Test your fan on the car at home overnight to see if it will kill the battery or not.

 

The HVAC system exhausts of many modern cars (I assume, all modern cars, but I can't prove it) are what you see when somebody is driving around after losing their plastic rear bumper covers due to crash damage - rectangles of about 1/4 square foot covered by fluttery thin rubber one-way flap valves which prevent water and rodents from entering the car.

 

rectangles of about 1/4 square foot covered by fluttery thin rubber one-way flap valves which prevent water and rodents from entering the car.

They do absolutely nothing to prevent rodents from entering. It's a common ingress point. Even a large rat could easily chew through the plastic frame to enter if they wished.

On my 2016 I found mice had entered via those vents. I removed the bumper skin and sewed (with brass wire) heavy stainless mesh over the exterior of the vents to prevent further ingress yet still allow the rubber flaps to function normally.

 

"Test your fan on the car at home..."
Yeah was hoping somebody had already tried this, going camping tonight so it's gonna have to wait. Wonder if it matters whether you have the stock battery or a premo one for overlanding.

 

A while back ago, I saw a car with this in summer. If you can rewire it to run off AA batteries. It may works. I have never tried this and has no idea on how long the battery will run on this.
Solar Powered Car Window Windshield Auto Air Vent Cooling Fan Car Window Air Fan (Black) https://www.amazon.com/dp/B07G2BLYTS/ref=cm_sw_r_cp_api_fabc_2YSRFb844QQ4M

 

nearly all modern cars (in at least the last 20 years) have flow through ventilation that is separate from the HVAC system. No car is fully closed off from outside air.

In the case of Foresters (at least on my SJ 2014 Limited) there is incoming air flow through the dash vents on the outer edge of the dash and the little triangular vents that reside above them. Then in the rear of the vehicle, there are panels (much like what @whobodym refers to) in the rear panels of the car - down under the cargo area, usually behind the rear wheels on each side... they're a plastic grill with a rubber cover on the outside. This is the exhaust for air in the car... The rubber is designed to seal over the grill to keep air from coming IN (or critters - more on this later).

So when you're sleeping in your car - windows closed - the air will exhaust out of there.

Now the plastic grill and the rubber cover do not really stop critters from getting in as some of those that have posted about mice in their Forester can attest.

A better option would be to buy one of those window vent things that are battery and/or solar powered and just sit on top of the window (when closed) and circulate air in and out of the vehicle. They sell them at WalMart from what I can see.

 

I bought one of these a few years ago from Amazon. It works off of 6 D-batteries or you can plug it into an outlet. I used it at night and it lasted us the weekend and beyond.

Personally, I would rather power stuff like this off of other batteries, solar, etc. than my car's main battery or accessories like the fan. As @FozzieBalou implies, your car isn't completely sealed. Air will get in and exhaust.

If you just want a nice flow of air, get something like we mentioned. There are also nets you can place over an open window or mesh socks (if your doors are framed) where you can crack the windows allowing some fresh air in while keeping bugs out.

 

I may try one of those fans, but I figured since I already have one built in maybe it would make more sense to use it, especially since it could bring in outside air. I have some of those bug/sun net things for summer and they work great. But in winter ideally the windows could be closed so snow/rain doesn't come in, I wouldn't want a window to get frozen open and I have some of those bubble things that totally block the window which I'd like to use. Another option is opening the moonroof with a cargo carrier to cover it, although if it was snowing it would still blow in. Keeping them shut all night long isn't an option because of moisture build up and VOCs. I guess the real question is leaving the fan on all night going to significantly drain the battery or not, will have to try in the garage some time.

 

You can always use a piece of tarp or similar and close it in the door frame to keep rain out if you want to open the window a bit to help prevent things fogging up, to get outside air, etc.

......maybe even using a tarp over the whole car with the cargo carrier creating a gap on top and maybe using poles on one side to allow easy access to the side door(s)? .....or maybe an EasyUp?

 

Just wondering if anybody knows if this can be done without killing the battery?

It most certainly will drain the battery. Your cars fan can run only when the ignition is in the "on" position, so more than just the fan will be drawing down the battery at that point. There's a caution somewhere in your owner's manual against leaving the ignition in the ON position for too long for this reason.

@Kean and @FozzieBalou's suggestions are good ones.

 

It most certainly will drain the battery.

I agree.

 

A lot depends on how strong your battery is. My wife's 2016 started up fine. Nothing to worry about. But recently I sat parked, listening to the radio with the engine off while she did a few transactions, and it wouldn't start the car when she got back. We had to rely on the beloved jump start box.

We then drove home, about a 100 mile trip. Next morning it started up just fine, first try! But who knew how much energy was really left there?

Next stop - Walmart, new battery!

Fan eats up more power than radio! Beware!!!

 

Depend on what model year you have, for our 2018 crosstrek and 2020 forester, once the ignition is in the on position (key turned or start/stop bottom is pressed) the radio is ON. Even if you press the power bottom on radio, it is still ON and running, just the monitor is off.

 

This was in Accessory position. But the real point here is that a weak battery can still got recharged well enough from normal driving that you don't realize how easy it is to suck it down to nothing with a rather minimal draw and no recharge from the alternator. Even though it seems good for a normal restart.

 

Cars are for driving; tents are for camping

 

I put those screen socks over my open windows and hung a rechargeable fan that works for 8 hours. Worked great many times.

 

Just wondering if anybody knows if this can be done without killing the battery? No idea ho much energy it would draw. I usually sleep with the windows down but was thinking extra air in summer would be nice and in winter if I want to keep the windows closed. Side question, is there an interior air "exhaust" somewhere?

Check this out. I have one, it runs off 2 D batteries, can hang from ceiling or use its own stand. Puts out a good amount of air directly where you need it.

https://www.amazon.com/Odoland-Portable-Camping-Lantern-Ceiling/dp/B016HM7QRE

Scot