Most radios use diode protection as a means of preventing further damage (by blowing the fuse - hooking up direct to a battery if something is blowing a fuse isn't a great idea) The diode will typically fail if a reverse polarity condition has been present (even for a split second) and in most sets if an over voltage condition has been present (same deal with the the time frame). When the diode fails it creates a short circuit across the power input which causes the fuse (and any subsequent fuses until the diode is replaced) to blow. Hooking the radio directly to the battery without a fuse while in this condition can cause the printed circuit board to be damaged if it is the "next weakest" current carrying device.