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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.
don't worry nothing happens if you charged your cell phone for a long time..... it uses electric current not battery.. so.. if you want to save electric current just charge half an hour only... mobile has the capacity to save battery in standby mode... if you can't unplug it in time off your cell phone and charge it phone switch on's itself... and stops charging....