I believe it's an explosive squib to disconnect things in case of an accident (i'm not kidding)
Here's another shot of the same area:
--dick
p.s. before your photo, i hadn't realized that it said "BAM" on the part... how appropriate.
Whenever you encounter yellow connectors in the Sprinter, they're part of the "supplemental restraint system".
p.p.s. i only have schematics for the 2007 model year, and that device doesn't show up on them ... neither as a fuse (the "300 amps" label hints at that) nor as a squib attached to the SRS module. There is a 300 amp fuse in the wire between the starter and the generator, but it's not at the battery post. It's part of the infamous "Y-cable".