The transition to electric vehicles (EVs) is seen as an important means to reduce global carbon emissions from the transport sector, but a number of barriers to mass adoption of EVs have been identified. Technical concerns such as battery range, charging time, and battery life are prominent among these, particularly for battery-only EVs as compared to hybrids (Biresselioglu, Kaplan, and Yilmaz 2018). Increased battery sizes increase the range of EVs and the provision of rapid charging infrastructure reduces charging time, but we ask what effect these have on the third concern of EV battery life?
We aim to answer this question, whilst considering the impact of charging speeds on battery life more generally. We also have identified a lack of clear guidance for prospective EV purchasers and for charging infrastructure providers on the interactions between battery types, range, charging times, rate of charge, and battery life. Therefore, we aim to fill this gap with a simple table providing this information for ten of the most common private electric cars on the market today.
We define battery life and the processes causing battery degradation, then review the sparse literature empirically testing battery degradation. As degradation and the impact of charging speeds are dependent on the size and type of battery, we use web searches to synthesize information on how choosing different charging options affect battery life for common EV models in the UK.
All batteries degrade with time and use. Most EVs have a warranty for eight years or 100,000 miles, whichever is earlier. An EV battery is considered at end of its life if it no longer maintains 80% of total usable capacity and has more than 5% self-discharge rate over a 24-hour period (Engel et al. 2019). Accelerated battery degradation can be caused by charging and discharging patterns, such as repeatedly using the entire capacity of a battery, or repeated rapid charging (IEA 2020).
Charging (and discharging) patterns are measured via ‘C-rates’ per hour, so that 1C-rate means that the battery will be completely charged or discharged in 1 hour at that level of current. Ignoring the conversion efficiencies, the C-rate can be calculated by dividing the charger’s power level by the battery capacity or size. For a given charging power, the larger the battery capacity, the lower the C-rate for charging.
Battery life is also dependent upon the type or chemistry of the battery used in the EV, which can be Lithium Nickel Manganese Cobalt Oxide (NMC), Lithium Nickel Cobalt Aluminum Oxide (NCA), or Lithium Iron Phosphate (LFP).
A review of literature on the impact of charging speeds on battery life was conducted using keywords of ‘Lithium battery degradation’ and ‘Lithium battery life’ on Google Scholar. Although there is considerable work on the different degradation mechanisms, few studies quantify degradation through empirical experiments on batteries suitable for EVs. Even less research maintains uniformity across discharge rates to enable a direct comparison of the impact of charging rates. The remaining literature is summarized in Table 1 and shows that for NMC batteries, charging rates above 1C rate adversely affects the battery life whereas, for LFP batteries, the battery life is not significantly affected by charging rates up to 4C.
Thus, rapid chargers can hasten the degradation of batteries in vehicles with NMC battery chemistry. However, the battery management systems in vehicles are set to limit the level of power received to prevent accelerated degradation. Therefore, the impact of rapid charging also depends upon these limits, which can be used as the numerator instead of the charger’s power level in calculating C-rate. Table 2 shows the battery chemistry, size and any power limits of the top ten battery-only EV models sold in the UK (Department for Transport 2020). The vehicle power limit or the maximum power capacity of the charger, whichever is lower, is then used to calculate the respective C-rate.
Table 2 shows that EV models with NMC batteries have DC charging power limits that prevent the C-rate from going much above 1C. Even without power limits set by battery management systems, none of these EVs come close to exceeding 1C whilst using fast chargers. For rapid and ultra-rapid charging, the exceedance is minimal due to the power limits. Consider the Jaguar I-Pace: the additional degradation is limited to around 3% in 300 cycles when using ultra-rapid chargers. Such charging rates can reduce the NMC battery life by up to 10% as against home, fast or rapid charging in 300 cycles. Thus, regular rapid and ultra-rapid charging does reduce battery life, but this is minimal due to battery management systems.
This research was funded by the ‘Park and Charge’ project, awarded to the University of Oxford by Innovate UK, under project reference: 105428.