My iPhone battery went down by 14% over two years with about 500 dis/charge cycles. Do you really think if it hadn't sat at 100% for maybe six hours every night (and in my usage also a couple of hours during the day as I often leave it plugged in while using Bluetooth headphones), it would have gone down only by 4%?a software switch isn't very "Apple style" so I don't think we will ever see one.
I really don't know if in 1 year the difference will be just 1%. Maybe you are wrong and it's 4/5%. Still not much, but over two year (typical life time before upgrade) is more like 10%, that is significant.
It has been known for a long time that if you don't use a rechargeable battery at all for a while, it's best to store it at maybe 70 to 80% instead of 100%. But before iOS 13 nobody had reported that not fully charging your battery makes a significant impact on battery life. If that 80% rule really helped significantly, wouldn't we have heard about it before? Why are there zero reports from actual experts and actual tests that this 80% approach really helps in a substantial way? If the effect were really as large as you suggest why are no other reports about it (besides Apple's feature in iOS 13)?
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