Questions

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.

Methods

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).

Findings

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.

Table 1:Literature on the influence of charging rate on battery degradation
Reference Type of battery Number of batteries tested Charging rate of tests Conclusion
Gao et al. (2017) 18650-type NMC 21 0.5C, 0.8C, 1C, 1.2C, 1.5C NMC battery degrades significantly on C-rates higher than 1. Battery degrades by 10% and 23% at 1.2C and 1.5C respectively at the end of 300 cycles as against degradation by 7% at 1C.
Somerville et al. (2016) 18650-type NMC 12 0.7C, 2C, 4C, 6C Increased charging rates negatively affect the lifetime. Charging at rates higher than 4C alters the chemical composition resulting in significant damage and reduction of life.
Anseán et al. (2016) LFP 3 1C, 4C Capacity degradation is 15% at 1C and 17% at 4C after 4,000 cycles. Up to 1000 cycles, the degradation from both charging rates are similar.
Wang et al. (2011) LFP 200 0.5C, 2C, 6C, 10C Experimental results indicated that the capacity loss was strongly affected by time and temperature, but minimally by charging rates.

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:Battery chemistry, capacity, charging power limits, and C-rates
EV model (market share of BEVs1) Battery chemistry2 Battery capacity or size3 (kWh) Charging power limit3 C-rate in per hour by charger type (charger power limit kW)
AC4
(kW) 		DC
(kW) 		AC4 Home charging
(7 kW)		 AC4 Fast charging
(22 kW)		 DC rapid
(50 kW) 		DC ultra-rapid
(150kW)
Nissan Leaf (22%) NMC 40 or 62 6.6 49 0.11C 0.11C 0.79C NA5
Tesla Model 3 (17%) NCA 55 11 170 0.13C 0.20C 0.91C 2.7C
BMW i3 (6%) NMC 42 11 49 0.17C 0.26C 1.17C NA5
Volkswagon e-Golf (5%) NMC 35.8 7.2 40 0.20C 0.20C 1.12C NA5
Renault ZOE (9%) NMC 55 22 46 0.13C 0.40C 0.84C NA5
Tesla Model S (7%) NCA 100 16.5 250 0.07C 0.17C 0.5C 1.5C
Kia e-Niro (3%) NMC 42 or 67 7.2 50 0.10C 0.11C 0.75C NA5
Jaguar I-Pace (6%) NMC 90 11 104 0.08C 0.12C 0.56C 1.2C
Tesla Model X (4%) NCA 100 16.5 250 0.07C 0.17C 0.5C 1.5C
Hyundai IONIQ (2%) NMC 40 7.2 44 0.18C 0.18C 1.10C NA5

1 Department for Transport 2020.
2 https://pushevs.com/2020/04/04/comparison-of-different-ev-batteries-in-2020/ Accessed January 2021.
3 https://ev-database.uk/ Accessed January 2021.
4 The electricity that comes into our households from the grid is Alternating Current (AC). Most modern electronics, including EVs, use Direct Current (DC) for their operation. During AC charging of EVs, current from the grid is converted into DC using the on-board inverter, whereas in DC charging, the rapid or ultra-rapid charger converts AC into DC.
5 Many models do not currently support the use of ultra-rapid chargers.

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.

Acknowledgements

This research was funded by the ‘Park and Charge’ project, awarded to the University of Oxford by Innovate UK, under project reference: 105428.