How to send back my phone?

To return your phone you contact support and if you are in a hurry you speed up the process by posting your request number here:

This should be fixed by now. Did you apply the latest update?

To me that sounds like your charger/cable is no good. If it lasted 48 hours the first time then the battery obviously works well and if nothing else changed then the only explanation to me is the charging process was deficient.