This week I realized that there is no constant for battery temperature cooldown.
And this came from next observations:
during above freezing ambient weather it is very hard to get more than 10C above
ambient for average battery temperature with only 3.3kW charge rate (no L3 charging).
On the other hand it is very hard to not get more than 10C above ambient if it is
very cold outside. This week we had -20C (aka -4F) and battery was +2C (above freezing)
after only one longer trip. It doesn't cool down as fast as in mild weather.
I though about that for half an hour and came to conclusion than humidity
plays a big role on cooling. And now it makes a lot of sense why battery pack hardly
cools below -5C even after cold soaking for 10-15 hours at -15C -20C.
As a sauna lover I know that it is not the temperature people feel, it is the humidity (aka ability
to get/lose energy). Same works with enclosed battery pack. If we submerge battery into
water it will have much better heat transfer. Air pouch in the pack would then be the limiting factor.
I can easily sit in 120C (250F) environment for 10 minutes if humidity is low.
Adding half a liter of water (½ gallon) into small room at that temperature will instantly burn my ears
I can not withstand two minutes in 60C (140F) if Rh is above 100% (misting vapor).
Therefore there is no constant for L3 charging. Heat generation can be calculated using efficiency figure.
That efficiency also changes but most likely very little (25kW rate vs 50kW rate).