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If what you son was using is a safety fuse, then it is designed to fit via magnet to the negative (-) end of the battery (see above).
However, if what your son was using is a Kick module, then it is designed to lay on the positive (+) end of the battery (see below).
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The first fuse is what he has. He wasn't using it in the mod tho. Only when charging - well, until I started searching around and found it belongs in the mod.
Thank you so much for the images! This really helps me understand things better.