What you want is an "acoustic instrument amplifier", not just any old amp. In terms of quality, there are lots of different levels available from cheap, inexpensive amps to top-of-the-line.
I have an Acoustic AG60 $299 at Guitar Center, Musician's Friend Online (60 watts, four inputs with individual volume controls on two channels with independently controlled DSPs). The amp sounds great with guitar and ukulele and it has plenty of power, but you need an electrical supply-line to run it. That works for me.
Crate and Behringer both have a reputation for lower quality. Most serious acoustic guitarists I know would never recommend Behringer amps. Interestingly, they don't recommend Marshall acoustic amps, either. They do recommend the Roland AC60 ($500) quite often and the new Fishman acoustic tower ($999.00). Fender Acoustasonic amps are popular for obvious reasons. Expect to pay $400 to $500 for one.
For battery powered amps, Crate does have a small amp with rechargeable battery (the Taxi) that will play for eight hours. It is only 15 watts of power, but that is how you can get eight hours from it. The Roland cube sounds pretty good, but it is not really an acoustic amp and when you play it side-by-side with a good acoustic amp, there is really a world of difference in tone. Still, if you play with the knobs a bit, you can get some good sound from it. It is a great value and sounds sweet with an electric guitar.
Over long use, the Crate won't hold up as long as Roland.
