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). We will be contacting Noctua to obtain a more accurate P-Q curve for their fan.
| Condition | Airflow (m3/h) | Airflow (CFM) | 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
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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:
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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.
Since our motors have a max continuous rating of 4kW, which equates to 30A at nominal pack voltage, we will be able to operate at maximum steady state condition without any issues.
Next, we will look at the conditions when climbing hills at higher current draws, and determine the maximum length of a hill that we can climb while staying under the 10 degree rise. For this however, we cannot necessarily assume that the pack will be at ambient before the start of the hill.
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.
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