Li-ion batteries in S-Pedelecs are usually equipped with cells of NMC chemistry 811. This chemistry currently achieves the highest energy density of common battery chemistries, which is currently the best option for S-Pedelecs due to the usually tight space conditions and the desire for a high range.
Nevertheless, the batteries are subject to quite high stress during operation. They are usually fully charged, exposed to wind, weather, and the corresponding temperatures during year-round use, and are also mechanically stressed. They do not have thermal management to keep individual cells within a comfortable range. All in all, therefore, quite adverse conditions. In the best-case scenario, the batteries reach a cycle count of 300-500 full cycles until End of Life (EoL), which corresponds to approximately 10,250-16,900 miles for a Stromer BQ983 battery (assumption: ST3, level 3, 100 pedal sensor, 34 miles range per full cycle).
Although Stromer uses high-quality cells in its batteries, defects cannot be ruled out. One such defect manifests itself in the battery being unable to charge fully, either completely or partially. The charging process stops somewhere between 80–90% or the charger displays an error. This error pattern is a sure sign that there is a problem with the battery—provided the charger is working properly.
This is usually due to so-called cell drift, which is a deviation in the charge states of one or more cells.
Cell Drift (Cell imbalance) in a lithium-ion battery means that the individual battery cells connected in series have different states of charge. Since a battery is made up of many small cells, unequal aging or slight differences in self-discharge cause the cells to gradually „drift apart“ during operation.1, 2]
How to detect cell drift on a Stromer with the battery's SELF_CHECK is here described. If cell drift is detected, a new or reconditioned battery is usually the result. The second option is the preferred one – however, there are a few things to consider.
See battery ,Repair / Refit / Used Equipment‘