Almost two years ago, I started an experiment to see just how well a random no-name power bank would survive under repeated cycling, and to chart its “demise”. While I had some notion that it might have failed early due to its unknown heritage, and some other preconceptions that it might conform to the 300-500 cycle “rule of thumb”, the reality had been quite different.
The cycle experiment continues, with the 14th installment of results representing a cumulative cycle count of 700 since the experiment had begun, or 716 since new.
From the last report, the trend seems to be holding mostly steady, with a post-rest downward trajectory that is slightly steeper and mostly “holding” along the trajectory. However, the past ten cycles or so have seen a slight “bump” occur where the capacities have trended upward slightly, but still within my margin of error of ~100mAh. This may be potentially related to the recent cold weather being winter, but is probably just a temporary deviation. The capacity lows and highs are still roughly headed downwards, although the mean capacity is still about 100mAh above the 80% “failure” point.
We can see on zero-scaled plot just how much capacity remains, and how much is lost.
The experiment continues, with another report to be posted in 50 cycles. The resilience of the power bank is rather astounding, for it indicates the cells, as well as the connectors, charging IC, etc are all functioning well with so many cycles despite the “average” looking design. I’m surprised my test-rig didn’t break either, although I suspect connector wear may have caused some increased variations in readings due to connector resistance.
I think this goes to show that lithium-ion/lithium-polymer cells aren’t strictly going to fail based on a high cycle count alone, and can outlive the “rule of thumb” 300-500 cycle count. They likely accumulate damage due to poor charge/discharge protection, as well as ageing and storage. Regardless, their mode of failure is reported to be increasing internal resistance, indicating the inability to service and sustain high load currents. As power-bank currents are relatively modest by comparison to other loads, it likely indicates that cells in such situations may last considerably longer than expected.
It also lends credence to the whole idea of using Li-Ion chemistry for electric vehicles, and once unsuitable, using them for stationary power situations where larger bulk and weight is not a problem, and having racks of them can reduce instantaneous peak current demands to something the packs can still handle to eek more life out of them.