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Recognizing weak cells in Li-ion battery packs

When cells are produced, they are tested. But a small percentage has weakened when they are assembled into packs, usually months later. This may be caused by impurities in the electrolyte, resulting in shorts, which in turn cause an increase in the internal resistance.

Thus, that cell will cause a larger part of the energy, compared to the other cells, to be transferred into heat. This article describes how to identify weak cells in e-bike battery packs and improve quality and yield.

 

Temperature sensors

Typical 10S4P e-bike batteries have 3 temperature sensors, with extremes of 1 and 5. In all cases this is highly insufficient to identify a weak cell, as the high temperature of one cell is distributed from one cell to the other and this makes it difficult or impossible to recognize. A dangerously high temperature of one cell would not be noticed, except when the weak cell is very close to the sensor. A good practice for e-bike batteries is to have one temperature sensor per 4 cells, as illustrated below.

cell-sensors

The red and orange dots represent the temperature sensors. Red means a significantly higher temperature. In addition to the temperature measurement the SoC (State of Charge) of each paralleled group of (4) cells can be measured with a battery analyzer that can read BMS data, provided the BMS was designed to be able to read individual cell group voltages. When the third cell group shows a lower SoC than the other groups, the weak cell will be in the upper half of that group in this (simplified) example. The SoC alone does not necessarily identify a weak cell, but the combination with temperature measurement does.

Identify weak cells during production

During quality control of a just assembled battery pack, it is important to determine the location of a weak cell and return the pack for correction. A weak cell initially reduces the capacity of the pack, but as weak cells tend to be weaker over time they may result in a warranty claim. It is therefore in the best interest of the manufacturer to perform a good, full charge/discharge/recharge cycle. This will identify virtually all weak cells.

Typically, the number of weak cells from a good quality cell supplier is a single-digit number per 10,000 cells. At 4 weak cells/10,000 and 40 cells per pack up to 1.6% of the assembled packs could have a weak cell. Identifying them and correcting increases the pack capacity and reduces warranty claims. The first step is to do an incoming QC check on the cells at the pack assembler, but not every company has the facilities hereto. The next alternative is then to do a check (on the internal resistance) on the assembled pack with a battery analyzer.

Weak cells and safety

Advanced battery packs such as the Tesla EV, not only recognize weak cells during quality testing, but can also be de-activated when an individual cell becomes too hot during operation. The e-bike industry is the other extreme, where safety is most ignored. The only solution to improve this, is increasing the number of temperature sensors. In applications where safety and reliability is of the utmost importance, every cell has a temperature sensor. For e-bike batteries, 1 temperature sensor per 4 cells is acceptable.

Safe to Transport Definition

Since a few years there is a law that requires batteries that are not “known to be safe” to be transported in a fire-proof container. But as this law does not define this, it is impossible to implement. The author of this article is involved in an initiative to define “safe to transport” that will ultimately lead to legislation. While still under development, it is clear that batteries with weak cells may fail this test. The exact criteria are to be determined by a European normalization institute, based upon risk analysis. Very likely it will become mandatory to have more temperature sensors and register the safety history in the BMS.

Other sensors

Few e-bike battery packs contain moist sensors. Like temperature sensors (NTC’s) they are very cheap, but the assembly cost prohibits wide use. Likewise, drop-detections sensors are currently rarely used. A drop detection, combined with moist registration would allow a warranty claim to be refused.

Wiring

A typical 10S4P battery with 3 temperature sensors has 14 wires that go to the BMS. With 10 temperature sensors and 2 moist sensors this would increase to 23 wires and thus significantly increase assembly cost.

The author of this article co-developed a low cost method to replace the manual wiring by flex PCB’s, which can be reflowed (soldering machine). This is not only much cheaper to produce but also far more reliable.

Conclusions
  • In order to recognize weak cells more temperature sensors are necessary. 1 per 4 cells allows proper temperature measurement.
  • A BMS must be able to measure the individual cell group voltages
  • Manual wiring can be replaced by flex PCB’s