I had to change my battery and also got the battery icon with “X” over it. After doing a lot of research and thinking I’d gotten two bad batteries for replacements (tested swapping back old worn out battery, then new ones, etc) I discovered there was a small piece of clear protective tape covering the battery contact - which prevented the phone pins from making the electrical connection for charging. The phone thought I didn’t have a battery installed. It took a bright light and magnifying glass to discover the tape. Once it was removed, I plugged in the charger and though it took awhile to gain enough charge to initialize the system, it finally worked. It’s a simple fix - just make sure the battery contacts are bare and not covered with a protective coating or tape. (mine was a Curve 8530).