Penn State team devises new control strategy to enhance EV battery performance when driving in cold temperatures
Researchers at Penn State, with a colleague from EC Power, have devised a new control strategy that can rapidly restore EV battery power and permit full regeneration while driving at temperatures as low as −40 °C. The strategy involves heating the battery internally during regenerative braking and rest periods of driving.
In a paper in the Journal of Power Sources, they report that this technique fully restores room-temperature battery power and regeneration in 13, 33, 46, 56 and 112 seconds into uninterrupted driving in 0, −10, −20, −30 and −40 °C environments, respectively. Correspondingly, the strategy significantly increases cruise range of a vehicle operated at cold temperatures—49% at −40 °C in simulated US06 driving cycle tests. Based on their findings, the team suggests that smart batteries with embedded sensing/actuation can significantly improve their performance compared to more conventionally controlled counterparts.
|Comparison of energy balance between the “Battery Heating While Driving” case and the base case during simulated US06 drive cycle tests at −40 °C. Zhang et al. Click to enlarge.|
Drastically reduced driving range is a major challenge for electric vehicles (EVs) operating at subzero temperatures as it exacerbate drivers’ range anxiety. Two technical problems of Li-ion batteries are particularly long-standing. First, regenerative braking is restricted or completely turned off at cold temperatures due to the phenomena of lithium plating that could severely reduce battery life and increase safety hazards. Second, there is significant power loss, up to 10 fold at −30 °C, due to sluggish reaction kinetics, slow diffusion, reduced electrolyte conductivity, and increased solid-electrolyte interface (SEI) resistance at low temperatures.
Great efforts have been made to increase battery power at cold temperatures, notably reformulating electrolytes, hybridizing batteries with high-power supercapacitors, and preheating batteries before driving. Among these approaches, battery preheating has been extensively investigated due to its relatively simple implementation. But preheating is slow, typically tens of minutes, and inconvenient, prohibiting instantaneous mobility of EVs.
Here we demonstrate an active control strategy that can rapidly restore EV battery power while driving, which eliminates any need to wait for preheating. This control strategy represents a new paradigm allowing batteries to be actively controlled and manipulated. We also demon-strate, through simulated US06 driving cycle tests and an energy balance analysis, that power restoration while driving could significantly increase EV driving range by fully recuperating braking energy and significantly increasing utilization of energy stored.—Zhang et al.
For the experiments, the team used self-heating Li-ion battery (SHLB) NCM cells with two embedded nickel foils. Each cell had a 152 × 75 mm footprint area, nominal capacity of 9.5 Ah and weighed 210 g. Two pieces of polyethylene terephthalate coated nickel foil, each with resistance of 78 milliOhm at 20 °C, were stacked at 1⁄4 and 3⁄4 of cell thickness for uniform heating. The two pieces of nickel foil are connected in parallel with their total resistance of 39 milliOhm at 20 °C. The added weight and cost due to nickel foils are about 1.5% and 0.4% of the baseline battery.
One end of the nickel foils are connected to the negative terminal of SHLB cell while the other end extends out of the cell as an activation terminal (ACT). A switch is placed between positive terminal and ACT terminal. When the switch is ON, the SHLB cell works at heating mode as high current passes through the nickel foils and generates heat very rapidly. When the switch is OFF, the SHLB cell works at normal mode just like a conventional cell without embedded nickel foils.
The control strategy activates self-heating during braking and rest periods of initial driving, thereby not compromising battery power delivery to the vehicle. The team found self-heating while driving to be very energy-efficient and fast, with the heating energy supplied from both stored battery energy and vehicle braking energy.
The strategy enables full recovery of regenerative braking energy and much increased utilization of available energy even at low temperatures, which in turn significantly increases vehicle cruise range. With the battery temperature remaining high, near room temperature, at the end of driving, the vehicles would also be primed for fast recharging after driving.
We showed that delivering 90% of the room-temperature driving energy in the −40 °C environment is highly possible with a future battery of higher energy density and improved thermal insulation of a battery pack, making functionality and performance of EVs truly weather independent like internal combustion engines. This work paves the way for specification of all-climate range (ACR) (−20 °C to 40 °C), a new criterion intended to measure cruise range of battery electric cars in real-world conditions.—Zhang et al.
Guangsheng Zhang, Shanhai Ge, Xiao-Guang Yang, Yongjun Leng, Dan Marple, Chao-Yang Wang (2017) “Rapid restoration of electric vehicle battery performance while driving at cold temperatures,” Journal of Power Sources, Volume 371, Pages 35-40 doi: 10.1016/j.jpowsour.2017.10.029