Unveiling the Mystery: Why You Might Not Get Rated Capacity from Your Lithium-ion Battery Pack

Welcome to my newsletter! I'm on a journey to uncover all the untold facts about lithium-ion battery storage in the industry. By subscribing, you'll get valuable insights, expert opinions, and up-to-date news about this fascinating technology. Join a community of like-minded professionals and stay ahead of the curve. Subscribe now !

When you purchase a lithium-ion battery pack, you're presented with its rated capacity, which represents the maximum energy it can hold. However, due to practical considerations, you're unlikely to extract every last bit of this energy. In this edition, we demystify the facts that are untold regarding a lithium-ion battery pack capacity.

Total capacity that the pack can deliver =? Major function { Total Usable Capacity, Storage time, BMS Balancing capability, BMS self power consumption,, Degradation behaviour of cells towards its end of life, End of life SoH}

Of course, there are other parameters that influence such as application parameters like C-rate, type of cycling, and other factors. In this edition, we are going to just focus on the specifications that are untold during the purchase of the battery packs.

To navigate at ease, let us assume that we are talking about a 10 KWh LFP battery pack in this edition and see how much we get as the actual capacity.

Total Usable Capacity (DoD limit) - The Realistic Benchmark

The total usable capacity of a lithium-ion battery refers to the amount of energy that can be safely and effectively extracted from the battery during its lifetime. This capacity is not the maximum theoretical energy stored in the battery, but rather the practical energy that can be used.

Major research papers and experimental studies do not recommend discharging the battery packs below 20%. Therefore, the Depth of Discharge is limited to 80%. It means with this specification, the capacity that is usable from our pack becomes 8KWh.

Shallower discharges, where the battery is not completely drained, tend to extend the battery's usable lifespan

Calendar Ageing: Time's Effect on Capacity

Lithium-ion batteries age even when they're not in use. This phenomenon, known as calendar ageing, results from internal chemical processes that gradually reduce a battery's capacity over time. It means that even if you rarely use your battery pack, it can experience capacity loss due to the passage of time.

Even at 25degC and with 40% SoC stored for 1 year would mean a capacity of loss of 4%. Assuming the different temperatures and 50% SoC at an average during the usage time, there is a clear loss of minimum of 2% during the shipment time and shelf time from the point of manufacturing to sales. Hence JIT is an important aspect in LiB production, a topic of discussion for another time. This means we are going to lose at least 2% of capacity in our pack when we start to use it. Hence, our pack capacity becomes 78% of 8KWh, which is 7.84KWh.

BMS Balancing Capability

As the Pack ages, the cells start deviating from each other. The work of the BMS actually starts after 500 cycles as some researchers say. However, the sizing of the BMS for its balancing current and the balancing strategy is a specification solution. Effective balancing contributes significantly to the usable capacity of a battery pack. When cells are unbalanced, certain cells might reach their voltage limits (fully charged or discharged) before others, limiting the overall usable energy of the pack

For example, The capacity of original cells with a cell imbalance of 0.5Ah would mean 0.25% for every 500 cycles, which means that there is a loss of 1% over 2000 cycles thus leading to? 7.76KWh.

On a side note, the self power consumption of the Battery Management System is critical to ensure that the capacity is not lost in self.

Health at the End of Life: The Final Chapters

A lithium-ion battery pack is considered to be at its end of life when it can no longer meet the performance and capacity requirements of its intended application. This transition is gradual, marked by a decline in capacity, increased internal resistance, and a decreased ability to hold and deliver energy effectively.

One of the telltale signs of an ageing battery pack is capacity fade. Over cycles of charge and discharge, the pack's capacity diminishes. A common benchmark is when the pack's capacity has degraded to around 80% of its original rated capacity. This means that the pack will move from 7.76KWh to 6.2KWh during its period of operation. This time to reach the same varies from manufacturer to manufacturer.

A 10KWh Pack that you purchase is supposedly giving us

Start of Use:? 7.76KWh?

End of Use: 6.2KWh

Perform Well to Wheel Analysis as being termed in the industry to look at the sizing of the pack first, when it comes to stationary applications primarily. The entire Total Cost of Ownership is dependent on these considerations.

It's important to note that the views expressed in this newsletter are personal and do not necessarily reflect the views of any organisation or company mentioned herein.

I hope you found this newsletter informative and thought-provoking. If you have any questions or feedback, please don't hesitate to reach out.

Best regards,

GM

About the Author

Ganesh Moorthi

I am an accomplished Lithium-ion Battery Pack Expert and Energy storage Expert with over a decade of experience in designing and developing EV and ESS battery packs. Throughout my career, I have spearheaded the technology development of Battery Packs and Battery management systems with different AFE's and architectures. I have also led the development of Hardware, Firmware, Packaging, and Software for Energy Storage Systems. I possess a deep understanding of Lithium-ion chemistry and have demonstrated expertise in the production of LiB's with proprietary technologies. With my extensive educational qualifications and vast experience, I believe I can bring unique perspectives about the Energy Storage industry.

Image by storyset on Freepik