With an internal battery, like the Boxster, I prefer to pull the battery from the car.
The Boxster uses a vent tube to vent away any accumulated Hydrogen or Oxygen gas. There can also be minute amounts of sulphuric acid in this vapor as well. Normally, the air rushing past the open end of the vent tube creates negative pressure (vacuum) pulling these gasses out of the tube and away to the atmosphere.
But, when the car is not running, no air is rushing past the vent tube and so no gasses are extracted and can buildup to the point, over a several month storage peiod, where they accumulate under the hood. The Hydrogen and Oxygen pose a fire and explosion risk, but the sulphuric acid can condense on cold metal parts (hood, firewall, electronics, etc.) and can lead to corrosion - rust. Also, the battery maintains it health better in a warmer environment than in the cold car.
The downside, besides actually having to disconnect and carry the battery away, is that the DME, Tiptronic Control Module (if equipped) and Radio lose their memory functions (and security code in the case of the radio - pre-'03) and will have to re-learn once the car is put back into service. But, this is a minor inconvenience and these things re-learn after just a couple drive cycles.
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