Anyone who takes their old cell phone out of the drawer after half a year knows the phenomenon: During storage, the battery has completely drained itself, and nothing works without a charger. The same can also happen with an e-car if it is not moved for a long time. So far, no one has been able to fully explain why this is so. Now researchers have found out how such “vampire losses” occur.
In relevant Internet forums, one reads again and again about e-car drivers with completely empty batteries. Or batteries whose state of charge is reduced from 100 to 0 percent in just a few weeks, as if by magic. Most seem to be isolated cases, some comments probably exaggerate mercilessly. In principle, however, it does exist, self-discharge. Even if the on-board storage is not used at all, it will eventually run out. Experts expect a loss of around 3 to 5 percent per month for lithium-ion batteries. Not a big problem for the individual e-car driver. Calculated on the entire fleet but quite a waste worth mentioning. However, the phenomenon becomes a real problem with stationary energy storage systems, for example to buffer wind and sun lulls. And the battery manufacturers also feel the loss when their newly produced batteries are charged for the first time and then stored. Enigmatic shuttle molecules Who or what draws the energy from the cells was an open question for researchers for a long time – until German and international scientists at the Canadian Dalhousie -University in Halifax could first observe the pure mechanism. What they saw: At high temperatures, so-called redox shuttle molecules formed in the cell, transporting electrons back and forth between the electrodes. These electrons should actually flow through the circuit and supply consumers there – i.e. above all the electric motor – with energy. If they take the shortcut through the electrolyte liquid inside the cell, they don’t drive anything, but only lower the voltage of the battery. At first no one could explain how these shuttle connections could form inside the cell – none of the commonly used materials and substances seemed to be the source. Homemade battery problem The solution finally appeared when one of the researchers came up with the idea cut a battery. An inconspicuous plastic adhesive tape was found that was used to connect the electrodes and the separator. The adhesive for a large number of cells consists of PET, which is familiar from beverage bottles. Under the effect of heat, smaller molecules are released from the chain, which then swim around in the electrolyte as electron shuttles and slowly continue to discharge the battery. The scientists have now published their discovery in the journal “Journal of The Electrochemical Society”. The first battery manufacturers have already reacted and are now examining whether they can somehow replace the plastic film. Possible alternatives would be welding the components together or using higher-quality film materials. So far, nobody had bothered much about the thin plastic layer – as a rule, the cheapest and not necessarily the purest product was used. The self-made chemical self-discharge of lithium-ion batteries should soon be a thing of the past. Nevertheless, parked e-cars will probably continue to lose energy. Especially for the battery monitoring systems: They switch themselves on regularly, but can also be activated by the driver, for example if he checks the battery status via the smartphone app. Discharging cannot be completely avoided And another phenomenon regularly consumes energy: that so-called balancing. Because the individual cells of the battery are discharged and charged to different degrees when driving due to the smallest differences in material, the control electronics must adjust the state of charge and the voltage of the individual cells from time to time. This is usually done by discharging the fuller cells until they are at the same level as the less full cells. The excess energy is simply “destroyed”. How long it takes for a parked e-car to run out of power depends on many factors. In addition to the filling level and the basic quality of the respective battery and the efficiency of the cell monitoring, the outside temperature also has an influence. Leaving an almost empty electric car in the blazing sun for weeks is definitely not a good idea. Anyone who does not want to use their vehicle for several weeks or months should therefore charge it regularly. Many manufacturers recommend a three-month rhythm, if in doubt, the user manual has advice. If you have the opportunity, you can also permanently connect your car to a wall box or another charger. There, the so-called trickle charge ensures that there is enough juice in the battery even after a long period of inactivity.
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