I have a 2004 GS ADV and a 2008 1200 RT (my wife's) parked in the garage as I wait for the winter to go away. I have to admit I do little winter prep on the bikes. The garage is semi heated. I do fire them up every couple of weeks (rightly or wrongly) but clearly the battery juice is low. What would you recommend for a charger that I could leave on and rotate between the two bikes or possibly purchase 2.