How to Prevent Recalls of Li-Ion Batteries?
HONGKONG – Reports on safety issues in the field require a recall of the whole batch. Look what happened with Dutch Gazelle as they had to issue a recall of thousands of batteries in our report last March 26.
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 distributed.
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. This test should be performed just before sealing the pack so that individual problems can still be corrected. This procedure has the 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.
Accelerated life cycle test
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 some 120 cycles can be completed.
This is about equivalent to one year of street use. By applying 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.
Center of Advanced Life Cycle Engineering
Considering the high impact of this publication I asked for second opinions and this lead to a response from the Center of Advanced Life Cycle Engineering. This authoritative lab specialized in the area of prognostics and early failure detection methods for batteries, confirmed, “It’s a good summary of the concerns and practices for battery reliability but accelerated life cycle testing has some open issues”.
I am aware that the proposed methodology described above needs to be confirmed in practice, but personally I am convinced that this structural approach will save the industry a lot of money, headache and improved reputation of brands who implement such a program. As I will meet the professor/director of this lab next Monday I am well positioned to recommend stakeholders hereto.
About the author
James Post, a technologist and high tech strategist since 1975, came to the e-bike industry mid-2011. By visiting major e-bike manufacturers he started to analyze their test needs, while he extensively evaluated the e-bike market and potential improvement opportunities.
From spring 2012 he became a frequent speaker at battery/EV conferences. 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 (www.batteryconditiontest.com), closely associated with a Dutch high tech electronics R & D company (www.engineering-spirit.nl). Industry stakeholders reach him at: email@example.com