When we see “tested under laboratory conditions”, we often assume real-world conditions will lead to faster degradation of a product.
But experts from Stanford University have found the opposite is true for electric vehicle (EV) batteries. Their new research shows traditional laboratory testing leads to faster degradation, while real-world use gives substantially more battery life, extending the lifespan of the entire EV. Researchers found the stop-start way we drive and the variable rate the battery discharges power actually prolongs battery life by up to 38% compared to traditional tests.
This is good news for EV drivers – and for efforts to electrify transport. This extra battery life would translate to more than 300,000 more kilometres an EV could drive before needing battery replacement, the researchers say.
Longer-lasting batteries would reduce the total cost of EV ownership – and benefit the environment by getting more use out of each battery.
How do we usually test battery degradation?
Common battery chemistries such as lithium-ion will degrade over time. As lithium ions shuttle back and forth across the electrode, some will be diverted or trapped. As batteries age, they don’t hold as much charge.
So how do you measure this?
When you make an EV battery, you don’t want to spend 20 years testing its longevity before release. To test batteries more quickly, researchers have tended to estimate battery degradation rates by using a constant rate of battery discharge. Studies of EV battery degradation are normally done in a laboratory environment under controlled conditions.
In the lab, researchers subject the battery to rapidly repeated charge-discharge cycles. Power is discharged at a constant rate. Observing the gradual drop in capacity gives us the degradation levels over time. This is how we get estimates such as “retains 80% capacity in ten years time”.
But while this method is widely used, it has limitations. Discharging power at a constant rate is not how we really drive. We might accelerate fast to get onto the freeway, spend lots of time accelerating and braking in stop-start traffic, or do a quick run to several shops. Plus, much of the time the battery is not being used. Instead of a constant drain on the battery, it’s a mix.
What the Stanford researchers have done is test EV batteries in realistic ways, imitating the way we actually drive. This is known as “dynamic cycle testing”.
Mimicking real-world use
To replicate real-world usage and driving patterns, the Stanford team designed different discharge patterns for EV batteries, some based on real driving data. The researchers then tested 92 commercial lithium-ion batteries for more than two years across the different profiles.
The results showed batteries tested using real-life scenarios degraded substantially slower than expected and had higher battery expectancy than those tested under lab conditions. Even better, the more realistic the battery use, the slower the battery degrades.
Battery researchers have long assumed rapid acceleration is bad for battery life. But this isn’t the case. Short acceleration and regenerative braking – where EVs charge their batteries during braking – were actually associated with slower battery degradation rates.
Is this backed up in practice?
A number of other recent studies have looked at how batteries perform in practice using data from EVs in operation, including commercial vehicles. These studies also found correlations between real-world use and lower battery degradation rates.
A 2024 report by GEOTAB researchers used telematic remote monitoring to get data from 10,000 EVs. The study found that improved battery technology is leading to slower degradation. Newer EVs lose about 1.8% of their health per year – a sharp drop compared to the 2.3% degradation rate in 2019.
Read More: Electric vehicles in Africa: what’s needed to grow the sector
Several factors influenced battery longevity other than use patterns. One of these is worth noting – frequent use of DC fast chargers by high-use vehicles is linked to faster battery degradation. The effect is more notable in hot climates. By contrast, slower “level 2” charging is better for battery longevity. Overall, the researchers found the best way to prolong battery life was to keep the charge between 20% and 80%, reduce exposure to extreme temperatures and limit fast charging.
Another 2024 report analysed the batteries of 7,000 EVs used intensively over 3-5 years. The report found lower degradation rates than expected.
This report found most batteries still had good capacity (more than 80%) even after propelling vehicles more than 200,000 km. Factors such as use patterns, advances in cell chemistry and optimised battery management were also found to influence battery ageing.
What does this mean for the EV transition?
These results suggest EV owners may not need to replace expensive battery packs for several additional years. Over the lifetime of an EV, this means lower operating costs.
The findings are also encouraging for fleet operators. Batteries in high-mileage commercial EVs should remain reliable even after heavy use.
Car manufacturers and technology providers can benefit by updating their EV battery management software to take these findings into account. This would help to increase battery longevity under real-world conditions.
Fewer battery replacements will mean fewer batteries to recycle. Once removed from the vehicle, EV batteries can be used to store energy for homes or businesses for years. These findings suggest a longer and more reliable second life for the batteries.
These findings may help reassure drivers interested in electric vehicles but unsure about battery lifespan.
- is a Professor of Future Urban Mobility, Swinburne University of Technology
- This article first appeared in The Conversation
