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Heating up and cooling down happens at the same rate if everything else stays the same. If heat is locked in a pocket below the vehicle (heat rises cold falls down) that may change things.
For those who have garages can use regular fan just to circulate air better under the belly. Weak 20-50W fan is all that is needed. Hardest part is to get air moving in the gap between battery and car. Also cooling down interior should help (open window).
30C is the optimal working temperature for battery. Pretty much ideal for hardcore usage. But 40C is the upper edge for Optimal working temperature, ok for use but not for rapidcharge. 30C is upper edge for storage. Nothing bad in 8hours of storage at 30C. 35C is above Optimal storage and 40C is upper edge.
PS! have an idea. I believe battery service plug cover is a direct access to that air gap between car and battery.
I believe that just removing carpet and metal cover will allow MASSIVE airflow improvement.
I ask somebody to do some measurements with hot battery (no need for hot weather)
If it works then it will drastically speed up battery heat dissipation.
It is even possible to push cold AC air from cabin onto the battery upper part if vehicle is stationary and air re-circulation is disabled. Tight space between car and battery is ideal for uniform distribution of airflow (stationary).
It is safe only if there are no children inside the vehicle. But for adults it is absolutely safe. Just don't remove the fuse.
Depending on airflow this might have to be closed when driving on highway.
If this is the case I guarantee a massive change in battery cool-down period while stationary (or heatup for nordic users): stationary in garage, windows ajar, heat rises by itself.
Source : Which factors have the biggest impact on battery life?
