Andreas Gutsch of the Karlsruhe Institute of Technology has enlivened the discussion about the quality and safety of lithium-ion storage batteries with new tests.
KIT also caused a stir last summer when it published tests on battery storage and a checklist to estimate the system safety.
In comparing battery storage, you will probably look at what kind of lifespan the manufacturer promises and what the system costs. However, there usually is no guarantee for the given lifespan. The results presented by Andreas Gutsch of the Karlsruhe Institute of Technology (KIT) at the PV Symposium in Bad Staffelstein, Germany, last week, are therefore sure to be quite explosive. According to his findings, some lithium-ion cells integrated in storage systems lost up to 30% of their capacity after just 1,000 cycles. That is a possible criterion signifying the expiration of cell’s lifespan. Other cells are well above that even after 5,000 cycles. The range, it turns out, is huge.
However, cells that hold fewer cycles are not necessarily of lesser quality, the KIT storage expert explains. “That depends on the price.” While this is known, the lifespan is not. "The market is completely opaque in terms of performance," Gutsch said.
He and his colleagues therefore put a large number of lithium-ion battery cells under a particularly stressful test, fully charging and discharging the batteries to 0%, as is common for tests in the automotive industry. In battery storage systems, the cells are usually discharged only up to a state of charge of 20%, so that the cycle life in absolute terms is likely to be longer than in the KIT-tests (typically referred to as 80% Depth of Discharge, or 80% DOD). While the KIT tests do not offer an absolute statement about the lifespan of the cells in a battery storage system, they do offer a good comparison between different cells as the test conditions for all of the cells were the same.
Differences by country of origin
Gutsch decline to present the results of individual cells. "It's not so easy to obtain the cells," he said. It was therefore necessary to agree with manufacturers’ wishes not to publish the results with specific names. He nevertheless revealed a regional distribution of the results that may surprise some.
In a graph, Gutsch offered a comparison of the maximum capacity of cells based on the number of cycles they had behind them. All four or five cells from China that were in the test ranked in the bottom third of the coordinate system, indicating that their performance declined particularly rapidly. All cells from Germany and Japan ranked in the upper third, while those from the United States and South Korea were either in the upper or middle third.
Gutsch did offer detailed information on one particularly successful model, the Tesla battery. It only lasted 400 cycles. “This battery is built specifically for use in a car," he said. If an electric vehicle owner is able to drive 500 kilometers with a fully charged battery, he can drives 200,000 kilometers with a battery that lasts 400 cycles. A Tesla battery can therefore not be used for stationary applications...
