The BESS Balancing Act

The most common cause of BESS underperformance?

Cell imbalance – when stored energy in a battery system becomes inaccessible due to individual cells developing unequal states of charge – has long been considered a solved problem in the EV industry. The consensus approach involves simple, inexpensive passive cell balancing circuits that slowly equalize cell voltage at top-of-charge while a vehicle is not in use. Boring? Perhaps, but very effective.

As such, it was a surprise for us to learn that cell balancing remains a major problem in grid-scale BESS today. In fact, across the several GWh·years of BESS field data we've analyzed, cell imbalance is the most common cause of BESS underperformance we see. And although most asset managers we speak with already know they have balancing issues, many still struggle to address them. How could this be possible?

The answer is so simple that sometimes it’s difficult to believe: contemporary BESS don’t track or report cell imbalance at the EMS level. And as the old adage goes, you can’t manage what you can’t measure.

Why the imbalance problem persists

Unlike EVs that regularly idle at top-of-charge, most BESS don't reach top- or bottom-of-charge during normal operation. To make matters worse, the LFP battery chemistry now universal in BESS maintains extremely stable voltage outside of top- and bottom-of-charge, defeating traditional voltage-based balancing mechanisms.

We’ve written all about the nitty-gritty in Demystifying Battery Balancing, but the takeaway is that BESS operators must deliberately take racks, blocks, or entire assets offline to balance, bringing them to charge states where balancing mechanisms can function. This leads to a painful “balancing act” for asset managers: more balancing time (increasing downtime) versus higher imbalance levels (decreasing available energy).

Common balancing techniques and their consequences

The obvious solution would be to balance only when necessary – that is, balance only the racks and blocks that are imbalanced.

Unfortunately, contemporary BESS control systems provide little-to-no information about imbalance levels. Without any visibility into the situation, operators resort to suboptimal balancing strategies:

• Symptomatic balancing: Initiate balancing only after observing problematic behaviors.
• Scheduled balancing: Balance all blocks/racks on a fixed schedule, regardless of actual imbalance levels.
• Round-robin balancing: Balance one block/rack at a time in a sequence, iterating over the entire project.

All of these practices result in excessive downtime, reduced energy availability, and missed market opportunities.

• Symptomatic balancing reduces capacity because minor imbalances go undetected.
• Scheduled balancing and Round-robin balancing are overly conservative, creating unnecessary downtime to run balancing routines on hardware that may already be well-balanced.

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Zitara’s approach to BESS balancing

Zitara addresses imbalance directly by making the imbalance level of each and every rack and block immediately visible and actionable. Our autonomous on-prem software package, Zitara Balance, offers two key capabilities:

1. Real-time imbalance reporting including impact on available energy at project, block, and rack levels.
2. Automated smart balancing strategy targeting only the racks and blocks needing balance, producing balancing priority signals that can be integrated with existing BESS controls.

The benefits are tremendous:

• 50-75% reduction in balancing-related downtime.
• 10-15% increase in total dispatchable energy.
• Precision balancing that happens only when and where needed.

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Get Your BESS In Balance

Don't let cell imbalance limit your asset’s potential. If you don’t know your imbalance levels, it’s time for better data. Contact us today for a free Insights Report to measure your asset’s imbalance, and what can be done about it.