On 11 Jan 2018 18:35:18 GMT, Jolly Roger wrote:

the batteries work perfectly fine in normal everyday use.

Which is why the iPhones suddenly shut down in use.

False.

"In other words, Apple decided on behalf of its customers that they'd
prefer an iPhone that performed worse for the same amount of time,
than an iPhone that performed just as well for a shorter amount of
time. It's a decision that does nothing to dispel the characterization
of Apple as a company that does what it can to push customers into
buying new phones."
https://www.theverge.com/2018/1/11/16878412/iphone-slowdown-battery-replacement-wait-times-6-plus-supply-shortage