Hello & Happy New Year

My Jeep (WJ) lives outside now & to preserve the battery, I've fitted a disconnect to the negative terminal, which to date, works a treat ! However, yesterday, after starting the Jeep as usual, I tapped in the radio code & nothing happened !
Is there some sort of battery which retains the radio code when the main battery's disconnected, which could have become discharged ? If not, anyone have any ideas what may be happening ? Cheers
