the 20 watt one.

or any battery powered portable bass amp you might have.

how long do the batteries last?

any downsides to these type of amps?

What's the reason you are looking into battery powered amps? I own a Pignose amp; its OK for bedroom practice, outside gigs with acoustic guitarists. But honestly 20W is not a good gigging wattage.

The battery on the Pignose H30 and 50s lasts about 6 - 9 hours. In the 6 years I've owned mine, I've had to replace the batteries once.