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Reasonable to vent a warm climate garage?

300TTto545 | Posted in General Questions on

I have a new house with a west facing garage in Raleigh, NC. There is a 40 foot shade tree with pretty good coverage. I insulated the garage to basically a code minimum for conditioned space with a R11 double door. It has one wall attached to house and nothing above it.
The house is 2×6 with Zip R3, mix of foam and blown fiberglass. HERS 50 (-7 with solar). 
The garage has 2 Teslas (as well as batteries for mower/edger/blower). 
This morning the garage was 83 with an outside temp of 68. No charging, no driving after 4:30. Yesterdays high was 85 or so. 
I frequently find myself leaving the garage door open to cool it off but there are lots of critters around including ticks. That doesn’t go over well.
So I have considered a fan to the exterior – with an passive intake. Not for the 50 cents a year saving on a/c of the house but to protect the batteries. Probably irrational but the last car was  a Leaf so I am a bit sensitive. Had some degradation but not warranty level. Sold after 6 years.
My experience with open windows at our last house was that you could easily get to within 10 degrees of outside temp but not much better. That was a much bigger volume of course and I was not using a fan. 
In the spring and fall of course the exterior air temp will be 50 in the morning and the garage will be 75 but probably up to 85 in the early evening. 
Most of the car charging is mid day when solar is fired up or early AM topoffs. Hard to say how much this contributes to heat load – certainly not the early AM charging.

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  1. Expert Member
    Akos | | #1

    Unless it is a big fan, it will not move enough air to cool the place.

    Charging cars does make heat especially if the car runs the AC system to cool the batteries down.

    There is a calendar life difference between batteries at 85 and 75 but it is small. Getting above 100F is where you start to see more significant effects. Batteries also have a huge thermal mass, their temperature lags ambient, a bit of extra night time cooling might not do much. I doubt the improvement will be the worth the energy cost of running that fan.

    You are better off to install a passive inlet and a couple of operable windows higher in the wall. The natural light will also make the garage much more pleasant.

  2. Jon_R | | #2

    Depends on the exact batteries (the Leaf was poor), but for graphs I see, life vs charge temp curve is very flat (ie, little effect) from 68F up to 100F (battery temp, air temp will be lower). Many curves show slightly *better* life charging at 90F.

    Do charge as late as possible (ie, just before driving). Less time at high state of charge is helpful.

    If you proceed, I'd use an ECM fan (more efficient) and a thermostat that compares outdoor temp to indoor.

  3. tommay | | #3

    Cupola vent? Solar powered vent with t-stat?

  4. Expert Member
    BILL WICHERS | | #4

    You could probably adapt a small whole-house fan for a garage cooling application. You’d want a thermostat arrangement that would only activate the fan if the garage temp was too high AND the outdoor temp was lower than the garage temp. No point running the fan just to bring in air that is even hotter.

    I’ve had my volt for 7 years now, and I still have factory new battery life. I’ve never done anything special to make it happy — it sits outside or sits in the garage and charges when needed. I’ve never had a problem. That said, the volt does run heating and cooling systems to maintain the battery at a good temperature for whatever it’s doing and I’d expect the Tesla does the same. This means the battery will be at whatever is it’s optimal operating temperature regardless of the garage temperature as long as the garage isn’t so hot as to be outside or design limits for the car (there is no way 83F is outside of design limits, design limits are probably at least 104F which is the NEC standard, and automotive limits are usually more severe, so probably even higher).

    Just for some perspective, in the datacenter facilities I design at work, I spec the battery rooms to be maintained at 78F which is the optimal temperature for the lead acid batteries that are typically used. All the datasheets give electrical parameters based on a 78F space temp for the batteries. The lithium ion batteries in the cars are likely different, but you probably have no need to worry about the temps your seeing in your garage.


  5. 300TTto545 | | #5

    Thanks all.
    The Volt is much more aggressive about maintaining near 70 when plugged in. Cooling on the Tesla only starts around 100. The 78 data is interesting.
    I figured the goal of near perfection was not worth it.
    My 2015 (75k) has around 8% degradation. Feels like it should drive forever and I know the curve has likely flattened considerably. Just want to be as good as possible.

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