They will try their best to follow the law in the most annoying way. You can do it on your own but your warranty is gone and important security features will be ‘unavailable’ like fingerprint on iPhones with replaced screens or the introduction of ‘registered USB-C cables only’.
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They’ll appeal this arguing that using their proprietary equipment that can be rented for €100/week, the battery is user replaceable
The text specifically disallows this, at least.
Though some sort of ID-verification so you can only put in official Apple batteries for a stupid markup seems likely.
They will try their best to follow the law in the most annoying way. You can do it on your own but your warranty is gone and important security features will be ‘unavailable’ like fingerprint on iPhones with replaced screens or the introduction of ‘registered USB-C cables only’.