There are many posts and request for a portable or outside speaker on ask.sonos and others for a battery powered Play 1.
The Play 1 is plugged in from the bottom with a very flush and tidy fitting power lead. This makes me think and others might agree that a battery pack could be developed that works like a drill battery and plugs into the bottom of the Play 1 - maybe the power lead could also plug into the bottom of the battery pack to recharge in situ so it is seamless in design and can be used whilst recharging or if battery power is low.
I would opt for a lithium-ion polymer battery as:
Excerpt from wikipedia
“The result is a "plastic" cell, which theoretically could be thin, flexible, and manufactured in different shapes, without risk of electrolyte leakage.”