I will have a 18 day period when both of the xk's will not be driven. I always lock them when I leave the car knowing that this shuts down most of the drain after 2 minutes instead of 30 min. but this also activates security system on the car with the blinking red light on the dash. Just wondering if not locking them and keeping security off for 18 days might actually be less drain on battery? Of course cars are in locked garage. Please no CTEK advice.

 

Locking them will reduce the power to a minimum. That blinking red light takes a an insignificant amount of power. If your battery is good, it will be fine after 18 days.

 

WhiteXKR is correct. A fully charged battery should easily hold plenty of charges after an 18 days "rest". To put that into numbers; A typical acceptable discharge (I measured this on my XKR) is about 40 milliamps. That means that during a full 24 hours it will loose close to 1 Ampere/hour worth of charge and in 18 days it will discharge 18 Ampere/hours. The factory battery is rated for around 90-100 Ampere hours, meaning that you only discharged the battery to the tune of 20 percent in 18 days. I had my battery-sucking XJL at an airport for 21 days last year and it started up fine. As long as no doors being open, which draws large amounts of currents.