Tough one to answer and I'm no expert on speaker design.
Small (and how small is small  ) sealed enclosure usually means low bass output.
(I once had a small pair of rear satellite speakers intended for a car parcel shelf... probably was a 3" driver and the box was sealed and only around 4" square. Those speakers on the end of a 150 watt Pioneer A80 amplifier sounded amazing for what they were but they needed plenty of clean power).
A battery puts limits on things. 12Volt ? SLA type (small sealed lead acid). The voltage puts a limit on the power available into a given load. You can't get more than a couple of watts RMS into a typical speaker from that. A "bridged" amplifier increases that by 4 times (in theory), in practice probably less.
Some very knowledgable folk over on the single speaker forum,
Full Range - diyAudio
so perhaps a new thread there might help on the speaker design aspect.
With a battery you need to make the most of evey volt so efficiency counts. I suspect an audio amp (probably a suitable chip amp) intended for car use might be the best bet for amplification. I'm assuming 12 volts though as you didn't say. For music a small SLA battery would easily last 10 hours on a charge.
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Small Portable 3" Speaker?
) sealed enclosure usually means low bass output.
