A battery powered bass guitar combo amp has many uses and if you're playing on the street where you haven't got a power outlet, you'll still be able to play. I've been on stages that didn't have enough power outlets and it can be a problem. The portability of these amps is also nice since they're usually very light. These are usually thought of as practice amplifiers, but some of the products below are more than that.
At one time I backed up a guitarist playing acoustic bluegrass or Americana type music on the street. We could do quite well, but I had the choice of playing an acoustic bass guitar or my electric. Neither was that effective. The acoustic wasn't really powerful enough and I had to play hard. The electric was played through a Pignose guitar amplifier with a battery option. The battery wasn't meant to provide power for very long and I had to play at low volume because the Pignose was made for guitar and not electric bass. The products in this post weren't available at that time.
I've played through all of these products except the Vox. I've never seen it in a store, but the reviews are good and I've talked to other players who said it was a decent practice amplifier.
The Phil Jones battery powered bass amp below is the top of the line for a battery bass amp. The sound is amazing and it's good for small gigs and the best in the business for busking.
The Roland below is a solid battery powered bass amplifier. It has four 4" speakers and this combo provides more than enough power to do small gigs and rehearsals.
The Blackstar Fly3 is perfect for rehearsing or playing on the street. The price can't be beaten for a battery operated bass amp.
Pignose created some of the first battery powered bass amps. They've been building these for a long time and many musicians like the Pignose sound. Check it out.