EX and SC models have a security feature that disables the radio once the power is cut—such as when a radio is stolen. So when you disconnect your battery for any reason, you have to enter a unique code, or the radio becomes a paperweight. More
here (linked on
FAQ page).
Disconnecting the battery also requires that you reset the driver's window auto up/down feature. For details, see owner's manual or search for relevant thread. (Hey, you got two free answers before being told to use search!)