Jam and JAMin batteries are not EXACTLY the same
Like lots of others, I bought a spare JAM battery with my JAMin. However, whilst it fits and works, they are not exactly the same. On the side opposite the power connections, there are two shallow recesses. These are designed to allow two small ridges on the case to snap into them, thus holding the left side (facing you) of the battery in place. For whatever reason, these recesses are in slightly different locations on the JAM and JAMin, meaning that the JAM battery does not lock in as the OEM battery does. It thus relies on the back cover to retain the battery in place. As I say, it works but....
I came, I saw, I got very confused!