Scaling Module Results to Pack

Based on our results from the battery module testing, we must figure out how we can scale these results to the pack level.

With 1 Noctua fan moving air through 1 prototype module at 15.52m³/h, we were able to remove 20W of heat (see this page).

This is with ambient temperature air entering the pack. When this air exits the pack, it will be at a higher temperature than when it exited the pack. This 'exit temperature' can be calculated using the specific heat capacity of air.

Temp Rise = (Heat Production / (Specific Heat Capacity * Mass Flow**))

**Mass Flow is not a technical term (or if it is, I'm probably not using it properly) - in this case, I used it to describe the value of flow rate * density.

Temp Rise = 20J/s * 3600s/h / (1005 J/Kg℃ * 15.52m³/h * 1.18 Kg/m3) = 3.912℃

Treating Each Module as a Separate System

From this, we can determine that there is approximately a 4℃ change in temperature from the inlet to the outlet, from before the module to after the module. This seems a little high, as over the 5 modules, we will see a 20 degree rise in the air temperature (if the rate of heat transfer to the air remained constant for all the modules).

Some more testing will be done to verify the 4 degree rise.

This approach seems more intuitive than treating the whole pack as a giant module, and saying that we can only remove 20W of heat from the line of 5 modules. When 5 modules are lined up, and we keep the same amount of air moving through them, the air should heat up more than through a single module as it spends 5x more time inside the pack.

We need to keep the exit temperature of the air at an absolute maximum of 45 degrees. At 45 degrees, this would mean that the cells at the rear of the pack are at a minimum temperature of 45 degrees, and will not get cooled because there is no temperature difference between them and the air (which is necessary for heat transfer). This is not a good situation to be in, as the cells at the rear will keep heating up as the pack gets used, and will reach over 45 degrees. So, we need to keep the exit air lower than 45 degrees. If we arbitrarily pick 40 degrees as the exit temperature, and pick our max ambient temp as 35 degrees, then we require a 5 degree rise over the entire pack. This would mean 1 degree per module, and thus we would require 4 times the amount of airflow through the pack. Pushing 4 times as much air through the pack will require more power for the fans, and I believe it to be unnecessary as we can take advantage of the termal mass of the pack to cover the transients.

A few things to keep in mind

• We will not be able to remove the same amount of heat from the last module as we will the first module.
• More airflow uses more power (but if we need it, its better than overheating)
• We can take advantage of the thermal mass of the pack