Using old adapter
as you know that the iphone 11 now came with only the charger wire , can i use the adapter of the iphone 6 to charge the iphone 11or will harm the battery ?
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It won't harm the battery. Apple encourages use of the old power adapter. See the following from Buy iPhone 11 - Apple
As part of our efforts to reach our environmental goals, iPhone 11 does not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.
We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones which are compatible with this iPhone. But if you need any new Apple power adapters or headphones, they are available for purchase.