To reassembling it, we should take special care on the battery contacts are they should be springy and put under the gray bottom plastic.

Bend the contacts a little bit to make this shape.

Insert the top part, this step is easy.

Now insert the bottom part, but half way.

And, with the help of something slim, push the bottom completely making sure the battery contacts are behind the gray plastic.

In the picture I show you how NOT to let the contacts, it is outside the gray plastic.