From Wired's (great) recent piece, "How Apple’s Lightning-Plug Guru Reinvented Square’s Card Reader."

But ditching the battery meant more than saving space. It was also a huge step towards that simplicity Dorogusker set out to achieve from the start. “With a battery, you have to somehow connect the battery to your circuit board,” he says. “You do that with two little wires. They have to be cut to length, stripped on both ends, tinned on both ends and hand soldered to the battery and then to the board. It’s a huge amount of labor.”

In my robot door life, we were lucky enough to have someone aboard to warn us of the physical danger of lithium ion batteries; we ended up finding some very powerful lead acid batteries to do the trick instead. But we spent a *lot* of time configuring our battery pack, which was a PITA to assemble and mount. Wires suck, and batteries often require them. Avoid if possible.