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Understanding the performance degradation of batteries: Main considerations for pack developers

2019-09-24
Understanding the performance degradation of batteries: Main considerations for pack developers

Understanding the performance degradation of batteries: Main considerations for pack developers

One of the main factors concerning batteries is their inherent performance variability depending on the use case. Additionally, the large variety in the existing chemistries, form factors, power vs. energy cells, makes an accurate determination of performances more challenging. Further complexing the situation is the large effect of ambient temperatures on performance, which can vary widely, based on geographical locations, physical locations (indoor or outdoor) and usage patterns. In this article, we highlight some of the key parameters and general relationships between operation and performance of battery systems.

What factors to consider while choosing cells to make a battery pack?

For any developer who is making battery packs for any application, it is very critical to have a complete understanding of battery performance. This is because the longevity or cycle life of the batteries is strongly dependent on the operational parameters. The operational parameters are DOD (depth-of-discharge), C-rate of operation and the average ambient temperature. It can be highly misleading to depend on the battery specification sheets provided by the cell manufacturer because their testing is always done under very standard conditions of temperature (25°C) and c-rate (C/2 or C/5), which will never be directly applicable to the actual operating conditions. In most parts of India the temperatures are in the range of 30-45°C. Under operation in EVs or stationary storage, the cell temperatures can often reach close to 50°C for extended periods. This has a tremendous negative impact on the cycle life of the batteries, which may be reduced by a factor of two or more as shown in Graph 1. For this system an increase in the ambient temperature from 25oC to 65oC, lowers the cycle life from 2500 to 750 cycles. A similar case may arise if batteries are being used for high c-rate application such as frequency regulation requiring the battery to constantly operate at 2C or more. In this case, the standard cycle life written in the data sheet will no longer be valid. The performance of batteries includes several other parameters such as energy density (Wh/kg and Wh/L), power density (W/kg and W/L) and energy efficiency (%). In case of EV applications, energy density is especially crucial because it directly relates to the maximum achievable driving range. As in any engineering system, but more so in batteries, these are strongly dependent on all of the operational parameters. The battery may be operated at a low DOD to enhance cycle life wherein a compromise will need to be made on the energy density and cost. If the battery is used for a higher c-rate application, a compromise will need to be made on the cycle life, energy density and the roundtrip efficiency. Due to this complex interdependence of operational parameters and the performance obtained, it become necessary for the pack developer to rely on internal testing of batteries as per the chosen application. Even if the same battery is used for different applications, differently conducted tests would be necessary. Other important aspects to consider with respect to battery performance are the battery chemistry and form factor. The battery chemistry and the quality of construction of the cell have a strong impact on the performance. The chemistry indicates the choice of anode (Graphite, Silicon or LTO) and cathode (NMC, LFP, NCA or LMO) used in the cells. Different cathode materials have different cycle life and it largely reflects on the overall performance of the cells as shown in the Graph 3 comparing capacity degradation of cells from four different companies. The construction quality refers to the consistency in the electrodes, if there are inhomogeneities then local hot spots are created which can lead to rapid capacity degradation or safety concerns and overheating of cells. Different form factors such as cylindrical, pouch and prismatic introduce different challenges in packaging and the design of appropriate cooling systems. An important distinction to know is whether you are buying energy cells or power cells. This distinction exists even within the same chemistry and the same form factor cells. It refers to the construction type of the cell and is controlled by the manufacturer based on the intended application. Energy cells have a lower cost (in $/kWh), lower cycle life, lower efficiency and higher energy density as compared to Power cells. Power cells are specially designed for applications where regular high c-rate operation is required (>1C to 4C). The most straightforward method for understanding the battery performance characteristics is to perform battery testing under a carefully chosen set of parameters based on the intended applications.

2nd use of battery packs: What are the indicators for battery degradation?

As in any engineering system, the performance of batteries degrades gradually with usage. Understanding of this process is important, also from the viewpoint of deciding on the 2nd usage of battery packs prior to recycling. We have already seen that capacity fading is one of the important factors to consider. In Graph 3, it can be seen that the capacity reduces at different rates for cells obtained from different companies. However, remaining battery capacity after the first use can often be misleading. This is especially true in case of the cell from Company 4 (in blue). It can be seen that after approximately 500 cycles, the cells exhibit ‘knee degradation’ or a rapid increase in capacity fade. If one is not aware of the existence of such phenomena to the cells of choice, it can lead to significant premature failures in case of the 2nd usage. This can result in significant economic losses in the form of having to replace systems. In addition to the capacity fade, another indicator of degradation is the cell internal resistance as shown in Graph 2. This can be measured as DC resistance (DCIR), which increases with usage and as the battery degradation progresses. The resistance can also be measured with impedance spectroscopy (EIS) that can give a more detailed account of the degradation. If we know the normal initial resistance of the cells (before use), we can estimate the extent of degradation by comparing it with the DC resistance after first use. Such measurements can be used to identify and eliminate ‘bad cells’ from a used battery pack. Accelerated Testing One of the principal drawbacks of battery testing is the long time required for the tests. For example if a cycle life test is being carried out at 0.5C/0.5C, a maximum of 6 cycles can be conducted in a single day. If this test needs to be run for 2000 cycles, almost 11 months would be needed. Often time constraints do not allow such long waiting periods and that introduces a lot of doubt in the mind of the pack developer regarding the actual field performance. How to give warranties on a product in such a case? These worries can be alleviated by accelerated testing. One of the most common methodologies for accelerated testing is the use of temperature. In Graph 1 shown above, we can clearly see that the cycle life obtained has a clear dependence on the temperature. This capacity (Ah) degradation data has been obtained from real testing of batteries. It can be seen that the cycle life is reduced and the rate of degradation is increased as we increase the ambient temperature during cycling from 25°C to 45°C and 65°C. By using this type of a known relationship, a complete cycle life test (upto 80% EOL) at 65°C would need only 1/3rd of the time needed at 25°C.

Battery Testing at CES

In the battery testing laboratories established at CES, we continue to investigate various battery chemistries and their performance under different operational conditions. Many of the graphs resulting from the ongoing testing have been shared to exhibit some of the clearly observable trends. For further information regarding your testing needs or if you need any information regarding interpretation of your battery performance related problems, please feel free to contact CES at the email mentioned below.

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