+1 These cars are meant to be driven thus storage and low usage slowly creates problems, I take mine out, in between major trips, once a week on a motorway to get the cat upto temperature and a blast, I also charge the battery once per week and the battery is now nearly 7 years old and shows no signs of issues.You're welcome!
I started using a tender when I was getting "Increased Battery Discharge While Stopped" error messages. It helped to "bridge the gap" until the drain issues were resolved.
I also got an OBD reader so that I could monitor overall battery state of health as well as a plethora of other information.
LOTS of info on this Forum Re: Battery Charge vs Driving Habits.
It's where I learned most about my car's charging system, IBS, etc.
@400 miles over 9 months I would most definitely consider keeping it on a smartcharger when parked.
As a side note, you may want to consider driving your car at least once a week, if possible (bring it up to normal operating temperature and drive for 15-30 mins).
This will help with engine longevity (keeps seals from drying up)