This week, the group completed test 4.1.4 - Crossflow Ventilation Comparison Test and made a slight correction to the battery monitor code.
Test 4.1.4 - Crossflow Ventilation Comparison Test
This test compares the cooling capabilities of the system with optimized crossflow to unoptimized crossflow. The procedure for the test is given in the Acceptance Test Procedure under 4.1.4. The data gathered in the test is plotted in the line graph below.
The results of the test are given in the table below. The average of the reference vehicle, optimized test vehicle, and unoptimized test vehicle temperatures are taken. The amount of cooling in °F provided by each cooling method is calculated by subtracting each average temperature from the reference vehicle average. To compare the optimized cooling to the unoptimized difference, the percentage difference between the methods is calculated using the formula for percentage difference. This shows a difference of 40% between the two cooling methods, which satisfies the High-Level engineering requirement stating that optimized crossflow should provide at least 30% more cooling than unoptimized crossflow.
Get Battery Mode Program Modification
Since it was observed that the MCU was not shutting off the load when the battery went below 12 V, the group set about troubleshooting the Get Battery Mode function. The group brought a DC power supply out to the car and connected it to the battery voltage divider circuit.
With the power supply set to 12.00 V, the voltage read by the ADC was higher than anticipated. The offset value for the input voltage calculation below was changed to calibrate the ADC reading such that the input voltage maxed out at the lower threshold of 2.641 V. Overshooting the calibration ensures that the load will not be active when the voltage of the battery is 12.00 V or below. To confirm the effectiveness of the calibration, a reading was taken from the MCU.
Vin = Vadc + Vadc*(-0.149 + 0.121*Vadc + -0.019*Vadc**2)
To make sure that the load shuts off when the voltage of the battery makes the transition from above 12.00 V to below 12.00 V, the group raised the voltage of the power supply to 12.30 V and slowly lowered the voltage until the MCU cut the load. Then, the following printout was obtained from the MCU. This shows that the MCU immediately switches the battery mode to zero when the input voltage falls to 2.641 V or lower. This switch occurred just before the power supply fell to 12.00 V.
The group also wanted to ensure that when the system is off, the MCU does not activate the load until the voltage of the battery rises to 12.15 V. Starting below 12.00 V, the group slowly increased the power supply voltage until it read 12.15 V and took a screenshot of the MCU printout. The printout shows that the ADC voltage maxes out at 12.15 V as desired. This set the battery mode to 1, which turned on the load.
Further testing will be conducted later in the week to verify the operation of the battery modes within the full system.
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