It’s been ages since I last posted about the fate of this power bank. When I last posted, we were up to 500 cycles in the experiment, and 516 in total from new, and the unit was doing very well. On the general rule of thumb of 300-500 cycle life for Li-Ion and Li-Poly batteries, we had gotten our money’s worth and then some! Find out what happens next.
The Test Continues?
As I had gotten into the habit of posting test results in blocks of 50 runs, I was continuing with the tests from cycle 500. By the time I had completed cycle 518, I had to dismantle the experiment. A new experiment for my PhD required the use of my best equipment to monitor a particular voltage over a period exceeding 5,000 hours. As a result, my power bank testing rig was out of commission for a total of 228 days.
Testing recently resumed after the PhD experiment was dismantled – and my multimeter has had barely any rest. This particular posting, as you may have guessed, corresponds to cycle 550 in the experiment, or 566 since new.
But there is a complication. It seems that the 228 days of storage may have affected the experiment. The power bank was stored at a 100% state of charge at 25 degrees C (air-conditioned) with no top-up charges until just before cycle 519. As we can see, a big change can be seen in the capacity, with about 200mAh of real capacity lost.
It’s amazing to see that storage at full state of charge has had this level of effect on the cell. I’ve tried to see whether other factors may be at play – but checking the calibration of the meter turned up no anomalies, and the whole test set-up is the same as before without any updates to the software or repairs to the devices involved. Even the cables were the same. It seems most likely the degradation is within the cell itself.
As a result of this ‘step change’, it’s no longer meaningful to try and fit the data. That being said, I will probably continue and see what happens into the future.
Overall, as compared with a zero-scale, the battery still has over 80% of its initial capacity, although at its low points, it’s only about 160mAh away from actually reaching that point which most manufacturers term the “end of life” for a particular battery.
I suspect this indicates what most have known about Li-based cells – their lifetime begins as soon as they leave the factory. Whether you use them or not, they will degrade, and ironically, this seems to show that prolonged storage at 100% state of charge resulted in about 200mAh capacity loss, which was nearly identical to the first 500-cycles placed onto the cell (about 260mAh capacity loss in ~ 300 days). From this, it almost seems the capacity degradation for this particular cell is independent of the number of cycles and may actually be better predicted by time.
That’s an interesting thought which probably deserves a little more work – maybe when I have the time, I will re-format all of the results by the date/time of their recording, and then plot it versus “real time” to see if this hypothesis seems true. However, I do think that without further extended breaks in testing, it would be hard to be certain as cycles are often run back-to-back in this experiment.
It seemed that the break in testing saw degradation levels similar to cycle testing, and that the age of the cell (time-wise, rather than cycle-wise) may be better correlated with its capacity decline. It still continues to function above 80% of initial capacity. It will be interesting to see what happens, if anything, by the next time I report.