When the lithium-ion battery overheats, the cell's contents vaporize.

CNN: Boeing announces changes to Dreamliner battery system

In addition to Walker, five witnesses testified Thursday morning, including officials with cell phone companies and a computer forensic specialist for the Ohio Bureau of Investigation who testified about her analysis of the contents of 17 cell phones examined by police.

CNN: STORY HIGHLIGHTS

On Monday, the California Supreme Court ruled that police in that state can search the contents of an arrested person's cell phone.

CNN: STORY HIGHLIGHTS