Earlier quoted context omitted.
Hi, Battery engineer here at an electric vehicle company. The crux of it is this: when you draw high current from a small battery, the voltage will sag (1). The manufacturer has an on-board battery management system that MUST turn off the battery if it goes below a certain voltage threshold. As the battery ages, the sag effect increases significantly, up to 2-3x. Thus, when your phone clearly indicates say 20% batter…
>The reason is technical, so it's hard to communicate this in a way that all consumers can understand "The battery in your iphone has degraded. Phone performance will be lowered to ensure stability. See an apple store or visit apple.com/battery to have a replacement battery installed" It would have been so easy.
"This iPhone has experience an unexpected shutdown because the battery was unable to deliver the necessary peak power. Performance management has been applied to help prevent this from happening again. [Disable…]"
Which still isn't perfect because "performance management" is such a weasely way of saying "reduced performance", but it's a lot better than nothing.