The Obstacle Course on the Path to Repurposing Used Electric Vehicle Batteries (EVB). Part VI: The Significance of Battery Balancing
This is the sixth blog post of our series, where we’re going to discuss the importance of balancing batteries. Balancing is needed as aging battery pack cells/modules develop current and voltage imbalances, even if they roughly maintain the same capacity of new batteries. These imbalances will reduce the efficiency of the system and cause faster aging, taking a toll on both labor hours and financial costs. As we prepare for the launch of our new VGT (Vehicle Grading Tool), we want to delve into the consequences of an imbalanced battery and the significance of an efficient solution.
In this blog, we’ll discuss:
Why is battery balancing important
Application to second-life batteries
ReJoule’s solution to imbalances
Why is battery balancing important
Battery packs are essential in many industries, including electric vehicles and energy storage. However, proper balancing of batteries is of utmost importance due to the nature of their degradation. Although new batteries are almost identical, used batteries cannot be paired together easily. The reason behind this lack of compatibility is that each battery ages at a different rate and degrades differently. This degradation leads to imbalances that can result in over voltage, under voltage, over temperature, or under temperature. For more on this, you can also read fourth blog on Cell Imbalance and how this can affect critical measurements like state-of-health (SOH) and state-of-charge (SOC)
As the battery cells/modules age, the occurrence of imbalances compromise the safety of the system and increase the risk of a thermal runaway, not only affecting the overall performance and capacity of the battery pack but also raising serious safety concerns. To ensure the optimal performance and longevity of battery packs, it is crucial to implement a balancing mechanism. Balancing involves managing the charge and discharge levels of individual battery cells or modules within the pack. By equalizing the state of charge among the batteries, the risks of battery imbalances can be minimized.
ReJoule’s project with the California Energy Commission seeks to reuse batteries from delivery vans paired with solar to provide resilient and clean energy to the American Museum of Ceramic Arts (AMOCA). Our efforts focus on the safe reuse of the batteries by extensively testing and matching them together. See the figure below on how this manifests across multiple battery modules. In the field, these voltage curve differences can have a profound effect on performance and safety.
The voltage curve above shows 28 battery modules being charged simultaneously, but with one outlier that reaches its max voltage before the others. In this case, the battery module needs to be replaced by another that follows a similar curve.
Application to second-life batteries?
Imbalance in batteries must ideally be addressed during their first life (we're actively working on a solution). Otherwise, in a second-life application, the battery might be rejected due to its imbalanced cells, causing faster capacity fade compared to a battery that is properly balanced.
One possible approach is to rebalance the cells to enhance the pack’s usability. However, this poses a challenge as it requires specialized equipment, labor, and considerable time, which adds to the cost of a second-life battery system. As discussed in our previous battery grading blog, determining a battery's State of Health (SOH) and State of Charge (SOC) is a demanding task. Decommissioned batteries, especially if they are coming from multiple vehicles, have lost a large portion of their history and data. To understand them, thorough testing is essential.
One of our engineers disassembling an imbalanced batter pack
ReJoule’s solution to imbalances?
The cost of replacing a battery pack in an EV can be exorbitant, ranging from $16,000 to $24,000, making it a substantial investment, especially for vehicles out of warranty which tend to be over 8 years old. While a more affordable option involves replacing a faulty module (a subset of a battery pack), this approach provides only a short-term fix and often requires effort that isn’t worth the result -tressing the importance of finding cost-effective and long-term solutions for managing imbalances in battery systems.
While some automakers aim to remanufacture and replace faulty modules, this solution requires advanced diagnostics to ensure effectiveness. For businesses with large fleets of batteries, dealing with imbalances in currents and voltages can has significant costs but limited options. Various balancing techniques exist, ranging from passive balancing (e.g., using resistors to dissipate excess energy) to active balancing (e.g., transferring charge between cells). However, the challenge with battery balancing is identifying which batteries belong in which group. The process is time-consuming and tedious, making it impractical if you are processing hundreds or thousands of batteries. Finding a cost-effective and sustainable solution remains essential for all parties involved in managing aging battery systems.
ReJoule is addressing the challenge of battery imbalances with a solution that involves calculating DCIR (Direct Current Internal Resistance) to assess the internal resistance of batteries. By analyzing DCIR, we can determine the age and wear of a battery, distinguishing older or more worn-out batteries from those in better condition. This process, although manual at first, has been developed to provide crucial insights into the health and performance of each individual cell automatically.
We analyze DCIR using proprietary software, utilizing it to process large datasets of battery performance data to identify patterns and correlations between battery age and capacity. Our machine learning algorithms can then use this information to create a predictive model that can identify which batteries are likely to have similar performance characteristics and should, therefore, be grouped together.
The graph above shows how a battery group that will be used in AMOCA can be separated into four similar clusters (blue, red, green, and pink).
Through ReJoule's advanced cloud-connected Battscan device and the powerful matching software, technicians can efficiently gather and store extensive data, including DCIR measurements as well as State of Health (SOH) metrics. This real-time data access enables informed decision-making and proactive maintenance strategies.
Contact us at info@rejouleenergy.com if you want to leverage our technology to test your batteries in or out of the vehicle. What are your thoughts? Did we leave anything out? Please comment and share so we can all engage in conversation and learn from each other.
If this project and/or our work piques your interest, please check out our open positions here.
Cheers,
The ReJouligans
Legal notice:
This document was prepared as a result of work sponsored by the California Energy Commission. It does not necessarily represent the views of the Energy Commission, its employees, or the State of California. Neither the Commission, the State of California, nor the Commission’s employees, contractors, or subcontractors makes any warranty, express or implied, or assumes any legal liability for the information in this document; nor does any party represent that the use of this information will not infringe upon privately owned rights. This document has not been approved or disapproved by the Commission, nor has the Commission passed upon the accuracy of the information in this document.©2021 ReJoule Incorporated. All Rights Reserved.
