BESS Operating Voltage Windows: What Happens to Operations (and Profits) When Your Battery Goes Over or Under the Limit

Operating voltage windows are key to ensuring safe, high-performing operations for battery energy storage systems (BESS). But when cells overdischarge or overcharge, it causes your battery to operate outside of the voltage window. This not only disrupts operations, it wastes time and resources and puts BESS asset operators’ profitability at risk. 

Here’s a closer look at what happens when your battery goes over or under voltage, what it means for overall BESS operations, and why conventional BMS fail to solve this problem. 

Understanding BESS operating voltage windows

Typically, battery energy storage systems (BESS) comprise thousands of individual battery cells, all placed in series and in parallel to achieve rated voltages and energies needed for modern electrical grids. Each individual cell has an operating voltage window determined by the fundamental electrochemical properties of its chemistry. 

Importantly, this voltage window defines the cell’s safe and repeatable operation and plays a critical role in determining overall battery health. 

What happens if your battery operates outside the voltage window? 

As operating voltage windows are intrinsically linked to a cell’s state of charge (SoC), exceeding these nominal voltage cutoffs initiates detrimental internal reactions that compromise the longevity, efficiency, and economic viability of the entire BESS. This is true for both overdischarging and overcharging. 

Over time, if the cells continuously overdischarge or overcharge, it leads to premature cell aging, in turn, compounding energy losses and increasing internal resistance. 

Here’s a closer look at what happens when cells overdischarge or overcharge and cause your battery to operate outside the voltage limit: 

Consequences of overdischarging

When a lithium-ion cell faces overdischarging, its internal materials undergo irreversible and damaging side reactions, impeding BESS operations revenue and eventual profitability. 

Here’s what happens: 

• The electrolyte (responsible for lithium-ion transport) begins to reduce into undesired side products. 
• Lithium inventory decreases, meaning a loss of available capacity.

At the same time, overdischarging also strains the anode, causing graphite disordering which renders lithiation sites unavailable, thereby shrinking effective capacity. 

In extreme cases, copper from the anode can even dissolve and deposit onto the cathode surface, blocking sites from repeated lithiation as well. 

Ultimately, these phenomena contribute to a significant increase in the cell’s internal impedance, resulting in more capacity loss. 

Consequences of overcharging 

Overcharging, unfortunately, isn’t any better than overdischarging, as it similarly triggers severe degradation mechanisms. 

When a lithium-ion cell is overcharged, any excess lithium remaining in the cathode structure gets forcibly extracted, leading to structural deformation of the cathode material. Meanwhile, if the anode’s available sites for lithium insertion become saturated, lithium ions are forced to metallize on the anode surface, forming dangerous lithium dendrites, i.e., dendritic growth.

In turn, dendritic growth can cause: 

• Loss of lithium inventory
• Internal short circuits
• Ultimate threats of thermal runaway 

Overcharging can also cause the electrolyte to oxidize, leading to cell swelling and possible off-gassing. 

Learn more about overcharging and overdischarging and how it can cause lithium-ion battery degradation: Understanding Lithium-Ion Battery Degradation: Causes, Effects, and Solutions.

Why voltage windows matter for BESS operations—and profitability

Operating voltage windows are more than a technicality. If cells regularly exceed nominal voltage limits, it can significantly disrupt BESS operations, unnecessarily waste time and resources, and limit profitability. 

When LFP cells overdischarge or overcharge, it causes unexpected performance degradation—and a domino effect of both short-term and long-term site-level consequences: 

• Wasted time and resources: Most immediately, performance degradation leads to inaccurate SoC measurements, which require extra energy capacity testing and rebalancing efforts. In turn, this forces BESS asset operators to spend more time and resources on maintenance and troubleshooting. 
• Lower ROI: If your BESS site can no longer deliver as much energy or power as you initially forecasted (or if its operational lifespan is cut short), then your financial projections will be off. This means a lower ROI and potential economic losses that degrade profitability. 

What makes cells overdischarge or overcharge?

