Hello, Galen Bird was asking about "a MIDI". If he means a unit which
will connect to a computer and output sound from a MIDI program, I can
recommend the Yamaha MU-10. This would work off a 12v battery (mine
came with an 11.5v fat plug).
It does a maximum of 32 notes at once, but some instruments use two
voices, which reduces the total number. It will connect to any make
of computer, PC, Mac, Amiga, Atari, etc, and to a MIDI keyboard.
The piano sound is particularly good, in my opinion.