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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.
Re: S3 Battery Taking Too Long to Charge. Try letting the battery fully discharge until it turns itself off. The keep it off & let the device charge back up too 100%. There is a app called 'fast discharge' too speed up the process of killing your battery.e: S3 Battery Taking Too Long to Charge. My phone is awful when it comes to this, it charged 14 hours and it went from 0% (it dies very quickly too) to 18%.