements are still undefined for high power levels ^[26][27]. Dimitriadou et al. (2023) categorized wireless power transfer as using either far-field or near-field methods, with each having advantages and disadvantages, as summarized in Table 2 ^[27][28]. Vishnuram et al. (2023) classified wireless charging systems more broadly as either static, where the vehicle charges while parked, or dynamic, where the vehicle charges while moving ^[28][29]. The main advantages of dynamic wireless charging include convenience and the potential for vehicles to use smaller batteries, which would reduce both their weight and cost. The main disadvantages of dynamic wireless charging are higher infrastructure costs and lower power transfer efficiency. Yang et al. (2023) discussed how in-pavement wireless chargers can dynamically charge vehicles as they move ^[29][30]. However, current methods require continuous operation at full power, which causes large standby currents and concerns of exposure to harmful electromagnetic radiation.