Leave the car parked with the traction battery at 60% to 70% SOC and not plugged in. Unplug anything from the OBDII diagnostic port, turn the car off, lock the doors, and enjoy your six weeks. The car will charge the 12-volt battery from the traction battery as necessary. I have parked my 2011 and 2015 up to 3 weeks with no noticeable loss of traction battery charge. I never connected a battery charger to my 12-volt battery in either LEAF.
Silver LEAF 2011 SL rear ended (totaled) by in-attentive driver 1/4/2015 at 50,422 miles
Silver LEAF 2015 SL purchased 2/7/2015