Nicholas V. Riedel and Professor Perry Carter, School of Technology
The BYU Electric Vehicle Competition Team (EVCT) elected to use Lithium-iron phosphate batteries as the power source for their streamlined electric race car. This car will be used to attempt a land speed record for electric vehicles under 1800 lbs at the Bonneville Salt Flats in August 2008. These batteries were chosen because they were light-weight and relatively durable. However, there was some concern about using them in place of a more durable battery, like lead acid, because of the extreme temperatures and high electrical loads which the batteries would experience during a run.
The concern about the battery’s operating conditions was influenced by recent incidents involving lithium-based laptop batteries. In most cases, those laptop batteries had insufficient cooling and fell into thermal runaway and exploded as a result. Thermal runaway happens when excessive heat generated by a battery’s internal resistance causes undesirable chemical reactions within the battery which often leads to fire or explosion. While most lithium-based battery packs made today have fuses or built-in pressure relief valves to help prevent thermal runaway, the risk of them overheating is present when packs containing these batteries are modified and the safety circuits are removed in favor of more power. This is the case in the EVCT car; there is a need for more power than the battery pack’s safety circuit will allow, so it has been removed.
This research centered on finding a passive (no energy input) method to cool the EVCT’s car’s batteries. Some benchmarking research was done to find out what methods other researchers had developed. All of the methods that seemed promising were tested and are listed below:
1. Natural Convection; 2. Aluminum Foam Heat Sink; 3. Paraffin Phase Change;
4. Aluminum Foam with Paraffin; 5. Forced Convection; 6. Evaporative Jacket
Experiments were conducted using data acquisition equipment and software to measure the temperatures of the batteries as well as battery voltage, and power output. The batteries were connected to a modified water heater to simulate the actual loading conditions expected from an electric car. After the power, voltage and temperature versus time were graphed for each cooling method, a table was made to compare temperature rises, energy output, maximum power, and average current and voltage. This table was used to determine which cooling method could control battery temperatures the best.
The results showed that the best overall method to keep battery temperatures low was the evaporative jacket method. This method employs blowing air over a water-soaked, cloth tube which is surrounding a battery (see Figure 1). The effect is similar to a person’s perspiring and cooling off when a breeze allows the sweat to evaporate off of the skin. The idea was put forth during an interview with Dr. Timothy Knowles, an expert in the field of passive heat transfer. He mentioned that the heat transfer coefficient, i.e., the ability to transfer heat, of evaporating water was much higher than that of forced convection (blowing air) or melting paraffin. He suggested that the batteries would be kept cooler than any of the other methods just mentioned as long as there was a good amount of flowing, low-humid air (which there happens to be plenty of in the Utah desert, where the car will be raced).
There was an interesting anomaly relating the average battery temperatures with the amount of energy output by the batteries. While the evaporative jacket method kept the batteries the coolest, they also kept the batteries below their optimum temperature range to the point that they could not expel as much electrical energy as, say, the paraffin methods allowed which kept the temperatures near that of the melting point of the wax. This anomaly was factored into the recommendation for which method should be adopted for use in the EVCT car.
Table 1 shows the results of testing each battery cooling method with separate categories and point values to rate various important factors.
Chief of these factors was the ability to keep the batteries cool; the evaporative jacket was the most successful in this category. Cost and weight were also factored as highly important based on the budget and physical constraints facing the EVCT. The aluminum foam scored low in cost because of the high manufacturing costs associated with its production, and both of the methods involving paraffin scored low in the weight category because using it made each battery pack about twice as heavy. Although the paraffin and aluminum foam methods kept the temperatures elevated, which allowed the battery to expel more energy, their cost and weight together thwarted the possibility of their application in the car. The only drawbacks to the recommended solution, the evaporative tube, are the dependency it has on vehicle speed and the initial cost for materials. That cost is somewhat made up for when comparing against the option of using the no cooling method; the evaporative cloth tube method also ensures almost no risk of overheating and damaging these batteries which are quite costly to replace.