You don't say how old your car is but modern batteries should last a while even when the car isn't being driven before going flat. But yes, portable jump starters are very useful in case your battery does go flat and are a useful alternative if you can't keep your battery conditioned while the car sits stationary. What I would advise is reading the car's manual just to ensure you're okay to jump start it. If you don't have a manual, I'd call BMW customer services to ask what they advise. Some manufacturers are very specific about battery charging and jump starting in regards to warranty issues if you cause damage.