How Long Do EV Batteries Last (or: Why Do EV Batteries Fail)?

How long do the high-voltage batteries in electric vehicles last? Well, let's start with the good news: most batteries are still in vehicles. Yes, replacement batteries can be expensive (unless you repair them like we do), but barring physical abuse like accidents the battery goes the distance. Hat tip to the team at Recurrent for this info: 

• Older vehicles have higher replacement rates. Alternatively, later-generation vehicles have lower failure rates
• Newer models have bigger batteries, which have greater range and the same level of degradation on a newer battery is more likely to give you the km's you need
• Battery management systems and, very importantly, thermal management of batteries continues to improve, thus extending the useful life of batteries

So Then, Why Do EV Batteries Fail?

Battery degradation refers to the gradual decline in a battery's ability to store and deliver energy over time. This process is influenced by several factors, including chemical wear and tear, environmental conditions, and usage patterns.

Chemical Factors

• Cyclic Degradation: Each charge and discharge cycle slightly reduces a battery's capacity. Deeper discharges and higher charge rates accelerate this wear, leading to diminished performance over time.

• Calendar Ageing: Batteries degrade even when not in use due to internal chemical reactions. Factors such as temperature and the state of charge during storage influence this aging process.

• Overcharging and Deep Discharge: Regularly charging a battery to 100% or discharging it to near 0% can accelerate degradation. Overcharging causes excessive voltage stress, while deep discharges strain the battery's internal structure.

Environmental Factors

• Temperature Fluctuations: Extreme temperatures adversely affect battery health. High temperatures speed up harmful chemical reactions, while cold temperatures impede efficient charging and discharging.

• Humidity and External Conditions: In industrial environments, factors like high humidity, dust, and vibrations can lead to corrosion and other forms of battery damage.

We'll get to what you can do to manage battery degradation a little later on in the article. First, let's unpack the technical workings of battery degradation. 

Understanding Battery Degradation

We'll take a brief journey into the physics of batteries. The end result of battery degradation is either power fade (ability to deliver power decreases), or capacity fade (ability to hold charge decreases), or both. These are the effects that we can observe. What we've already outlined above are some of the factors that drive these possible outcomes. Here, we take a closer look at what's happening inside the battery to understand the working of cause and effect of why EV batteries fail. 

Researchers from the Universities of Oxford and Warwick (Birkl et al, 2017) studied the complex interplay of physical and chemical mechanisms that lead to loss of power or loss of capacity in lithium-ion battery cells. The diagram below lists the most common degradation mechanisms. We'll save the detail on each of these for another post. For now, suffice to say there is a complex interplay of degradation mechanisms at at work. 

Degradation mechanisms in lithium-ion cells (Birkl et al, 2017)

Understanding what happens is one thing. Understanding why it happens is another. The figure below shows the cause and effect of degradation mechanisms and the associated degradation modes. By measuring Open Circuit Voltage (OCV), the difference between the anode and cathode voltage, researchers effectively capture a thermodynamic fingerprint of a cell's electrodes at any point in time. Changes in this fingerprint offers valuable insight into degradation pathways of the individual electrodes and the cell as a whole. The short of it, there are three commonly reported degradation modes:

1. Loss of lithium inventory: Lithium ions are consumed by parasitic reactions such as solid electrolyte interface (SEI) growth, lithium plating, and decomposition reactions. These ions are therefore no longer available for cycling between the cathode and anode, thus leading to capacity fade.  
2. Loss of active anode material: Degradation mechanisms like particle cracking and loss of electrical contact can reduce the active mass available for the insertion of lithium. If that happens, it can lead to both power and capacity fade. 
3. Loss of active cathode material: Corrosion of current collectors and electrode particle cracking, amongst others, can lead to a reduction of active cathode mass available, which results in capacity and power fade. 

Cause and Effect of Battery Degradation

Bringing it all together, we have chemical and environmental factors that represent causes of battery degradation (e.g., high temperature), which triggers degradation mechanisms (e.g., electrolyte and binder decomposition), resulting in loss of lithium inventory and loss of active cathode material, and ultimately capacity and power fade. Below diagram shows how it all connects.

Cause and effect of degradation mechanism and associated degradation modes (Birkl et al, 2017)

How You Can Extend the Lifespan of EV Batteries (or: How to Mitigate EV Battery Degradation)

Mitigating battery degradation in an electric vehicle requires a combination of mindful driving habits, proper charging practices, and regular maintenance. Here are some practical tips:

1. Mindful driving: avoid aggressive acceleration and hard braking, as this can cause thermal stress on the battery. Avoid parking in areas with prolonged exposure to extreme heat. And if you live in a cold climate, pre-condition your battery for driving or charging. Extreme temperature, both high and low, is linked to multiple degradation mechanisms. 
2. Smart charging: maintaining the battery charge between 20% and 80% is one of the simplest and most effective ways to prevent overcharging and deep discharging. Limit DC fast charging because this generates excess heat and accelerates lithium plating (dendrite formation). Slower charging rates, especially at a high state of charge ("% full") can mitigate this effect. Also, minimise the depth of discharge (DoD, how much you deplete a battery) because frequent deep discharges reduces the number of usable cycles. 
3. Regular maintenance: keeping your SV's software up to date will allow you to benefit from the latest improvements in battery management. Servicing your EV and monitoring battery health regularly can help detect early warning signs and allow you to take preventative measures. You can always get a HealthCheck done by one of our Authorised Fitters if you're not sure what to make of the data on your dash. 

Okay, so it's good know what can cause EV batteries to degrade over time, but we should not be alarmed. For one, knowing why EV batteries fail helps us understand what we can do to extend their lifespan. And car and battery makers are in our corner here with advances in battery management systems and new materials that increase battery longevity. 

What's more, the push toward a circular economy for EV batteries includes advancements in the repairability and recyclability. Modular battery systems that allow for individual cell replacements further extend overall battery life.