To prevent your battery from operating outside the voltage window, you first need to know what causes cells to overdischarge or overcharge. 

These are the two main reasons why cells exceed nominal voltage thresholds:

1. Poor mitigation of cell imbalance

The main reason why cells operate outside the voltage window is cell imbalance. 

Despite sophisticated mechanisms within Battery Management Systems (BMS), inherent manufacturing variations, design factors (e.g., interconnects), and localized thermal gradients can cause individual cells to deviate from the SoC—even within the same module or rack. 

This is where the trouble begins. When a module undergoes charge or discharge cycles, some of its cells may reach their “full” or “empty” voltage limits too early; others, too late. If the balancing system is unable to compensate for these discrepancies quickly or effectively enough, then the too-early or too-late cells will end up overcharging or overdischarging. 

2. Risky operational conditions

While overarching system controls are designed to prevent extreme overcharging or overdischarging, certain operational conditions can push these boundaries: 

• High loads can cause transient voltage excursions.
• Rapid charge or discharge can result in brief voltage spikes or wells across battery terminals. In most cases, protective circuits don’t have enough time to fully react and intervene. 
• Low-temperature conditions can inadvertently cause cells to momentarily exceed voltage limits during attempts to extract or store slightly more energy. 

It’s worth noting that LFP has become the dominant lithium-ion battery choice for BESS in recent years—and for good reason. LFP cells boast longer lifetimes, better operational safety, and lower cost per unit of energy. Still, although LFP cells are safer and more robust than other lithium-ion chemistries, they too can lead to the site-level consequences of operating outside voltage windows. 

Learn more about LFP cells and why they’re the best choice for BESS.

Why conventional BMS and EMS can’t prevent overdischarging or overcharging

Although cell overdischarging and overcharging is a pressing problem that mires performance, weakens longevity, and threatens BESS financial performance, most conventional BMS and energy management systems (EMS) fail to offer a solution. Here’s why: 

BMS limitations

Conventional BMS often rely on simplified models that may not account for the premature aging of individual cells caused by repeated overdischarging or overcharging. Unfortunately, what seems like a simple oversight has outsized consequences. 

Unable to consider this premature cell aging, conventional BMS can end up generating inaccurate SoC measurements, exacerbating imbalances and contributing to energy and power losses for the entire module or rack. In the end, this spells operational inefficiencies, inaccurate financial projections, and potential profit loss for BESS asset operators. 

It’s true that conventional BMS may be able to react to a cell hitting a hard limit, but these systems often lack both the predictive capabilities and the granular control to prevent cumulative damage from sub-threshold stress.

How can you make conventional BMS better? Learn how in our Comprehensive Guide to Common BMS. 

EMS limitations

The EMS operates at an even higher hierarchical level than the BMS and therefore does not possess the granular visibility or control required to directly counteract individual cells’ premature aging. This means that if the BMS is already struggling to accurately assess individual cell aging and SoC, then the inaccuracies will snowball. 

In other words, the EMS will use the inaccurate information to make suboptimal decisions for the entire BESS site, resulting in efficient operations and a reduced system lifespan, leaving operators unable to deliver energy and power services as forecasted. 

A new approach to avoid overdischarging and overcharging

When cells overdischarge or overcharge and exceed your battery’s voltage window, the effects are damaging. By degrading performance, it disturbs operations, eats up time and resources, and can even put BESS profitability at risk. 

Zitara’s “cell-to-site” approach offers a new way to overcome the challenges of cell overdischarging and overcharging. 

Zitara for BESS is a purpose-built software solution that leverages advanced battery modeling and precise analytics to ensure cells operate within safe voltage limits—and keep track of when they don’t. Our “cell-to-site” approach means we construct reliable energy and power signals from the bottom up, i.e., from precise cell-to-site models that account for each and every level of your BESS project. 

Learn how Zitara for BESS enables you to get more from your BESS assets for better availability, reliability, and profitability. Download the technical white paper: Maximizing Asset Availability with Zitara for BESS.