If a battery is exhausted, it does not automatically have to go into the shredder. It is possible that only individual cells are defective. But in order to find out, all cells usually have to be measured. This is usually too expensive for recycling companies.
Researchers from Bayreuth have now found a way to assess the aging of the individual cells in a battery more quickly and easily. They published their results in the journal “AppliedEnergy“.
They determined the condition of the cells with the “Electrochemical Impedance Spectroscopy” (EIS). The battery is exposed to alternating currents of different frequencies. The reaction to this indicates impedance and consequently aging.
The three main problems:
- Firstly, it is not enough to measure the current values of a cell – its aging can only be assessed if you also know the comparative values in the new condition.
- Secondly, the characteristic values are never completely identical, even with cells fresh from the factory, but are statistically distributed due to the manufacturing tolerance.
- Third, it is important to find out whether all cells in 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. Then 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 at the Bavarian Center for Battery Technology at the University of Bayreuth, compared various characteristic values for aging in an experiment. To do this, he connected twelve new Samsung cells in series to form a battery module and measured them. Then one of the cells was replaced by an aged one.
The result: The method with which individual aged cells betray 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 for the cells examined was 6.86 Hertz. “The value is of a similar order of magnitude for other lithium-ion cells,” 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 that there is inhomogeneous aging of the cells – but not about which cells are affected. To do this, 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?
One question remains: How should a recycler of an old battery get the comparative data from the new condition? In theory, he could get new cells and measure them himself. “But that’s impractical,” says Rüther. It would be better to have the data somewhere ready to hand – for example in the form of a battery pass.
This is where the EU comes to the rescue. In your Battery Policy Among other things, it wants to require manufacturers to provide certain information for recycling. “The first draft contained exactly the data that we need,” says Rüther. “Unfortunately not in the current version.” However, the exact technical formulations are still being discussed.
Actually, the “Battery Management System” (BMS) built into most batteries should also know with sufficient accuracy about the condition of the individual cells. “Yes, but this data is sometimes difficult to read. It varies from manufacturer to manufacturer and sometimes does not contain any information about individual outliers.” Here, too, the EU directive could provide a remedy by imposing an open interface to the BMS data on manufacturers. “We are very happy about this EU directive,” says Rüther. “It addresses a lot of what was on our hearts.”
Measure while loading
The directive will probably come into force later 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 the entry into force and a transition period – not for all the batteries that are already on the market.
“The next step would be to integrate our algorithm into a standard loading process,” says Rüther. In addition, the researchers want to use further measurements and artificial intelligence to extend their method to larger batteries with more cells.
The process is particularly interesting not only for the batteries of electric cars, but also for electric bicycles and heavy-duty power tools such as cordless screwdrivers or electric saws. “The remanufacturing of batteries has not yet been researched very much, but the volumes involved here are huge,” says Rüther. His colleague Gregor Ohnemüller from the Chair of Environmentally Friendly Production Technology adds: “We are laying the foundation for product recycling with strong economic and ecological consequences.”
(grh)
