2000 S80:
Can a "Fuse Box Kill Switch" stop battery drain?
The classic anti-theft install for fuse-box kill-switch only cuts-off the starter system; this will not save the battery, just stop theft(in theory).

Problem:
The sensors allow the AC to run when the car is off, which kills the battery.
Disconnenting the battery sucks when you have cargo and a disabled remote control trunk-lock.
Luckily, the 2000 S80 fuse box is inside the passenger compartment on the driver-side of the dashboard.

Non-optimal Alternative:
Most 'Remote Battery Isolator Switches' (classic theft protection) are made for smaller side-post batteries, they're a drag to install, and they all get bad performance reviews. They seem to only last a month or so if one uses them a lot. The mechanics and electronics are cheap and they don't tolerate heat or moisture.

Cheers.

 

Are you saying the heater fan blows when the car is off? And runs until the battery is dead?

If so you have a bad heater fan resister - simply replace the bad part. (it's mounted under the heater fan motor)

 

Well, the heater/AC blows sometimes. It does not seem to be related to the outside weather. Rather, when I shut off the car and return sometimes, the blower is on.
I've been told it's a faulty sensor and that the AC is designed to adjust humidity and temp even when the car is off.

I'll check your suggestion. Thanks.

 

Yes, about 50 minutes after the car is turned off the heater fan will blow for ~7 minutes. This does not happen if the battery is weak. That's called "afterblow" and is designed to hopefully prevent the stinky sock smell that happens in ac systems sometimes. That function can be turned off using VIDA in the advanced or programming section of the Climate control module.

 

In short, that's not what is killing your battery.. Your AC isn't running, only the fan. it's not "adjusting" humidity or temp. It's just drying out the ductwork to prevent mold growth.
The drain is minimal. It's not killing your battery.
A 2000 S80 has a number of battery killing problems, the HVAC blower is not usually one of them.
Number 1 - how old is the battery?