Due to the internal resistance of battery cells, they produce heat when a current is applied to them. We must dissipate this heat somewhere to stop the cells from heating up.
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We measured the airflow of our Noctua IPPC fan when pulling air through 1 or our prototype modules (see this page), and have an airflow of 15.52m3/h (or 26.3cfm). Looking at the fan curve for the Noctua NF-F12 IPPC 3000 fans mentioned above from Cooling Technique, we see a pressure of about 5.4mmH2O. We will be contacting Noctua to obtain a more accurate P-Q curve for their fan.
| Condition | Airflow (m3/h) | Airflow (CFM) |
|---|
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| Static Pressure (from Cooling Technique Testing) | Static Pressure (from Noctua P-Q Curve) | |||
|---|---|---|---|---|
| Just fan duct | 26.97 | 15.87 | 5.55 | |
| 1 Module and fan duct | 15.52 | 9.13 | 5.6 |
Converted to CFM using this online conversion tool: https://www.convertunits.com/from/cubic+m/hr/to/cfm
So it seems as if there is not much added static pressure by introducing a module into the airflow path. There was more room for the air to move over the top of the modules than there will be in the final pack, but this gives us a good baseline. We should test again, but forcing the air through the cells with extra airflow guides.
I presume that by forcing the air through more modules, the static pressure that must be overcome will be added. The easiest way to figure this out would be to test it with some more modules in series, but some calculations may also be possible.
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Now we need to figure out how much air we need to move through the battery box. We will be using this page as a reference for these calculations: http://www.arx-group.com/airflow.html
The first step is to figure out how much heat is produced in the battery system. See this page for the estimations of the amount of heat produced. Our total pack resistance is around 57mOhms.
Temperature Limit of our system - air output temp: 45 degrees Celsius (Tc)
Surrounding Air Temperature - air input temp (summer, middle of US): 35 degrees Celsius (Tamb)
This page goes through the rest of the calculations required for the flow rate required. The equations below are copied from the page: http://www.arx-group.com/airflow1.html
H = Cp × M × ∆T
| Variable | Description | Value (Units) |
|---|---|---|
| H | Least amount of heat removed | (W) |
| Cp | Specific heat capacity of the air | 1005 (J/Kg℃) |
| M | Mass of the air | (Kg) |
| ∆T | Temperature difference | Tc - Tamb (℃) |
M = Q x ρ
| Variable | Description | Value |
|---|---|---|
| M | Mass of the air | |
| Q | Flow rate of the air | |
| ρ | Density of the air | ρ = 1.18 Kg/m3 |
Rearranging the equations above, we get the following:
Q = H / (Cp × ρ × ∆T)
Q = H / (1005 J/Kg℃ x 1.18 Kg/m3 x 10℃) Units: m3/s
With 15.52 m3/h of airflow (tested value with 1 fan, 1 module), we should be able to remove 50W of heat.
With 4 fans (planned number in the battery pack) generating 60 m3/h of airflow, we should be able to remove around 190W of heat.
This amount of heat removal will allow around 50A of continuous to be drawn from the pack, and keep the temperature rise to under 10 degrees celcius.
Now we need to ensure that the fans we spec will be able to produce the required amount of airflow with the amount of static pressure they must overcome.
Might be useful, sometime and thought I would throw a link here - an interesting way to measure airflow in a closed pipe or duct:
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