Stress Level: The Key Drivers of Battery Degradation [The Battery Cycle #6]

In 2021, Claudius Jehle, CEO of volytica diagnostics GmbH, and Sustainable Bus launched a series of articles around the “The Battery Cycle”. They shed light on the complexities of Li-Ion batteries and provided valuable insights for anyone involved in electric mobility. In 2025 and 2026, the articles were updated with real-world data and lessons learned to understand how battery chemistry affects real-world operations. (The article was first published in June 2022, this is a revised version.)
For questions and contributions please reach out to us via contact@volytica.com.

The last article on State of Health was dedicated to (one prominent) effect of battery degradation, the fade of capacity. Everything in the business evolves around that aspect – but what we did not touch: what does actually hurt? What influences the rate of degradation and which measures can be taken to decrease the progression of capacity fade?

Recap: What is “Capacity” and “SOH”?

As we learnt in the last article, the capacity what one “experiences” in daily operations is often referred to as the “operational/ net capacity” of a battery, and it is programmatically made available by the BMS software. As such, it is not per-se automatically proportional to the technically available, theoretical electrochemical capacity of a battery system. The former might even be kept constant (“Eat the buffer” philosophy) over years, while the latter, the technical capacity, does degrade, albeit unbeknownst to the user.

The fact that the BMS might or might not release the full potential of a battery – there are good reasons for not doing it! – shall never mislead an owner or operator to not treat and operate a battery in a suboptimal, high-stress way, however!

This article is thus dedicated to learning about the factors that directly influence degradation, and thus at least indirectly also the capacity and performance available in every day’s life.

Battery Degradation: Influencing Factors and How to Minimize Capacity Fade?

The short answer is: there are only a few universal rules. Generally speaking, each battery cell type has their unique susceptibility to usage and external influences. Some cell types are tailored to withstand low temperatures, others high currents etc., and as most often in life there is no such thing as a free lunch. An optimization in one aspect comes at a price in another region – more on that later.

Calendar Aging vs. Cycle Aging in Lithium-Ion Batteries

First of all, let’s introduce the concepts of calendric (or calendar) aging and cyclic aging. While not used, i.e. no current is flowing in/out, a battery is degrading solely calendric. And as soon as current is flowing, this is overlayed by cyclic aging. Both modes lead to capacity fade (and resistance increase) by ions falling irreversibly dead by becoming trapped in side reactions, those reactions obviously being influenced by different factors. Therefore, we call these factors influencing factors. Typically, the ions react irreversibly with other materials in the cell (such as the electrolyte), forming passive residue*. There are also other forms of residue that can become dangerous (see inlay and a future article).

The influencing factors are quite universal, only their individual impact differs strongly:

• Temperature – i.e. cell temperature and not the temperature of the environment
• SOC – during idle (calendric aging) the idle SOC and during cycling, the SOC window (the range in which the cycling takes place) is relevant
• Current (only during cycling aging) – the charging and discharging current

Often mechanical stress like vibration and pressure is (correctly) included into the list; for the moment we will ignore them.

Temperature Effects on Battery Degradation and Lifetime

How do these factors influence the rate of capacity fade? It depends on the cell chemistry’s susceptibility, and this dependency is often highly nonlinear. Most cell chemistries appreciate being stored a low temperature, slowing down calendric aging (lowering the probability of side reactions).

However, particularly NMC cells on the other hand suffer heavily from being charged at low temperatures (<5-15°C), especially at high C-rates – which of course is a conflicting requirement! For more information see the figure and the inlay.

One example of a high-quality NMC cell: The slowest rate of degradation here is at 15°C, slightly better than at 25°C. But going another 10°C colder, to 5°C, literally destroys the cell.

Lithium Plating, Thermal Runaway, and Safety Risks

Li-Ion batteries can be regarded safe. “Self ignition” can, if at all, only be assumed if internal microscopic defects accumulated to critical levels. In simple terms Li-ions travel from one pole to another inside a cell, while electrons take that journey through the electric circuit of the vehicle. Under normal conditions, the ions migrate into the opposite electrode to be reunited with the well-travelled electrons.

Yet if it is cold, the current is high (fast-charging) and/or that host electrode is already well-filled, this migration becomes as cumbersome as boarding an already packed plane with another three dozen shivering passengers. So, under such conditions, it is electrochemically often more favorable to form metal deposits on the electrode – “Lithium Plating”, the “bad form” of residue.

A continuous repetition of this process stacks up such defects, potentially up to the point of penetrating filaments that cause internal short circuits (“dendrites”). Those in turn may generate heat, which is vital for speeding up chemical processes, leading to even more heat and gases being generated – a self-sustaining process called “Thermal Runaway”. Such a situation becomes ballistic in less than 5 minutes, eventually violently venting gases in the form of electrolyte-soaked, thick white vapor clouds.

Note: the conditions that facilitate plating are only partially controlled by a vehicle’s battery management system. It is the operator’s responsibility to monitor and foster their batteries, just like careful storage, handling and sensible monitoring of flammable liquids is unquestioned best-practice.

SOC Comes into Play – State of Charge (SOC) and its Impact on Battery Lifetime

So while C-rate (current) and temperature typically form a complex interplay, and susceptibility to temperature often differs from calendric to cyclic aging, the real surprises are brought to us by: the SOC!

