SageBrush wrote:^^ The 62 kWh pack is proportionally more dense than the 40 kWh pack; its heat removal per time will be impaired compared to the 40 kWh pack. Moreover, the 40 kWh pack takes the better part of 18 hours once parked to drop below 80F *outside* of the summer.
I'll be happy to make a friendly wager that the 62 kWh pack continues to heat up as you drive unless it is cold outside; that the first charge brings it up to a toasty temperature of ~ 120F, and that the pack stays there on subsequent charges with the charging rate then limited by the heat removal accomplished by the AC cooling
Yes, it's all about simple thermodynamics and the battery's thermal resistance to ambient, given no significant battery thermal management.
Since all batteries have an internal resistance, battery heat will always be developed whether driving or charging. So it's a simple function
of how much heat is developed, e.g. via vehicle speed and/or charging level, the temperature delta between the battery cells and ambient,
and the overall thermal resistance of the cells to ambient. Thus, the temperature delta is the key controllable variable!
Temperature delta is only a key in how fast the pack cools. If the delta is huge, cools faster, if small, cools lower, however how much the battery pack heats up, is depending on the resistance of the cells (as you wrote), but it is also depending how many current you push through the cells. If the current is low, the heat is low, if the current is high, the heat generated is high. And this is where the C rates come in question, if the C rates are low, current drown from each cell are low, therefore the heat generated is low.
There are 288 cells instead of 196 (or so), so double amount, and the charging power is 70kW (400V 175A) this 175A current is divided into 288 cells so each cell will get 0,6A or 600mA current. In the 40kWh leaf the charge rate is 50kW (400V 125A) this 125A current is divided into 196 cells, which means 0.63A or 630mA current, which is similar for charging. But for discharging (as you drive), if we say 20kWh/100km is a quite faster driving pattern than normal the 40kWh cells are drained with 0.25A or 250mA and the 62kWh batter cells are drained only with 0.17A or 170mA which will result in a much lower heat build up (current was calculated with constant 400V, of course current will increase as battery pack voltage drops, this is only for demo calculations).
Actually the 40kWh battery pack is not heating up, if you drain constantly less power than 12kW. With 62kWh pack this is around 20kW probably, so this means if you drive normally, and drain less power from the battery pack, it can cool down instead of heating up, like the 40kWh pack. If you achieve an average consumption of 15kWh/100km, then you already heated up the 40kWh pack a little bit, however the 62kWh pack could cooled down a little bit with this. Check out on youtube Jame's (Lemon Tea Leaf) video about how he achieved to keep the temp down. He managed to cool down the pack by several °C through not taking more power from the pack as 12-15kW.
How fast it's cooling down, depends on the temperature difference (delta). But the heat build up depends on the current that flows through the cells.