The specific heat capacity is a measure of how much energy is necessary to raise the temperature of a system a given amount. A system with a higher specific heat capacity is said to have a higher thermal mass, meaning that with the same amount of heat, it will have a lower temperature rise.
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So, the thermal mass of the pack being 35.5kJ/deg C lines up with the testing results pretty much perfectly.
With this heat capacity of the pack, we are able to calculate some of the transient effects of going up hills, etc. Our strategy lead (Clarke) says that while going up a hill, a 10kW power draw for 5 minutes would be a conservative estimate for the length of a hill.
At 10kW, we are drawing 76A at nominal voltage and producing 329.2W of heat.
1W = 1J/s
329.2J/s * 5min * 60sec/min = 98.76kJ
This amount of energy will give a temperature rise of:
98.76kJ / 35.45kJ/degC = 2.79decC
Keep in mind, this is a very reasonable estimatewithout active cooling. With this result, we are able to say that any hill that we go up will raise the pack temperature by a maximum of 3 degrees Celsius. So during cruising (before the hill) we require the pack to be under 42 degrees Celsius in order to keep the max temp under 45 degrees Celsius.
Given our conservative estimate of a cruising current of 20A which equates to 22.8W of heat. Looking at the measured results of the rate of temperature rise with the fan running (below), we were able to remove 20W of heat anywhere above roughly 7 degrees above ambient (look at where the curve flattens. So, we can essentially keep the pack at a roughly constant temperature of max 7 degrees above ambient. This allows the 3 degree rise of a hill while still maintaining temp under 45 degrees in a 35 degree ambient environment. Also, keep in mind that this 20W heat removal is assuming worst case module result scaling to pack environment.
The above discussion has concluded that we have enough cooling power to keep the pack at a maximum of 45 degrees given our worst case conservative estimates. However, the cooling and temperature distribution within the pack is NOT uniform (see below). We need to ensure as much uniformity as possible in our pack in order to be able to operate in more comfortable regions, and not having to worry about hitting a temperature limit - but also keep in mind that the 45 degrees limit will just disable regen and solar for a short time until we can cool the pack, we still have 15 degrees more headroom before hitting the discharge max temp.
This discussion is continued on the Battery Pack Temperature Distribution page.