The lithium battery pack for a Nagra 6 digital recorder costs $700. Markus Fuller opened one and found out it has a small battery in it (with 6 cells worth a total of $30), and a large sponge. It must be an awesome sponge! As he opens it, he gives an interesting and entertaining lesson on the history of lithium batteries.
Caveat emptor: Nagra sells a more expensive battery pack, which contains more batteries, so you won't get as much of the magical sponge if you buy it.
Fuller has a good video about capacitors, too: