2nd 12V battery for HAM radio use

I am a HAM radio operator and would like to install a removable HF radio in my Prius. I am aware of all the problems using the radio while driving, but only want to use the radio in a static parked mode anyway. I am not coordinated enough to drive and operate the radio at the same time! What I would like to do, however, is to power the radio with an additional 12V battery secured in the rear passenger side corner of the car.
My question is, is it ok or not ok to connect this battery in parallel to the stock 12V battery and have both batteries receive a charge from the car when driving. Again, I only intend to operate the radio (max 100 Watts) when the car is turned off. In case you are a HAM and wondering why I would do this, I live in an apartment that has no capability of setting up a decent antenna for com. Trust me I have tried. I recently laid out my radio system in the car and parked up the hill from where I live and had the most amazing reception! I don't want to use just the stock 12V for power while the Prius is off for fear that I could run the battery down too much.
Any suggestions or ideas would be appreciated.