Battery Degradation in Electric Vehicles Explained
Battery Degradation in Electric Vehicles Explained
Recently on our AVID Learning podcast, we covered the topic of battery degradation and how the state of health of a battery vehicle is determined. Our listeners specifically wanted to know if there was anything they could be doing to minimise the battery degradation in their new electric vehicles. First of all, it might be helpful to understand what we mean by degradation in an electric vehicle battery and how this happens.
Electric vehicle battery packs are made up of hundreds of lithium battery cells, and when an EV is built with a nice new battery pack it has a certain capacity, but in use, over time this capacity will slowly reduce, this is called degradation. In the EV world it is generally considered that when a battery reaches 80% of its original capacity it is no good for use, so in a 40kWh pack, this would be degradation to 32kwh of capacity. It is helpful to remember that a kWh is the unit of energy storage and equates to 1kw of power for a period of 1 hour. Meaning a 40kwh battery will take 5.7 hours to fully charge from a 7kw charger. BMW still express their battery capacity in terms of Amp Hours (Ah), this is a similar unit of measure and to convert Amp hours to KWH you have to multiply the Ah value by the voltage of the pack kw = volts x amps. BMW prefer this unit of measure because the voltage changes depending on the state of charge of the battery pack.
Many electric vehicles report a battery State of Health parameter, and we have had some of the AVID Technology podcast listeners asking questions about this as well. The State of Health is calculated by the Battery Management System (BMS) comparing the actual amount of electrical charge required to move the battery from one point on the charge curve to another vs the amount it would have taken when the battery was new. This is done by accurately measuring the current flowing into the pack by coulomb counting whilst charging and the voltage of the pack and comparing this to a look table. The State of Charge of the pack is simply based on the cell voltages, lithium cells have an S curve where the cell voltage changes with the state of charge, but as the cell degrades it will take less charge to move the cell up to its charge curve which is indicated by the state of health.
This gradual reduction in capacity over time is a key design consideration when developing EV battery systems. With a huge amount of investment going into making sure this does not happen too quickly. There are several mechanisms that cause degradation in the cell, such as lithium plating and dendrite formation, and the growth of the solid electrolyte interface (SEI) layer and copper and carbon stripping. These mechanisms are due to the electrochemistry in a lithium battery and all result in the loss of active material inside the battery cell and the formation of inactive compounds. So a key question is what causes or can accelerate these degradation mechanisms?
Battery Management & Cooling Systems
The BMS in an EV’s battery pack is a sophisticated management and measuring system which is there to protect the battery and make sure it cannot be damaged. For example, if the BMS detects that the battery temperature is getting too high it will restrict the rate at which it is possible to charge the battery. This is what is behind the Charger-gate issue with the Nissan Leaf. Because the current Leaf batteries are all passively cooled the ability to get heat out of the battery is quite limited. When either charging or discharging the battery hard it will cause the battery to heat up, due to the current flow and the internal impedance of the battery giving ohmic heating which is current squared x resistance. The BMS is monitoring the battery temperature and will see this happening. In order to ensure that the battery is not damaged, it will then slow the charge rate until the temperature has dropped.
If it is a hot day, the ability to get heat out of the battery pack will be further reduced meaning it is more likely that the car will restrict charge rates when we have high ambient temperatures.
Many EV’s including the 2019 65kWh version of the Leaf are coming to the market with active liquid cooling, sometimes this even uses the vehicles A/C refrigeration system to achieve subcooling of the battery. This ensures that even when the ambient temperatures are very high the battery temperature can be maintained at its optimum level below the ambient temperature. Obviously, this comes at a cost of a more complex system and the energy required to operate the refrigeration and cooling systems.
So in most cases, the BMS is working to ensure there is no excessive damage caused to the battery pack, but it does have an operating window in which it will allow the pack to operate. If as an owner of the vehicle you really wanted to ensure you maximised the life of the battery by preventing degradation there are steps that you could take.
Degradation mechanisms happen more readily at elevated temperatures, rapid charging, and hard driving will result in raised pack temperatures due to the high current flows. If the EV is equipped with a subcooling system that utilises the A/C refrigeration system this should not make much difference to the pack temperatures, although it will have an impact on efficiency as we need energy to operate this chilling system which you would see in a reduced driving range and increased charge times. However, if the EV is equipped with only a basic liquid cooling system or a passive cooling system minimising activity that would cause the pack to be in the upper quartile of its operating temperature range will result in improved degradation performance. For example, if the option is there to slow charge (3-7kw) overnight when the ambient temperature is preferable this should be done in preference to fast charging during the day.
Degradation also occurs more readily at higher SoC levels, meaning there is some sense to the commonly heard advice about not keeping your car battery continually topped up to 100% i.e. plugging in every time you park the car. You should never top the battery up if it is not necessary.
In reality, manufacturers are generally finding that battery system degradation is lower than they had been predicting. This is due to actual real-world vehicle use being easier on the battery than the prediction models. Transient, stop-start driving in real-world traffic with lower acceleration rates and average speeds allow the battery opportunity to cool down and the relatively infrequent use of fast chargers because of the convenience of simply plugging in overnight.
There are also some myths around EV batteries connected to temperature which can be explained by some of what has been mentioned already in this white paper. In low temperatures, for example, energy is required for heating the cabin for the occupants, this uses energy from the battery which will reduce the driving range, but there is no decrease in battery capacity. It is just that rather than all the energy being used to drive the vehicle some of it is also being used to heat the occupants. In very low temperatures the battery cells themselves will need to be heated to allow charging. This will mean it will take longer to charge the car because some of the energy going in is being used to warm the battery system rather than going into the battery, again no actual change in battery capacity. In an ICE vehicle efficiency is worse when the engine is cold, but once up to temperature, there is generally sufficient waste heat from the engine to keep the occupants warm even on the coldest of days.
At high ambient temperatures, energy is needed to operate A/C systems for cooling the cabin, occupants and possibly battery depending on the vehicle. Again this will reduce driving range but not pack capacity. At high ambient temperatures, energy will be being used during charging to cool the pack, which will reduce the rate at which the pack is charging, but is not a problem with the pack. In this way, it is not much different to an ICE vehicle, which will use more fuel to drive the A/C system on hot days.
Improved performance can be achieved by minimising solar gain while the car is parked, windscreen blinds and tinted glass will help this. Also making full use of preconditioning features to bring the car to temperature while plugged in so it does not drain energy from the car’s battery which would affect the battery range.
In summary, battery degradation is a lot less than manufacturers had predicted it to be, and essentially that is because EVs are not being used as aggressively in the real world as the test cycles that the manufacturers were thinking about when these cars were being developed. Also, a vast amount of money has been used to make sure that the degradation has been absolutely minimised. There are a few things you can do to make sure you personally can minimise this further, such as avoiding fast charging on hot days when the ambient temperature is high, driving your car sensibly and not aggressively driving, charging your car from a 3-7kw charger whenever you can. All of this will help your battery in the long term.
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About the Author
The Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified powertrain systems for heavy-duty and high-performance vehicles.