Summary

This battery cover allows a two-cell 18650 holder to be used in a Lego EV3 programmable controller unit.

The depth is increased due to extra space needed for the larger cells - but it can give much longer runtime than the original AA cells, or with small lithium cells that can fit within the original battery space.

Using a holder means cells can be swapped and recharged externally, rather than having to be charged in situ.

(I made this in 2018 for a young relatives Mindstorms kit).