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Also via the web - https://getsupport.apple.com/
select the repair -> battery replacement - then there's an option for "reserve a battery".
You will be prompted for your phones IMEI/MEID and you will be notified when the battery is in stock and to come in for the replacement.
This will save the hassle of coming in, diagnosing, and finding that there's no battery in stock, and to have to come back later. So this eliminates an extra visit/time.
My own experience (before this was available) was 3 trips - 1 for diagnose, place an order for iPhone 6 battery due to out of stock, return a second several days later to drop off phone for repair, then next day to pickup (as I didn't want to wait the 4 hour turnaround)
This should eliminate the first trip, saving my time (and Apple employee support time),