Battery Electric Vehicle (BEV) charging time is a critical concern for driving long distances. DC fast charging is an important factor for raising vehicle value and increasing BEV market potential. For this reason, infrastructure, vehicles, and batteries are being designed to achieve shorter charge times. One industry objective is the ability to charge vehicles in under 15 minutes. However, achieving high BEV charge rates is a complex interrelated multidimensional problem. It starts with having the proper high power capability infrastructure. Vehicle systems also have fast charging capability limits. Additionally, the overall system must meet vehicle cost and performance targets.
High battery charge rates result in heat from atomic charge transfer at the cell, pack, and system levels. Increasing battery charging current increases the need for cooling, as well as heating, and puts more of a premium on waste heat recovery to condition the battery.
The thermal characteristics of battery cells are the core challenge. The battery thermal management system is not designed to dissipate maximum battery thermal losses for economic value (cost) reasons. A portion of a battery’s losses are absorbed by its thermal mass, therefore the thermal system typically has a significantly lower capacity than maximum heat loss. Furthermore, during fast charging the battery thermal mass becomes saturated, and heat loss must be reduced. This is done by lowering the charge current toward the thermal system capacity to limit cell temperature rise.
Many Techniques to Improve Vehicle Charge Rates
The ITB Group examined the value of 22 rapid charging technologies in five technology pathways as shown below. New battery pack developments with the latest cell designs show high potential for improving DC fast charge time. The highest value fast charge technology is shifting to solid-state cell technology, but with a low technical readiness, this will not have great commercial impact until around 2030. In parallel, the value must be further increased by reducing cost.
Tesla, for example, has chosen to focus on cell and pack cost reduction but has also made thermal changes for performance improvement. Tesla raises the bar marginally in performance but uses cost leadership to compete with other rivals. ITB expects other OEMs will deploy cell improvements as their baseline to lower costs and also make charge rate improvements.
22 Fast Charging Technologies in 5 Pathways
Source: The ITB Group
Developing Solution Sets
The core to faster charging is improved cell design, which is complemented by improvements outside the cell – electrical system improvements, as well as battery pack cooling and heating. All of the domains – electrical, chemical, and mechanical are being improved for better performance at low cost. One example is the importance of higher voltages (like 800V or more) to reduce current by half which can reduce joule thermal effects by three quarters with the same system power delivery.
After cell improvement directions are chosen, then other pathways enter into the picture. There are several electrical charging, battery pack, cooling plus cell, and pack heating technologies with decent value. When we step back, many of the technologies involve a relatively low cost and a modest improvement in C-rate. To have more impact, they could be combined but may be more costly than changes that reduce cost. The key question when developing a technology to improve charge rate is how cost can be minimized or offset.
A significant portion of cell heating comes from the external cell interconnects (the low-voltage busbar). As internal cell electrochemical and joule losses are reduced for solid-state and conventional lithium batteries, interconnect and busbar thermal management become more important, particularly for fast charging. Therefore, there is significant development into busbar thermal management for improved performance, and to reduce cost.
One notable technology alternative is dielectric fluid battery immersion cooling. Our estimate is that the theoretical C-rate potential of immersion cooling is 3.5C. Alone, increasing cooling capacity for a cold plate solution cannot compete with immersion cooling, but in combination with other techniques may be able to offer better value. On the other hand, the challenge for immersion cooling is to lower cost. One way to offset the cost of an immersion cooling system is to immersion cool the busbar to reduce busbar size and cost which could raise the value of immersion cooling significantly.
To summarize, there are many pathways ways to improve vehicle charge rates. ITB’s customers are developing solution sets that will be applied to electric vehicles as their volume increases. Contact The ITB Group to gain insights and construct strategies for solution development and commercialization.
