Those 20mm battery holders don't actually fit under a normal 25mm base. They're too tall, not to mention the 20mm is in reference to the battery which they hold, not the holder itself - they're actually slightly larger than 25mm rounds, and they have a weird square peg on one side that the contact clip holsters into, making them impractical to use as a base by themselves.
I only know this because I've bought a bunch from an alternate source in the past and been a little dissapointed.