ok now i see.....when the engine is running the alternator is charging the battery....disconnect the battery negative and leave the engine running the radio still has 12 volts from the alternator and will keep its memory.(not good for your alternator though).
Hi Hightower - not a good idea - a battery acts like a voltage regulator to give out 12 volts whereas an alternator will vary the voltage on demand (up to 14.5 Volts) - and this wouldn't do the onboard electronic systems any good - if you end up frying a few circuit boards it could prove to be very expensive. If you want to keep from loosing the radio code try connectng another battery positive terminal to the red cable in the under bonnet fuse box and the negative to earth. Even if renault charge you a tenner for the code it would still be cheaper than trying to replace the ECU for example.