Back in June I ordered a Zeus starter kit. At that time the small was the only choice. Anyway, I missed that order before I took off, but it caught up with me and I must say I am a very happy vaper. Last week I ordered the medium Zeus battery......just to see.....When I opened the package I noticed the medium battery is the exact same size as the small battery I received earlier. That doesn't seem right, but if it is how in the world do you tell them apart without test equipment? I simply marked one "S" and the other "M."