the problem with varistreams is like jeff has said, they are set up to operate from a regulated power supply, a battery is not a regulated supply as it can discharge at a high rate and provide over 12V, a fully charged battery will actually supply about 13.6V.
Jeff's also right about the inline fuse, its to protect against a surge of current,Amps - not volts.
Jeff, if someone was to use a discharge capacitor for say, an amplifier, then surely this would validate the warranty?