How To Prevent Recalls of Li-Ion Batteries
HONG KONG – Reports from safety issues in the field require a recall of the whole batch. This is a not only a costly and complex operation, but also hurts the image of the e-bike manufacturer and the industry as a whole. There are a few simple steps e-bike manufacturers/importers can take – in close cooperation with the battery pack vendor- to recognize issues before the batteries are in distribution.
First and foremost, every single battery must go through a full charge-discharge-recharge cycle and it is important that the voltages of each cell are individually monitored. When cells are measured as paralleled groups of i.e. 4 cells it is not possible to accurately determine weak cells and the only good action is to immediately order a redesign of the BMS. Many -but not all- performance issues will be recognized during this test, which should be performed just before sealing the pack so that individual problems can still be corrected. This procedure has the added advantage that the exact capacity of the battery is known and thus allows inclusion of a test report with the battery, which is a definite benefit.
At the same time the battery can be recharged to the most optimal storage charge percentage (40-50%), which increases lifetime.
The second important step is to perform an accelerated life cycle test on samples of the batch. Battery condition test systems can provide such tests automatically. Typically, a complete cycle will take about 6 hours, which means that within one month after production date ~120 cycles can be completed. This is about equivalent to one year of street use.
By using this simple methodology, weak cells will be recognized, which can be a safety issue when the cells have growing internal shorts (i.e. caused by impurities). Ideally, this test should be performed under climate conditions that are harsher than in the field. As an added bonus, this test also provides useful statistics on battery quality, especially when a smaller sample is subjected to a full life cycle test. In an industry where lifetime specifications are wishful thinking in extreme cases, this is an opportunity for the better brands to distinguish themselves by accurate, realistic and well documented life time specifications.
About the author
James Post, a technologist and high tech strategist came to the e-bike industry in 2011. He is the driving force behind the “Better BMS” initiative and Executive Product Manager and director at Battery Condition Test International Ltd. in Hong Kong.