SageBrush2 wrote: ↑
Wed Aug 05, 2020 7:15 am
My inclination would be to measure the battery temp in the early evening and then in the am. If I know the average ambient then I presume a simple calc to tease out the heat transfer coefficient could then be used for measures that reduce ambient temperature. Since I leave our LEAF outside for ventilation and I live in a dry climate, I've been toying with the idea of soaking the ground under the battery after a long drive. I think that would give me a ~ 10C increase in temperature delta. If the battery is 38C and
C = centigrade
K = kelvin
18C = 290K
28C = 300K
38C = 310K
Then the temp delta would increase from 10K to 20K, implying a 50% increase in heat transfer initially.
By the time the battery reaches 28C the cooled ground would still have a 10K temp gradient, implying an average gradient of 15K instead of 5K with no cooling.
Sound ballpark ? Is the physics correct ?
I'll go back and find the calcs, but they were strickly theoretical using a significant overall delta T ( requiring plate refrigeration), with estimated film coefficients of still air at the surface of the cold plate and the suface of the battery case itself. The "resistance" to heat transfer is so great at these thin films that I ignored all other heat transfer limitations a negligable - including the air itself, and the inside battery case to LI cells (likely not negligable - but would make it worse). I ignored the radiant heat transfer - that would need to be checked actually as it could be significant?? With just those simplified assumptions, the actual heat transfered was very low - definitely not worth refrigerating a garage floor as an example.
I do like your idea of a cold wet ground (in your area, not mine - humidity at 90%+) that is outside where some air movement could up the limiting transfer coefficients of the battery "skin" itself (thin film resistance is much lower with increased air velocity), However, between the protector plate and battery case will likely be an extreme limitation (again stagnant thin films of air) unless you put some heat transfer stuff between the two or temporarily remove the skid plate. Since it would be quite simple to moniter with a infared heat gun, why not give it a try? Back calculating at different temperatures would likely be more accurate than any "theoretical" calcs. At the least you would have a good idea of the actual cooling capability that the "cold ground" can impart to the hot battery. An accurate calculation of actual overall heat transfer coefficients is quite difficult due to so many variables.