in the case of the furman unit I linked to above, you can buy extra batteries and, assuming you still have power, change them while everything is fired up playing. They also offer external battery packs, but that would probably be a pain imo. There is also a larger unit
I've had my car audio do some strange things, even w/ an aftermarket battery. I'd trust a power conditioner w/ voltage regulation to supply a more steady stream of juice than any battery, especially since there is no memory.
I've been looking at the APC UPS's but I like the aesthetics of the furman better, so I'm probably going that route myself.