Then ran diagnostics and advised it wasn’t in bad shape and to charge it twice a day. Not very practical for me, so I chose to go ahead with the replacement instead before said they didn’t have time. I’ve gone through the specialist battery replacement team now so hopefully the problem’s solves. How hard can it be to change a battery - not very if you design it right. Rant over.
It's not necessarily that easy, especially starting with the iPhone 7 and later models that use adhesives to form a water resistant seal. I think it's pretty straightforward with an iPhone 6. The main issue now isn't "having enough time". They can make time and have a small army of employees in the back doing this all day. The main limitation is limited supplies of the batteries. 2.5 months ago Apple wasn't anticipating that they would need tens (maybe hundreds) of millions of replacement batteries. However, there's the reduction in cost, published limited-time nature of this promotion, and that they don't require that batteries be under the normal requirement for the replacement service. Apple is going to their suppliers say "pretty please can you make millions of these things on short notice?"
Most people can live with a battery that is above the industry standard threshold of 80% (or higher) of original battery capacity. But in this case Apple is apply a considerably different standard. I've heard from some sources that about 90% of the people going to certain Apple Stores for the battery replacement have batteries that pass the diagnostic. This is creating considerably higher demand