Problem Statement:

In a polar environment such as Antarctica, electronic equipment must be able to withstand extremely low temperatures and severe winter storms. The performance specifications of electronic devices such as the temperature range of nominal operation must be seriously considered when a system needs to operate reliably in polar climate conditions. Since the availability of energy is a major concern in Antarctica, battery charge controller systems are highly desirable. Currently, commercial battery charge controllers function poorly in extremely cold climates. One solution consists of a mostly analog circuit design coupled with simple digital-logic subcircuits. However, this design is expensive and is not very flexible when extended features need to be added. A less expensive solution offering increased flexibility can be realized when a microcontroller is employed in the design.

Our objective for this project is to design a battery charge controller implementing a microcontroller from the PIC 16Fxxx family (Microchip Technology). The core system is comprised of an analog shunt regulator, which handles current from the wind generators and photovoltaic panels, and a controller that monitors several battery parameters and in turn directs the current from the shunt regulator to charge the battery when necessary. Therefore, the primary focus of the project is to design the analog shunt regulator with appropriate schematics and to develop the software for the PIC microcontroller. The final part of the project consisted of constructing and demonstrating a working engineering model of the system.

The battery charge controller consists of two parts: an analog shunt regulator used for allocating current to charging the battery, heating the system, and shorting the wind and solar generator terminals; and a microcontroller (PIC 16F877A) used to determine how the current is allocated based on measurements made by external subcircuits. The analog shunt regulator was conceived based on the notion of a simple voltage-to-current converter. In this manner, control signals from the microcontroller can dictate where the available current from the wind and solar generators are used. External measurement circuits consist of temperature sensors applied to the system plate and battery, and a voltage reference device that measures the battery voltage.

The previous design used analog circuitry to regulate battery charging. Several of the components were inefficient and needed to be replaced. A large portion of the circuit needed to be eliminated and replaced by the PIC microcontroller to obtain a smaller, expandable, and less expensive product. An efficient method of battery charging is programmed into the PIC rather than relying on the analog circuitry to regulate the voltage. The analog circuit is incapable of changing to accommodate the temperature dependence of the battery float voltage, whereas the PIC has been programmed to respond to the thermal changes. The analog shunt regulator has been rebuilt, resulting in a compact product that is more effective due to the upgraded parts, reduced logical complexity, and use of thermal dissipation technology.

We incorporated IRFP044N power-MOSFETs from International Rectifier to provide heat to the charge controller system in the event that the batteries risk overcharging. The previous design used four power MOSFETs capable of dissipating 25 W each. The new design uses two MOSFETs capable of handling 50 W each . This reduces parts and labor costs. The Bergquist Sil-Pad® 900-S, a non-conductive thermal dissipation pad, has been placed beneath each power MOSFET. This isolates the device cases from ground, while improving heat dissipation . For the measurement devices, we used LM335 temperature sensors for their ability to measure temperatures that occur in arctic environments. In order for the battery voltage measurement to be used by the PIC, an LM4040-10 voltage reference is used to scale down the battery terminal voltage.

The control code is written using the native PIC assembler language and is written to the flash memory on the PIC using the PIC-START PLUS serial programmer. The code consists of one main program , which monitors plate temperature, battery temperature, and battery voltage. The second section of code is a "diversion" subroutine used to divert current in such a way as to short the wind and solar generator terminals. The feedback elements contain the data necessary to control the current flow. The plate temperature is measured first, and if it exceeds 50°C, the program will call the diversion subroutine. The diversion subroutine increments the output control signal (IdivertC) until the maximum output is achieved. In other words, the generator terminals cannot be suddenly shorted; therefore, the diversion subroutine slowly diverts current until the terminals are fully shorted. After this routine, the temperature of the plate and battery are checked, in that order. If either signal is too high, the diversion continues. If both are below the threshold level, the program exits the subroutine. The main program repeats the process, except it next measures the battery temperature and calls a diversion if the temperature exceeds 40°C. The final step in the main program is to measure the battery voltage and increase the current to the power MOSFETs if the terminal voltage exceeds the temperature-dependent float voltage (14.1 V nominal) - a condition in which the batteries risk overcharging. If the measured voltage is less than the float voltage - a state where the battery needs charging, the shunt control signal (IshuntC) is decreased, thus increasing current to the battery. The program repeats in this measuring loop, keeping track of the pertinent system parameters.

IshuntC and IdivertC are the two main digital control signals generated by the PIC. Since these signals are fed back to the inputs of the analog shunt regulator, they must be converted to analog signals. The internal pulse-width modulation capability of the PIC is used to vary the IshuntC signal. This conveniently eliminates the need for an external D/A converter circuit. On the other hand, a simple resistor ladder is implemented for converting IdivertC into an analog signal because it is required that this signal slowly ramp up to its maximum value and slowly ramp back down to zero. Employing pulse-width modulation for this case would be difficult to realize in software.