The SOC can have a significant impact on battery lifetime, both the idle SOC during parking (calendric aging) and the SOC window or range in which an asset is operated (cyclic aging). While again only one of hundreds of examples, it is quite telling which potential lies hidden in that factor: in the following figure, the degradation evolution of two almost identical use cases of a hi-quality NMC cell is shown.

Both use only c. 50% of the battery (not uncommon), and the first (green) does so in the upper half (100 > 50 > 100 > …%, i.e. returning half full to the depot), while the second one (purple) has a daring driver: starting at 50% and returning virtually empty (50 > 0%).

The impact on battery degradation is incredible: the second battery will live 2-3 times longer than the first one, reducing total cost of ownership by more than 50%.

In the graph to the left you can see another – quite extreme – example of the same NMC cell: Both cases have the same energy throughput (i.e. result in the same vehicle range); yet a daring driver that doesn’t operate in the “safe zone” (100>50% SOC), but dares to reach the depot at c. 0% in the evening (starting at ~50%), will extend battery life by c. 2-3x!

Who dares, wins? Really?

When it comes to these susceptibilities, generalizations are very dangerous. There might be ample amounts of cells that react completely differently – one often quoted rule of thumb is that by avoiding high (>90%) and low (<10%) SOC regions, lifetime can be greatly extended. However not in this example – it’s not depicted, but an operation “in the middle” (75 > 25 > 75 > …%) even exhibits a slightly faster degradation than the blue one!

Battery Stress Level: Quantifying Battery Degradation Risk

That sounds complex, confusing and almost unmanageable. And yes, not even a trained expert can assess how stressful a given usage profile is even by analyzing it manually, i.e. by assessing C-rate profile, temperature evolution and SOC ranges and comparing them to the multi-dimensional susceptibility matrix of the cell type.

To reduce complexity there is the concept of the “stresslevel”, a simple number that tells the observer whether a given use case is more or less stressful than a nominal reference case (such as the default design and warranty conditions like “End of life after 8 years”) – a stresslevel of 1.0 means that a given case is circa as stressful as planned, and a stresslevel of 2.0 means that the battery under the given influencing factors is probably degrading c. two times as fast as planned.

There are tools that can automatically analyze stresslevel in a monitoring platform. These tools are useful for assessing risks and potential, as well as adapting use cases.

The Role of the Battery Management System (BMS)

There is a discussion ongoing – for years now – of whether or not it’s the battery electronics’ (BMS’) task to control the influencing factors and keep them under control. Yes, it is. It will prevent detrimental usage that might cause imminent threats for safety (“very high stress”). But except for very rare instances, it is neither designed nor intended to make recommendations for the ‘most optimal’ stresslevel.

In other words: I will prevent you from slurping 2 bottles of red wine, but I will not recommend drinking 1 cup of ginger tea  instead.

Outlook

Interestingly, the global trend is towards ever higher energy densities (longer range), which is often dearly paid for by sacrificing lifetime. Yes, average stability is going down! This is particularly true for passenger car automotive cells, but might, sooner or later, also affect heavy duty vehicles.

In any case, the severe and profound susceptibility of all Li ion cell types to the influencing factors should alert us all and raise the awareness for the huge optimization potential that might lie dormant in so many use cases.

Stress Level at a Glance

• Each subtype of Li ion cells has their unique and often complex susceptibility to external influencing factors – and calendric aging (parking) must be distinguished from cyclic aging (operation).
• Both result in different forms of Li ions becoming inactive – always leading to a more or less pronounced fade of capacity (and increase of resistance), and sometimes even leading to dangerous residue building up (Li Plating).
• Different usage conditions thus lead to different stress levels (or degradation evolutions), and just counting cycles or age can never result in an adequate assessment of degradation!
• Slight adaptations to usage profiles can reduce battery stress significantly and help to extend asset lifetime by 10 to 50%, and in some extreme cases even by the factor of 2 or more!

Example of a Treatment Recommendation: Value Retention & Lifetime Risk detected.

This example of a handling recommendation from a recent electric bus battery use case (2025) shows how elevated stress levels are translated into operational guidance.

Symptom:
High Stresslevel detected

Impact:
Continued operation under current conditions accelerates battery aging and reduces lifetime.

Treatment:

• Rule out sensor and data issues to avoid misinterpretation of the situation
• Check the usage profile and conditions for changes compared to the previous weeks
  • e.g. increased average current, SOC-range, average SOC, or temperature
• Check utilization of the unit for changes compared to the previous weeks
  • e.g. energy throughput per week
• Based on the findings, adjust the usage profile

Looking Ahead in the Series

In our final article of this cycle, we will tackle Data Transparency, Ownership, and the Battery Passport – showing you how to unlock and secure your fleet’s most valuable asset: its data.

All knowledge articles of the battery cycle:
Intro – The Battery Cycle – opening the black box
1 – NMC, LFP, LTO: What’s the Difference in Battery Chemistry? – energy density, safety, lifetime, cost
2 – State of Charge: Why It’s Harder to Measure Than You Think – about really knowing how full your battery is
3 – Fast Charging Explained: Why More Power Doesn’t Mean Less Time – how to keep a battery healthy
4 – Why an LFP Bus Can Suddenly Stop: The Battery’s Weakest Cell – why imbalances define the limit
5 – Battery SoH: The Number That Doesn’t Tell the Whole Story – State of Health is mostly misunderstood
6 – Stress Level: The Key Drivers of Battery Degradation – what really hurts a battery
7 – Battery Data: Are You Seeing the Full Picture? – use and interpret your data correctly