I wouldn't be too worried. My CL9 had a flat battery once. It was because I left the key on the "ON" position without the engine on, while talking with my mate, door open (so interior lights were on). Talked for about 15-20mins. Car didn't start. Jump started it, and it's been 3 months since, haven't had any problem similar. I'ld say just leaving the headlights on wasn't the killer cause, but if you leave your key on the "on" position, all the pumps and other devices are all turned on which really drains the batteries.