8.4V zinc-air batteries. 
These batteries are the size and shape of a standard 9V battery, but they are air-breathing half-fuel cells instead.  See the previous sample for more information about how zinc-air batteries work.  Note that these come sealed in a metalized Mylar vacuum pack: As long as the package remains unopened their shelf-life is virtually unlimited, because one of the components needed to make the battery work, air, has been excluded.  My understanding is that after you open the package it takes a couple of minutes for the voltage to build up, then the battery is ready to use. 
Source: eBay seller dmhsgeneralstore 
Contributor: Theodore Gray 
Acquired: 15 January, 2004 
Text Updated: 29 January, 2009 
Price: $6/3 
Size: 2" 
Purity: 10%
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