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.
Several methods for cooling the pack have been discussed and some were tested.
| Method | Choice order (by performance) | Result | Notes |
|---|---|---|---|
| Liquid Cooling | 1 or 2 | Would be great | Too complicated, too much risk, would take too much time. If done properly, it could work very well. |
| Phase Change Interstitial Material | 1 or 2 | Would work amazing | Unable to acquire / make, would also absorb thermal runaway heat and prevent propagation. Would also mean that we have a fixed amount of thermal mass to heat up, as the pack airflow would be terrible. We can seal the pack completrly with this option though. |
| Forced air cooling | 3 | Works well, must have approriately specced fan | If we get enough air through the modules, it can cool it properly. The fans will require some power when cooling, probably around 20-30W or a little more. We tested the prototype module with a standard 120mm case fan and a Noctua IPPC 3000 fan. |
| Conductive cooling to catamaran | 4 | Ineffective | The catamaran of the car is carbon fiber, and graphite has amazing planar thermal conductivity, but when you will it with epoxy it is terrible. From our testing, it is more conductive through the Z-axis then across the plane |
We selected Forced Air Cooling for our pack.
With this selection, the next challenge is to figure out how much air we need to move through the pack, and how we will move that air through.
We will not be doing simulation of any of our battery pack cooling due to lack of experience and lack of time, but will be doing hand calcs and basic Python modelling.
How to Move Air
The obvious answer here is to use a fan, but which fan to use and understanding the specifications is a whole other beast.
Let's start with the manufacturer specs.
A fan manufacturer (at least in the PC case fan world) will generally give 2 specifications for their fans:
- (Maximum) Static Pressure (usually in mmH2O)
- When amount of pressure generated when there is no airflow
- Ex - fan sitting horizontally on a table with the air being pushed in to the table
- (Maximum) Airflow (usually in CFM - cubic feet per minute, or in m3/h - meters cubed per hour)
- When there is no restriction to the airflow, this is the amount of air that the fan will move
- Ex - fan sitting on a table with nothing in front and nothing behind
Neither of these specifications are useful on their own to tell us how much air we will be moving through the pack, as we will have both airflow and pressure. So, we will be somewhere in the middle of the Static Pressure - Airlfow (P-Q) curve of the fan. This curve is different for every fan, and changes based on blade design, frame design, and many other factors. Reputable fan manufacturers will either publish their P-Q curves or will be able to provide one upon request. If a manufacturer cannot supply a P-Q curve, I would not buy from them as it may be a sign that they do not completely understand their fans (apparently some PC case fan manufacturers are in this boat). For more info on the P-Q curves, see the following links:
Resources - P-Q Curves of Fans
P-Q Curves explanation and measurement principles: http://www.arx-group.com/pq.html
P-Q Curves of some Noctua fans: https://noctua.at/en/nf-a12x25-performance-comparison-to-nf-f12-and-nf-s12a
Google spreadsheet comparing P-Q curves of various computer fans (made by the guy in the following youtube video):
YouTube Video explaining some of the P-Q curves of fans, and manufacturers:
3rd party testing of some P-Q curves of many fans - NF-F12 iPPC 3000: http://www.coolingtechnique.com/recensioni/air-cooling/ventole/1532-noctua-ippc-nf-f12-e-nf-a14-3000rpm-pwm-tabelle-prestazionali-analisi-spettrometrica-e-video-in-hd.html?start=1
Fans in Series and Parallel
Fans in parallel (side by side) will double the airflow at a given pressure.
Fans in series (stacked) will double the pressure at a given airflow.
https://www.sodeca.com/Content/img/en/InformacioTecnica_01_EN.pdf
We will have multiple fans in parallel to cover to whole width of the battery box, but might have more if required.
Pressure in our application
We are interested in finding the amount of pressure that pulling air through our battery modules adds to the system. Once this is figured out, then we can figure out the airflow required with some heat calculations, and then move on to choosing a proper fan for the battery box.
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.
So this is the static pressure with 1 module. How does adding more modules (5 modules stacked per fan in our application, with 1 fan pulling air through 5 modules), affect the static pressure? 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.
Anything in the airflow path is essentially an impedance to said airflow, and can be thought of as a resistor in an electrical circuit, with the current being the airflow.
Required Airflow
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
Might be useful, sometime and thought I would throw a link here - an interesting way to measure airflow in a closed pipe or duct:
Thermal Flow Measuring Principle: http://www.coolingtechnique.com/70-guide/metodologie-di-test/399-galleria-del-vento.html?start=1