The Cubelet kit we purchased via Amazon had an earlier version of the battery cube. That battery cube is hard to open. The screw had an unusual head making it difficult to open.
Not so the new battery cubes! The new one is much easier to open — they changed the size of the screw. Such good news! Standard itty bitty screwdrivers will open it with very little problems
Here’s the pro tip. Because the cubelets are big chunks of magnets, attach the screw to the magnet while you’re charging the batteries. That way you won’t lose it.