Another simple tool, the telescopic, collapsible cup found in many a camper’s gear, provides an example of recursion in action. To make this device, the manufacturer need only program a simple rule add a segment of increasing size to the last segment and repeat it until the desired size is reached. Humans use recursive operations such as this in virtually all aspects of mental life, from language, music and math to the generation of a limitless range of movements with our legs, hands and mouths. The only glimmerings of recursion in animals, however, have come from watching their motor systems in action.

The choice of separator is an important factor in both the manufacturing cost and the safety of the cells. An interesting recent development is a ceramic separator called Separion already in use in cells by German battery maker Li-Tec. The separator can withstand high temperatures, and can thus help to prevent an internal short-circuit in the cell. This is a particularly important aspect as fully-charged lithium ion cells, in contrast to NiMH cells, cannot continue to be accept current without damage, including risk of explosion. The other side of this coin is the very high efficiency of the cell (90 % to 95 %). Safer types are the lithium iron phosphate (LiFePO4) cells from A123 and GAIA, at the cost of around 10 % less energy density and terminal voltage.