IF you can I'd really suggest a CTEK3300 battery tender.
not a trickle charger, but a battery tender.

you can even set this model of battery tender to de-sulfate your battery, and it prolongs the life.

I store my 850i for 4-5 months, during winter, under a secure parking garage 30 kms from my house and this battery tender has NEVER caused me issues. Ive been using it the last 3 winter seasons.

not expensive to buy, and not expensive to run/on electricity. Only kicks in when needed. Research it if you can.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S