Why Does My BESS Stop Operating Before 0% SoC at High Discharge Rates?

When operating at high discharge rates, it’s not uncommon for battery energy storage systems (BESS) to halt discharge before reaching 0% State of Charge (SoC)—but it can be perplexing. 

Although your BESS has stopped operating, the battery isn’t truly “empty.” Instead, the battery voltage has dropped too low for safe operation, triggering a system shutdown. This voltage loss is a phenomenon called overpotential.  

In this blog post, we’ll break down what overpotential is, what causes it, and why it matters at high discharge rates. Then we’ll explore the impact of overpotential on BESS operations and what you can do to mitigate its effects. 

What is overpotential? 

Overpotential is the difference between a battery’s theoretical voltage under ideal conditions and its actual, real-world operating voltage. 

The theoretical voltage is the ideal voltage a battery would produce if there were no inefficiencies (such as resistance or slow reactions) causing energy loss. Under these conditions, the voltage would only be determined by the materials inside the battery. This is why the theoretical voltage is typically close to the open circuit voltage (OCV) when the battery is at rest. 

But real-world operating conditions bring unavoidable inefficiencies that lead to energy losses, collectively referred to overpotential. 

Do you speak battery? Check out our Battery Glossary for an easy round-up of essential terms, concepts, and acronyms. 

3 main types of overpotential and what causes them 

Each type of overpotential comes from a different source of inefficiency: 

• Activation overpotential: Reflects the energy required to initiate electrochemical reactions at the battery’s electrodes. Think of it like the “startup” cost for the chemical processes that produce current. 
• Ohmic overpotential: Results from the battery’s internal resistance, including the electrodes, electrolyte, and connections. It’s akin to the voltage drop across a resistor in an electrical circuit. 
• Concentration overpotential: Occurs when the concentration of reactants (e.g., lithium ions in lithium-ion batteries) at the electrode surface changes during operation. At high discharge rates, reactants are consumed faster than they can diffuse to the electrode, causing a significant voltage drop. 

Dive deeper into performance loss over time with our blog post on Understanding Lithium-Ion Battery Degradation: Causes, Effects, and Solutions.

Why overpotential matters more at high discharge rates

Activation, ohmic, and concentration overpotentials are always present in battery operations, but their impact grows significantly as discharge rates increase. 

During high discharge rates, the battery must supply a large current, requiring rapid electrochemical reactions. This high demand exacerbates all three types of overpotential, particularly concentration overpotential. 

Here’s what happens: 

1. Reactants rapidly deplete

At high discharge rates, reactants (like lithium ions) are consumed at the electrode surface faster than they can diffuse from the bulk electrolyte. This creates a concentration gradient, reducing the availability of reactants and increasing concentration overpotential. 

2. The voltage drops 

Thanks to the increased concentration overpotential, the battery’s actual voltage drops more rapidly than it would at lower discharge rates. This phenomenon can be described by the Tafel equation, which shows that overpotential grows with current density. 

3. Cut-off voltage is reached earlier

To protect the battery from damage, BESS systems are designed with a minimum safe operating voltage (i.e., a cut-off voltage). If high overpotential causes the voltage to drop to this level, the system stops discharging—even if some charge remains in the battery. 

For example, if you have a lithium-ion battery with a 100 Ah capacity at a 1C discharge rate, it might only deliver 80 Ah capacity at a 5C discharge rate if increased overpotential causes the cut-off voltage to be reached sooner than anticipated. 

Get a closer look at BESS operating voltage windows. Learn what happens to operations (and profits) when your battery goes over or under the limit. 

A closer look at how overpotential reduces usable capacity at high discharge rates

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This plot shows voltage versus cumulative discharge capacity at different C-rates (0.5C, 1C, 2C, and 4C). 

Unlike traditional voltage versus SoC plots, this chart uses cumulative discharge capacity (in AH) to directly show how much capacity is discharged at each voltage level, giving you a clear picture of usable capability under varying discharge conditions. 

In this plot, you can see that higher C-rates lead to a steeper voltage drop, causing the battery to reach the cut-off voltage of 2.9V at a lower cumulative discharge capacity. 

How overpotential impacts BESS operations

The increased overpotential that comes with high discharge rates directly impacts BESS operations, reducing usable capacity, lowering efficiency, and influencing system design: 

• Reduced usable capacity: Because voltage drops more quickly, the system reaches the cut-off voltage before the SoC is fully depleted, effectively reducing the usable capacity. 
• Lost efficiency: The energy lost to overpotential is dissipated as heat, lowering the system’s overall efficiency. 
• More complex system design: For applications requiring high power output (e.g., grid stabilization), overpotential can limit performance, necessitating careful battery selection and system design to avoid premature shutdown.  

How to mitigate the effects of overpotential 

While overpotential is always present, there are strategies you can strategies you can use to mitigate its impact: 

1. Advanced electrode design

Electrodes with greater surface areas or improved porosity enhance reactant diffusion, thereby reducing concentration overpotential. For instance, porous graphite electrodes in lithium-ion batteries can improve performance. 

2. High-conductivity electrolytes

Electrolytes with better ionic conductivity reduce ohmic overpotential, allowing more efficient current flow. 

3. Temperature management

Higher temperatures can reduce overpotential by accelerating reaction rates—but you need to balance this advantage against the risk of potential battery degradation. 

Avoid early cut-off voltage with Zitara

Zitara for BESS is a purpose-built software solution that uses advanced battery modeling to enable smarter balancing and more accurate state estimates. It goes beyond traditional BMS functionality by leveraging real-time data and predictive algorithms to precisely track voltage, current, and SoC, so you can better mitigate risks like early cut-off. 

With proprietary algorithms, Zitara for BESS also enables adaptive control, balancing power demands across modules to enhance system longevity and reliability—ideal for demanding applications like grid stabilization. 

Download the technical white paper to learn more about how Zitara for BESS can optimize BESS performance for maximum availability and profitability.