No. There's no way to get rid of the warning besides turning the car to "ON" mode. The 12V battery gets recharged from the traction battery but only in "ON" mode not "ACC" or "Off" modes. You can do a couple of things: 1) Ignore the warning. If you're only drawing radio power you can go quite a while before battery drain becomes a problem in "ACC" and even then you can bootstrap a charge to get going with a push of a button. 2) You can turn the car to "ON" mode briefly then back to "ACC". That will give you a fresh charge on the 12V bus. 3) You can leave the car in "ON" mode with the parking brake set. That will shut the headlamps off and you can stay in that config for quite a while before the engine starts and runs.
