This device wasn't in a battery enclosure. The ELT is a stand-alone device installed in the crown of the aircraft. According to the article, it was produced by Honeywell and there are 6,000 currently installed in aircraft of various types.

ELTs used to use NiCd batteries. I know there have been fire problems with ELT batteries on Cessnas and other GA aircraft in recent years. It sounds like the 787 was just doubly unlucky to be the commercial frame to get an ELT fire.