Lithium ion/polymer Battery Cycle Life and what might influence that.
Battery Cycle Life is defined as the number of complete charge-discharge cycles before we have 20% drop on battery capacity. User might say: big deal, I still have 80% left of useful capacity available on that fallen battery. I have a theory that after this 20% drop, battery degrades much faster then the rate before.
Normally Li-po or Li-ion batteries have around 360...600 cycles. Most of manufacturers claims this number at least 600. But it is not always clearly stated at what temperature, discharge or charge rate and very importantly the depth of discharge.
Also another parameter will influence Battery Cycle Life, which is Battery Shelf Life.And again it is defined by the time, when we have 80% available of initial capacity (or 20% drop in capacity).
Many parameters influence each other: for example Battery Cycle Life is influenced by Storage or Shelf Life and by temperature or by usage parameters. This is why there is no definite answer for most common question: “how long my battery could work for me?”. In order to answer, one must mention the usage environment (temperature, humidity), how far you wish to discharge it, what your charge and discharge typical cycle would look like, will it be sitting on the shelf for sometime? And so on and so forth. Most of battery manufacturers won’t even bother with the Cycle Life tests. This is not only because it takes many months sometime, but also too many parameters to consider. For example: shall we not only run 600 charge-discharge cycles but also put 4-5 different temperature tests?
To make my point clear, let’s consider this particular case. Battery GM403040, 460mAh according to factory labeling.
Date code from Aug of 2014. This makes it almost 6 years old battery. It was sitting in the warehouse in storage at 16...28C, with 23C on average. Recommended charge rate is 0.2C to 0.8C. It is around 1C discharge rate model.
Here is our modified Cadex 7000 battery tester, for Li-po bare cells and we let it run 3 different situation tests.: Charge 0.5C, 1C, 2C and all three sets running at 1.2C discharge.
The 1C and 2C charge rate gave us only 4 Cycles before it failed to 77...78% initial capacity.
Another test was done with the same model and same batch battery but with 0.5C charge rate. The life cycle test gives well over 120 cycles before we stopped the test due to time limit on the research topic. Here we can see the effect of long storage time on reduced life cycle.
Just to make sure the Cadex tester didn’t make mistake, I run the capacity measurement on the 4 cycles battery with computerized Battery Analyzer CBA-HR. And confirms the 77% of initial capacity.
Next test was done with newer 2019 batteries GM, which makes them to be only 1 year old.