Range anxiety is perhaps the greatest barrier to the rapid transition from vehicles with internal combustion engines to electric powertrains. Range anxiety refers to the perceived limited driving distance of EVs before batteries need to be recharged. Although the energy density and capacity of batteries is increasing, extending the distance to in excess of 500km on some vehicles (or more than 99% of daily driving distance), the recharge time is still substantially greater than the time to refill a vehicle propelled using fossil fuel. Indeed, even at a fast charge station, a typical battery EV will take at least 30 minutes to reach 80% capacity. Charging overnight will be satisfactory for many consumers negating the need for fast charging. Transport and logistics, however, are two areas where continuous use of vehicles is demanded, and electrification is critical as they have historically had high carbon footprints.
So, the solution seems straight-forward: simply charge the battery faster. Charging infrastructure providers are actively increasing the power however the batteries themselves are the limiting factor. This is because lithium ion batteries get very hot and can lead to catastrophic failure at high temperatures. So why does the battery get hot when charging and also on discharging? The main reason is that the battery is a bit like a toaster.
The flow of electricity in a circuit is not perfectly efficient. That is, as the electrons move along they encounter some resistance from ions resulting in collisions and the generation of heat. The greater this internal resistance is, the greater the amount of heat generated. This phenomena, known as Joule or Ohmic heating, has been exploited in many technologies of the past 100 years including the incandescent light bulb and in the heating elements of electric toasters. In these applications, the high resistance is necessary to produce the desired heating effect however in many other applications, including Li ion batteries, any increase in temperature is problematic.
It stands to reason then, that battery producers aim to keep the internal resistance of the circuits as low as possible but unfortunately the resistance cannot be completely eliminated. So batteries heat up. This is most problematic when the batteries are subjected to high effective currents (or flow of electrons). For instance, doubling the current increases the heat generated by a factor of four. As modern batteries become more energy dense and the desire to get vehicles back on the road quickly increases, it can be seen that the heat problem needs to be addressed.
As heat cannot be avoided with this current generation of EV batteries, how can it be managed effectively? Some EVs have used very basic cooling systems, such as air. Whilst this is simple and cheap, it is not effective for very high heat loads experienced with larger batteries or extremely hot environments. The current standard is to use liquid cooling, similar to that for internal combustion engines. Typical water-glycol coolants can either be passed in between cells, battery packages or even through a cooling plate to manage the heat generated. One big advantage of liquid cooling is that the same cooling loop can be used to direct heat away from other hot sources in the power electronics such as the motor or inverter. Furthermore, the “waste” heat can be recycled for heating of the passenger cabin reducing the load on the battery system.
Improvements in cell packaging and battery management systems are also areas under intensive research to reduce the effective heat load during charging. Furthermore, modern coolants designed specifically for battery systems rather than the more demanding internal combustion engine formulations are also under development. Nanofluid coolants that break the previous thermal limitations of water-glycol systems can make a real difference in cooling performance as well.
Ultimately, all of these strategies treat the symptom and do not directly address the root cause of the heat generated. Developments in new cell chemistries as well as solid state batteries may lead to a substantial reduction in the internal resistance of the circuits resulting in less heat generated during charging. The large scale investment though in Li ion gigafactories suggests though that these next generations of batteries may be some time off.