Why an LFP Bus Can Suddenly Stop: The Battery’s Weakest Cell [The Battery Cycle #4]

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 November 2021, this is a revised version.)
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System Topology

A “battery” is not one cell, although we might want to reduce it to that in everyday conversation. Ideally, it should behave like it, but as always, things are more complex in reality.

In a battery system, dozens or typically hundreds of single cells are connected in series (that is, the plus of one is connected to the minus of the next and so on), forming a string. Each additional cell raises the total system voltage. As “power” is “voltage x current”, doubling the voltage (by putting cells in series) cuts the necessary current half, and as cable diameters is mainly determined by maximal current, raising system voltage by putting cells in series basically helps to reduce cable size, weight and costs. As simple as that. If we want more capacity, we add whole strings in parallel.

But by this simple “trick” new problems occur: disbalance, inhomogeneity, inaccessible energy and the necessity for balancing.

Inhomogeneity: A Thought Experiment

If we depart from a system where all cells are the same, we run into serious problems that we will explain here. As usual, we start with simple analogies that are perfectly suitable for everyday conversations. Consider a single string system (works perfectly the same with multistring systems) and figure it as a cascade of the already introduced glasses, discharging into one another.

Would you agree that, as long there is no leak, the amount of liquid going in on the top completely passes unchanged through each and every single cell?

So, if now all have exactly the same capacity (SOH), and all start at the same filling height (voltage), then all of them have the same starting content (SOC). If current flows through the cascade, each cell in this perfect example is being filled/emptied by the exact same amount of liquid. If you keep discharging them, they all approach 0% SOC simultaneously. Vice versa, if you keep filling them up, they all will be 100% full at the same time.

In this ideal world we need no balancing, as there is no inhomogeneity whatsoever. Of course, this is not the case in the real world – there are many imperfections. Let’s take a closer look: leave the above thought experiment as it is, but for what reason ever, 1 of 100 cells has a slightly lower SOC, only 45% instead of 50%. The average SOC is still c. 50%, but as the cascade is being discharged, clearly the “lemon cell” will be empty before the others. This will be detected by the battery management system’s safety features, and the complete flow will be interrupted to not deep-discharge and potentially damage that lemon – while 99 cells still have c. 5% SOC. Think of the lost potential! There is still 5% in the system, but it is inaccessible or trapped, just because of a single lemon cell!

Suddenly Empty: Such system emergency shutdown we sometimes call a sudden depletion event (SDE) – a vehicle that until minutes ago displayed a substantial and unproblematic SOC suddenly stops and the SOC drops to 0%. By the way: If you followed all articles so far, you should have a feeling why especially LFP is prone to that effect.

Balancing is Key!

What is battery balancing now doing? In very simple terms: The balancing circuit in the Battery Management System must take care that all cells have the same SOC (filling content), ideally at all times, so that the weakest link doesn’t limit overall performance in the described way.

The simplest, but also the most common, approach is often called passive balancing, and it seems wasteful: a master logic determines the SOC of the “lemon” (or lemons) and discharges all other cells. Yes, in above’s example, a passive balancing system would start leaking ca. 5% from all the 99 good cells, almost “bleeding” them out. This energy is lost.

There are also active balancing systems in place, which are way more sophisticated and themselves prone to failures due to their complexity, and more invest intensive. The result is also that all 100 cells would end up at the same SOC, but by actively shifting excess charge from the 99 others to back up the lemon, no charge is lost in the process.

When Does Balancing Happen?

In large and very busy stationary systems, active balancing is gaining some foothold. However, the simplicity and robustness of passive systems still make them the means of choice in almost all applications, bus and truck included. So, to keep it simple, the following is true only for passive battery balancing systems.

As said, a balancing system must first determine the SOC of every cell. As you learned in the last article, this is particularly difficult for electronics in dynamic or “foamy” situations, i.e. during operation or charging, and for battery chemistries with flat OCV curves – like LFP. So the balancing system will want long, “relaxed” idle periods without any power flowing, and is even more happy if the SOC is very high, as the “glass shape” of most chemistries allows for an accurate voltage-to-SOC-conversion when almost full. The longer the idling, the more disbalance can be “heated away” by leaking.

Now you understand why OEMs recommend an extended idling phase directly after charging to 100%? Exactly: Idling at high SOC facilitates balancing – the longer the better.

Reasons for Inhomogeneity

We have not answered the questions why at all the cells don’t behave the same. We brought up the example where one cell is slightly emptier than the others (i.e. SOC). The reality is very complex, with many interactions.

To highlight one very mean — because self-enforcing — reason: due to manufacturing imperfections, uneven cooling/heating flows, welding issues, electronics imperfections etc. some cells degrade slightly faster than others – really only slightly – leading to a spread in capacity (SOH). When continuously charged and discharged, the differences in SOH will lead to the SOCs of all cells starting to diverge in a manner comparable to the first thought example.

To make things worse, a cell that is slightly smaller than the others experiences more current per capacity (C-rate), even increasing degradation stress, and making it even smaller than the rest. Also, it is likely to have slightly increased resistance, leading to increased temperature. Also, the SOC window is affected. A “pre-lemon cell” is thus likely to continue an irreversible downward spiral.

While the battery balancing system is always trying to level out SOC, it cannot affect the irreversible SOH spread. This can be mitigated by a balancing system for some months or years, at least the operator will not directly feel the ongoing process. But at one point in time, balancing times would take so long to completely level everything out, that operation would be severely affected. Or, if it was kept still too short, the risk for SDEs would significantly increase.

Conclusion on Battery Balancing

The OEMs and manufacturers are very aware of what we wrote above, and the systems are typically well-equipped with the right balancing setups. It is however instrumental that you are aware of the (dis)balance and risk of system underperformance due to it for each asset, and that a proper balancing strategy and adherence to the OEM’s recommendations are met.

Battery Balancing at a Glance

• Battery systems are complex. They consist of hundreds of cells “pretending to be one”: If they don’t, one cannot extract all seemingly available energy.
• In worst-case situations, unbalanced systems can suddenly appear to “self-deplete” in “Sudden Depletion Events,” leading to sudden, costly, and potentially dangerous en-route standstills.
• Balancing requires time and specific conditions. It most often occurs during extended idle phases after a full charge.
• The reasons for imbalance and inhomogeneity are complex. A pre-imbalance can lead to self-reinforcing internal degradation, which worsens the problem.
• It is advisable to adhere to OEM recommendations and independently keep track of inhomogeneity (reversible and irreversible) and balancing effectiveness.

Looking Ahead in the Series

In our next article on battery health, we will dive into the most discussed and critical metric of all: State of Health (SOH) and the secrets of battery degradation.

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