Are you charging your EV battery in the right way? Do you charge often at home or at charging stations? In this article, we discuss the factors that affect the performance of an EV based on charging practices.
Background on Battery Characteristics
Usually, the life of the battery is the life of an EV because it is the most important and expensive part. Over the course of its life, the battery in electric vehicles undergoes chemical degradation that decreases its capacity until it is unable to power the vehicle. Generally, an electric vehicle is said to have less than 80% of its capacity at the end of its life (EOL). EOL is also assumed to be reached when the battery has more than a 5% self-discharge rate compared to its state when purchased.
Lithium-ion (Li-ion) batteries are the most commonly used battery type in electric scooters. In these batteries, Li-ions move in-between the electrodes through the electrolyte to either discharge current (to move the EV) or charge the battery.
Three main degradation phenomena occur in the battery namely:
- Loss of Lithium Inventory (LLI) is all processes contributing to the depletion of Li-ions in the electrolyte. Occasionally, the Li-ions get deposited on the electrodes of the battery making it less porous. This is known as Lithium plating.
- Loss of Active Material (LAM) is all the processes contributing to the dissolution of the electrode.
- Cyclic Lithiation and Delithiation (CLD) Electrodes are subject to cyclic volume gain and loss under charging and discharging cycles. This creates mechanical stress in them over a period of time leading to cracks and loss of active material.
Impact of Battery Degradation on Vehicle Performance
The 2 main parameters that quantify the above degradation at the vehicle level:
- Capacity fade
- Power fade
Capacity fade is the gradual decrease in the capacity of the battery over continued usage. That is the amount of charge it can store to discharge later.
This decreases the expected range and increases the energy consumption of the vehicle
Power fade is the gradual decrease of the power a battery can deliver over continued usage. Power fade affects EVs in several crucial areas, such as:
- decrease in top speed
- decrease in capability to accelerate
- increase in charging times
- decrease in the capability to travel on unfavourable roads
LAM and LLI affect capacity fade based on whichever is the dominant mode of battery degradation whereas power fade is affected in an additive manner of all three degradation phenomena.
Effect of Charging Profiles on Battery Degradation
Now that we understand the impact of chemical degradation of the battery and in turn on the vehicle performance, let’s look at how the way we charge the battery contributes to such degradation.
1. High charging currents
Initially, higher charging currents were used to decrease charging times. This was done with the hope that EVs would rival Internal Combustion Engines (ICEs) in refuelling times. However, higher charging currents induce Lithium plating, LAM and larger cracks on the electrodes.
2. Operating in buffer zones
Operating a battery when it is nearly 100% and 0% charge, adversely affects the health of the battery by causing LAM of the electrodes.
Nowadays, electric scooter OEMs usually retrofit a buffer zone comprising an area of state of charge (SOC) usually greater than 90% and less than 10%. The customers cannot access this area of SOC. Other OEMs have slowed down the charging period in the buffer zones which means that the time taken to charge after 90% is prolonged.
Even with the buffers configured in the EV, it is advised to stop charging beyond 80%. Also, akin to this, it is advised to stop using the EV below 20% to maximize the life of the battery.
3. Calendar ageing
Calendar ageing is the ageing associated with the battery that does not undergo any charging or discharging cycles. It happens due to LLI and Lithium plating that occur due to the electrolyte slowly decomposing with time.
Parked EVs have a tendency to self-discharge over time. This happens due to minor electrochemical reactions occurring between the electrodes. The greater the age of the battery, the greater its tendency to self-discharge.
EVs parked at a charge greater than 70% increases the calendar ageing process by about 7-8% every year. This is because having a high SOC when parked, puts a greater strain on the battery to stop itself from self-discharging. Calendar ageing contributes to a significant loss in SOH as the electric vehicle is going to be parked for about 95% of the time.
Looking at the various factors that negatively affect the SOH of an EV battery, it is imperative to minimize these effects as much as possible.
A heavily damaged battery cannot be used in second-life applications and must be discarded, polluting the environment in the process. Thus, the environment-friendly tag on the EV is lost, apart from the economic loss incurred for an early battery replacement.
Future Prospects
Many strategies have been recently developed to manage these challenges effectively. Nowadays for preventing higher charging currents a power limit is established in the EV’s battery management system (BMS). This prolongs the life of the battery significantly.
Electric scooter OEMs have given customers the option to decide the charging limit of their electric vehicles. User manuals to extend the life of an EV battery are also given to customers which elaborate on proper parking strategies that need to be used.
Soon, there will be batteries that are capable of getting charged in five to ten minutes. This will become available which employ ultra-high currents with minimal impact on the health of the battery. To realize this, companies are performing state of the art research towards using semiconductor nano-particles as electrode material into which Li-ions can pass through easily. They are also looking at increasing the operating battery temperature slightly to increase the movement of Li-ions.
Nesh connectivity solutions for EV manufacturers retrieves and analyzes the battery and vehicle data discussed above to predict the battery’s state of health (SOH) and state of charge (SOC) at a given time. This allows the prediction of range and timely notifications to the user for charging best practices. Such continuous monitoring of performance helps manufacturers to drive safety and efficiency of their vehicles along with a seamless customer experience.
