Longer life for used batteries

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If a battery is depleted, it does not automatically have to go into the shredder. It is possible that only individual cells are defective. But in order to determine this, all the cells usually have to be measured. This is usually too time-consuming for recycling companies.

Researchers from Bayreuth have now found a way to assess the aging of individual cells in a battery more quickly and easily. They published their results in the scientific journal "Applied Energy".

They determined the condition of the cells using "electrochemical impedance spectroscopy" (EIS). This involves exposing the battery to alternating current at different frequencies. The reaction to this indicates impedance and thus aging.

The three key problems here:

• First, it is not enough to measure the current values of a cell - its aging can only be assessed if the comparative values in the new state are also known.
• Secondly, the characteristic values are never completely identical even with factory-fresh cells, but are statistically distributed due to the manufacturing tolerance.
• Thirdly, it is important to find out whether all the cells of a battery have aged evenly - in which case it would be a case for recycling - or whether only a few cells have suffered particularly badly. In this case, it would make sense to replace these cells and continue to use the battery.

Measurement experiment: Twelve battery cells connected in series

The research team led by Tom Rüther, a research associate at the Chair of Electrical Energy Systems and the Bavarian Center for Battery Technology at the University of Bayreuth, has experimentally compared various characteristic values for aging. To do this, he connected twelve new Samsung cells in series to form a battery module and measured them. One of the cells was then replaced by an aged one.

The result: The method by which individual aged cells betrayed themselves most clearly was the impedance measurement at the "low-frequency minimum". Here, the range of variation of new cells is lowest, so that weakening outliers produce the clearest statistical signals. The low-frequency minimum was 6.86 hertz for the cells studied. "For other lithium-ion cells, the value is in a similar order of magnitude," says Tom Rüther.

In the laboratory, the method worked with up to ten cells connected in series. With more cells, the statistical signal becomes too weak. In addition, the method only provides information about the fact that there is inhomogeneous aging of the cells - but not about exactly which cells are affected. For this purpose, all cells must be measured individually. But at least the Bayreuth method makes it possible to quickly assess whether this effort is worthwhile.

Help with a battery directive from the EU?

An open question remains: How is a recycler of an old battery supposed to obtain the comparative data from the new condition? Theoretically, he could get new cells and measure them himself. "But that's impractical," says Rüther. It would be better to have the data available somewhere - for example in the form of a battery passport.

This is where the EU comes to the rescue. In its Battery Directive, it wants to require manufacturers to provide certain information on recycling, among other things. "The first draft contained exactly the data we need," says Rüther. "In the current version, unfortunately, it doesn't." However, the exact technical formulations are still being discussed.

Actually, the "Battery Management System" (BMS) installed in most batteries should also know sufficiently precisely about the condition of the individual cells. "Yes, but this data is sometimes difficult to read out. They vary from manufacturer to manufacturer and sometimes do not show information about individual outliers." Again, the EU directive could help by imposing an open interface to BMS data on manufacturers. "We are heartened by this EU directive," says Rüther. "It addresses a lot of things that have been on our minds."

Measure while loading

It is likely that the directive will enter into force this year. However, this would not make the Bayreuth procedure superfluous. After all, the EU regulation only applies to batteries that come onto the market after it comes into force and a transition period - not to all the batteries that are already on the market.

"The next step would be to integrate our algorithm into a standard charging procedure," says Rüther. In addition, the researchers want to use further measurements and artificial intelligence to extend their method to larger batteries with more cells.

He says the process is particularly interesting not only for the batteries of e-cars, but also for electric bicycles and high-load power tools such as cordless screwdrivers or electric saws. "The remanufacturing of batteries is still little explored, but we're talking about huge volumes here," says Rüther. His colleague Gregor Ohnemüller from the Chair of Environmentally Sound Production Technology adds, "We're laying the groundwork here for product recycling with strong economic and ecological consequences."

(grh)