The two composite sides of the rover's battery pack will interlock, using teeth on the upper portion to latch into openings in the base. This will clamp the lithium ion cells from A123 into position to provide the 273 W-hr capacity the rover will use when it passes through shadow (during flight to the Moon or on the surface) or when it needs a supplement to the power from its solar panels. The composite battery pack parts created earlier were this week robotically shaped to create the interlock structure.