Buy a battery charger/maintainer and leave it hooked up while you are gone. Your battery will be fully charged when you return. You can buy the chargers for as little as $20. Well worth it IMO.

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'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4