Why take a chance on blowing the expensive electronics just so you don't have to enter the radio code. To use the power socket you have to have the ignition on enabling all the electronics to failure fron a noise spike. As for leaving it running the battery acts as a noise filter for the alternator generated voltage. Without it severe noise spikes are generated in the system and fed back to the computer sending it haywire. The simplest and safest way to change it is without any gimics. Just remove the negative lead first and then the positive and reverse to install the new battery.