The 2 AAA batteries in the $20 transmitter from radio shack last about 2-4 hours IIRC. Also, don't leave your batteries in the Radio Shack transmitter when it's off...I had bad luck with things accidentally bumping the transmit switch and running my batteries down.

For a long trip you should be good with an 8pack or so.

Opposed to buying electronics? Try making one that runs off 9 or 12 v

or buy a similar one pre-made
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~ John