Batteries

For the batteries considered, the power discharge curves are to be examined before a final selection can be made. Batteries do not consistently provide current throughout its entire run time; as time progresses, current output goes down. Battery discharge curves show the voltage drop that occurs if a constant current is output for a given time; by using these curves, the current that could be drawn for the endurance event (a 2 hour event) can be determined in order to see if it could power the motor at the required input power.

From the selection criteria, the Kung Long 12V 50Ah SLA Battery was chosen. The battery weighs 29.1 lbs, with a capacity of 50Ah. The discharge curve for the battery is shown to the right

Solar Array

From the selection criteria, the KingSolar 120W Flexible Solar Panel was selected for use in the solar array. The panel was 18V and weighs 4.76 lb; at 120W each, four panels were needed, giving a total weight of 19.04 lbs. In order to prevent a stepping-up of voltage from the solar array to the battery bank, it was decided to run the solar array as a 36 V system (as the maximum battery bank voltage is 36 V). This was achieved by connecting two strings of panels connected in parallel, each string consisting of two panels in series.

In order to determine the possible power that could be achieved from the panels, the solar insolation for the days of the event were calculated. Using data given for Dayton, Ohio in late May and late June, the insolation for June 7-11 was interpolated. The results were collected in the graph to the right, shown both with and without the Clearness Index for Dayton, Ohio of 0.53

TECHNICAL POWER SYSTEM DESIGN

Power Systems Schematic

To the right, the power systems schematic is shown, broken up into 4 sections:

Section A:
This section is the solar panel array. Here, there are four solar panels in total, two strings connected in series, each string consisting of 2 panels in parallel. Each panel has an efficiency of approximately 23%, and an area of 0.6254 m^2, according to the manufacturer's specifications. With these specifications, and the values for solar insolation obtained above, the final values for power output would range from 233.3 W - 263.8 W, dependent on the time of the heat. This power would be transported to the charge controller.

Section B:
This is the Maximum Power Point Tracker (charge controller). The maximum power point is the point on a power curve that has the highest product of current and voltage. The MPPT constantly tracks the maximum power point and optimizes the power to best charge the batteries. For the system, the MPPT chosen was the BlueSolar 150V/35A charge controller, with an efficiency of 98%. The MPPT takes the power given by the solar panels and passes them along to the battery bank.

Section C:
This is the battery bank, consisting of 3x 12V batteries in series, resulting in a . The power coming from the solar panels through the MPPT provide charge to the battery bank, while the battery bank discharges approximately 12.5 amps constantly at the bank voltage.

Section D:
This section is the final stage of the power system. The DC/DC converter takes the input from the battery bank and will either boost or buck down the voltage, adjusting the amperage as necessary, and sending the output to the motor control. For the endurance heats, the DC/DC converter will be bucking down the 36V voltage from the batteries to the 24V input of the motor. This can ensure a lower amperage draw from the batteries, in theory prolonging their life; ergo, allowing a higher speed for a longer duration. Zahn Electronics has agreed to provide various electronic components, including the DC/DC converter. The specific DC/DC converter will be decided upon with them when part is to be shipped; in general, their DC/DC converters have a 97% efficiency.