Swollen battery

Strange, my 4-year old iPhone 6 is suffering since Friday from a ‚swollen battery‘, even lifting the screen from the metal frame. I have passed lunch time today in the Apple shop in Zürich and staff decided quickly to exchange it for a new phone at the cost of a normal battery replacement (only CHF 29 as a special program is ongoing this year). Fine with me, again on the road with a new iPhone 6.