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No, not necessarily, but it can cause a minor issue. iPhones use a Lithium-Ion battery for its power, using while it's charging won't necessarily ruin the battery, but if you're using the phone while the battery is in the charging process, it can cause the phone to think it has more power than it actually has.
Fast charging is not that. But since fast charging is only for the beginning of a charge cycleâand phones and their chargers are smart enough to only apply the extra voltage when it's usefulâthe damage isn't too bad. Still, if you're not in a hurry, it's probably better for your battery to apply a slow and steady charge through a low-voltage charger.