I'm having the same problem with a recently purchased '04S. PO replaced battery and failed to swap out vent tube adapter leaving the battery unvented.
Check out this link:
https://www.amazon.com/BMW-Battery-Vent-Hose-Elbow/dp/B01L7U4MUU/ref=sr_1_fkmr2_1?ie=UTF8&qid=1503787574&sr=8-1-fkmr2&keywords=battery+vent+tube+elbow
The listing is for BMW's but I cross referenced battery requirements for the compatible BMW's in the ad and the 986. They were the same.
I'll be ordering a pair.
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2002 Boxster base - Seal Grey, 5spd trans.,ROW M030 upgrade, FVD Brombacher software tune
2004 550 Spyder Anniversary Edition, #1541,Tip, TechnoFix DOF IMSB upgrade
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