Battery Decay
At the heart of all electric vehicles is the battery system that deliver the desired power to the cars rather than the traditional fuel sources that power the car, petrol and diesel. However, these systems have a finite amount of life cycles before they are no longer provide the desired output required for that vehicle. Each life cycle is defined as the amount of complete charge-discharge cycles that one battery can achieve before its capacity decreases by a minimum of 20% of its original rated capacity [1].
There are three main types batteries used for electric vehicles, these being Lead-acid, Lithium-ion and Nickel-metal (NiMH). These batteries are primarily used for numerous reasons, such as performance potential, safety, life cycle and cost. The Lead-acid battery is the most developed and oldest of batteries used. The majority of electric vehicles use this type of battery due to its developed technology and low cost. However, despite the advancement in Lead-acid batteries and the large volume of production, they have a poor cycle life when compared to other batteries. NiMH can also be considered as a well developed battery, with a much greater energy density than that of Lead-acid. However, it is far less efficient in charging and discharging (60%-70%) [1]. Although, despite the poor efficiency the NiMH batteries, when used effectively, can have an extremely longer cycle life. Although Lithium-ion batteries are most commonly known for their use in laptops and other electronics, they are becoming evermore present in electric vehicles. This is a result of the batteries achieving the highest power density of all batteries per unit volume.
There is a large array of factors that affect the cycle life of an electric vehicle battery. These include the rate of discharge, ambient temperature, depth of charge, maintenance carried out on the battery and many others [2]. The battery lifetime that manufacturers produce assumes complete charge and discharge each cycle at ambient temperature. However, in V2G applications, batteries are only discharged partially each cycle and are exposed to severe ambient temperature. Ambient temperature and the depth of discharge have a much greater impacting in the batteries cycle life during V2G. The greater the depth of discharge and the greater the temperature results in the battery having a significantly shorter cycle life. Therefore, if a thermal management system can be installed into the electric vehicles, the ambient temperature could be more readily controlled resulting in an optimal temperature value and could see the battery life increase [2]. The effects of battery wear in V2G will also vary greatly with driving efficiency (how many miles are achieved per kilowatt hour). The greater the driving efficiency, the less effects it has on your battery. This is a result of less energy being consumed, with a greater driving efficiency, over a certain distance and therefore more residual energy stored in the battery leading to less depth of discharge in V2G and subsequently an increase in battery cycle life.
Lead-acid and NiMH batteries are not cost effective for V2G use due current electricity tariffs. However, lithium-ion batteries are cost effective due to their extensive cycle life. Due to the ever increasing development and advances in Lithium-ion batteries, future research into V2G technology using this battery will be more beneficial [4] [6].
Some V2G opponents claim that using vehicle-to-grid technology makes the car batteries less long-lasting. The claim itself is a bit strange, as car batteries are being drained daily anyways – as the car is used, the battery is discharged so we can drive around. Compared to this, vehicle-to-grid discharging doesn’t affect the battery life, as it only happens for a few minutes a day. However, EV battery lifecycle and the impact of V2G on it are studied constantly [3].
References
[1]. Zhou, C., Qian, K., Allan, M. and Zhou, W., (2011). Modeling of the Cost of EV Battery Wear Due to V2G Application in Power Systems. IEEE Transactions on Energy Conversion, 26(4), pp.1041-1050. [online].
Available from: https://ieeexplore.ieee.org/document/5958591 [Accessed 05 Apr 2020]
[2]. Bishop, J., Axon, C., Bonilla, D., Tran, M., Banister, D. and McCulloch, M., (2013). Evaluating the impact of V2G services on the degradation of batteries in PHEV and EV. Applied Energy, 111, pp.206-218.[online].
Availablefrom: https://www.researchgate.net/publication/257157677_Evaluating_the_impact_of_V2G_services_on_the_degradation_of_batteries_in_PHEV_and_EV [Accessed 05 Apr 2020]
[3]. Europian Commission (2020). Batteries & Accumulators [online].
Available from: https://ec.europa.eu/environment/waste/batteries/ [Accessed 05 Apr 2020]
[4]. Dubarry, M., Truchot, C., Devie, A., Liaw, BY. (2015). State of Charge Determination in Lithium-Ion Battery Packs Based on Two-Point Measurements in Life" Journal of Electromechanical Society, 162 (6) A877-A884 [online].
Availablefrom: https://www.phmsociety.org/sites/phmsociety.org/files/phm_submission/2014/ijphm_14_011.pdf [Accessed 06 Apr 2020]
[5]. Hajo Ribberink, (2015). Battery Life Impact of Vehicle-to-Grid Application of Electric Vehicles, KINTEX, Korea, 3(6), pp 528-547 [online].
Availablefrom: http://www.evs28.org/event_file/event_file/1/pfile/EVS28_Hajo_Ribberink_Battery_Life_Impact_of_V2G.pdf [Accessed 06 Apr 2020]