Posts have been getting a little thin from time to time because of the crazy amount of work I’ve got, and also the fact that I’ve spent the past week or so battling numerous illnesses. Anyway, today I’ve finally felt well enough, so back to blogging we go!
The endurance test keeps trundling away, and I’m sure you know what this post means – yes, another 50 runs has been completed. The unit has now completed a nice 466 cycles from brand new, or 450 cycles in this investigation. This means it has lived from about 93.2% to over 100% of its lifetime, using the 300 to 500 cycle rule of thumb for lithium-ion based cells. Lets see how it did.
A further 50 runs of data was completed, with no loss of data in this set. The data seems to show increased variability, although a downward trend is still evident.
The variation in the data seems larger than in previous runs, and spans about 100mAh, which is within my estimated margin of error. The reasons for this could be several fold:
- Increasing internal resistance is a normal issue with cycled lithium-ion based cells, and the increasing internal resistance could cause earlier termination of the power bank’s boost circuitry due to low voltage being sensed, but this might not be consistent on a charge to charge basis. Likewise, it could cause an early full-charge indication, which may not be consistent on a charge to charge basis.
- Contact resistance due to oxidation or wear of contacts, after all, the rig itself has been through a lot of insertions and removals and some banana plugs in the system did get unplugged and re-plugged in a slight room rearrangement.
- Temperature shifts, as my room is subject to weather more than usual. However, the temperature always stays within the stated temperature range for the accuracy of the meter (i.e. 18 degrees C to 25 degrees C, but normally a much narrower range).
Regardless of the cause, the amount of error in the variation is still mostly about 100mAh to 150mAh which is really within the realm of run to run variations, but we are still seeing a slow decrease in capacity.
Using a linear fit estimation, and the original capacity reference rather than the fit’s estimation of original capacity, the lifetime would be 1526 cycles. Using the power-fit, the result is still an admirable 1152 cycles. All of which is much higher than the normal rule of thumb of 300-500 cycles. Maybe this means lithium-battery based storage in vehicles and grid usage might be more realistic than imagined, as the cycle life has been conservative?
In the grand scheme of things, plotting it against an axis starting from zero, the battery still has a long way to go before it is “unusable”. I don’t think anyone really has to worry about cycle life if they’re using a quality cell, which hasn’t been subjected to abusive temperatures.
The experiment continues, and the next report will be due when we reach 500 cycles in the experiment (or 516 cycles in total). So far, I can definitely say that the cell has performed much more consistently over so many cycles than I would have initially imagined based on the “rules of thumb”, but it seems this one defies death and continues a steady and slow decline. Maybe the cycle-life mode of failure of lithium-ion cells is a little overblown especially for better made cells …