had this same problem; ive got an underground garage which sees wide temp fluctuations in ohio springs/falls so i kind of tested this over the course of a few mos; the pcm reads ambient air temp and cca's in the battery probably every few minutes. whenever the pcm thinks cca's are too low to start the car (it factors in ambient air sensor reading) it sets off the alarm to let you know that you better come start your car before your a_s_s gets stranded. i reccommend getting the 750cca battery. def need the vented one as everyone else said. i would especially recommend the 750 if you frequently park anywhere with frequent fluctuations in temp bc the 650 still had plenty of life left, just barely would keep enough cca's to start it when it got cold. the ambient air sensor doesnt always read correctly, so it would set the alarm off before it probably needed to, esp when the sensor came back colder than actual temp. it kept sounding the horn at night and waking everyone up, so i pulled the horn relay... ha nice try. when the security system trips, it drains the sh*t out of the battery quite quickly (lights, horns, sensors/modules/etc all run more than 'normal'). long story short, my battery was on its last leg, but the car completely killed it bc of the system constantly tripping out (usually unnecessarily due to inaccurate ambient sensor reading in the constant temp fluctuations in the garage). the 750 cca battery cured everything noted above, plus a problem with the car saying a door latch was open when it wasnt. I had replaced the trunk latch switch/sensor when i narrowed it down to that switch being the one that would phantomly open/close. i even went as far as putting the old switch back in to test the theory: not a single problem with the alarm or the open latch since the new battery installed.