Is there such a thing as a portable 22 volt battery pack?

I just got an iPod dock - the Kicker IK501 - and realized that portable does not mean that it has a rechargeable battery. It uses a 22 volt DC adapter, so i am wondering if it is possible to buy a portable 22 volt battery pack so that i can use my dock without it being plugged into the wall. If there is such a thing, then please list a place where I can get one. Thanks so much!