> One lesson I learned (after burning my fingers to the metal spring of one of the batteries) is that lithium cells can generate high currents when shorted out. Because the enclosure covers part of the battery holder, it's much harder this way to short out one of the batteries while replacing them.<p>I always wondered why many parallel AA battery holders have adjacent batteries facing in opposite directions. The little diagram and discussion here made me realize it's not just about shortening the wires on the outside, it's also a safety issue. Even in the revised design that he used, there is still one diagonal battery position that would short out the circuit.<p>Never discount the battery holder in any electronics project: it's one of the few mechanical parts that you are guaranteed to interact with every week or so. If they are always corroding or losing contact (or burning your fingers), it really takes the joy and polish out of the product.