MOBILE POWER STATION

ABSTRACTThe Mobile Power Station (MPS) is a device that manages and stores solar energy in a lithium ion (Li-ion) battery pack while tracking the maximum power point (MPPT) of the connected solar array. The MPS utilizes a DC buck converter design to control the impedance of the Li-ion battery pack and external load, allowing maximum power transfer from source to load. Our client, PowerFilm Inc., expressed a need for the MPS in military and consumer markets.

PROBLEM STATEMENT & SOLUTIONPhotovoltaic (PV) solar arrays can be used to power mobile devices; however, solar array output power is non-constant and most mobile device chargers require a constant voltage and current source.

Team dec1013 purposed building a MPS that would supply a constant voltage and current to mobile devices, while tracking the maximum power point of a PV solar array.

MAIN PROJECT REQUIREMENTSFR-01 Solar panel input – 1.2A @ 5.4V (20W) Amorphous Silicon PanelFR-02 100W minimum Lithium-Ion battery capacityFR-03 15V DC input (with AC-DC Adapter)FR-04 12V DC outputFR-05 Operation in temperature range of -20°C and 60°CFR-07 Charge Balancing Circuitry to keep Li-Ion Batteries balancedFR-08 Achieve 80% or greater efficiencyNFR-01 MPS should have a weight of less than 5 poundsNFR-02 MPS should be manufactured for a cost of under $500 per unitNFR-03 MPS should easily fit inside a military backpack

MPPT & LI-ION CHARGING CONCEPTLi-ion battery charging cycle includes thee phases: trickle, constant current, and constant voltage. The charging cycle must be strictly followed to prevent battery pack overcharge and fire.

Measuring the output voltage and current of the buck converter, the MPS changes the duty cycle of the buck converter to follow the Li-ion charging cycle. When the MPS enters the constant current charging phase, it will also track the MPP, providing maximum power transfer until the battery pack voltage reaches 12.6V. The MPS will continue to hold the output voltage at 12.6V during the constant voltage phase until the battery pack is fully charged.

PROJECT DESIGNThe basis of the MPS is a buck converter, which controls the voltage to the battery pack, and thus the current flowing to pack. The buck convertor is controlled by varying the duty cycle of a PWM signal generated and controlled by the MSP430. Programmed in C code, the MSP430 follows a charging algorithm based on feedback voltages and currents. UART communication to a PC provides diagnostic information, and the charge controller balances the Li-ion battery pack. Li-ion Batteries

(3 series cells x 8Ah)

Buck Converter

Microprocessor (MSP430)

Solar Panel Source

5VDC (USB)12VDC120VAC

Serial Communication (UART) to PC

Voltage and Current Sense

PWM Control Signal

Charge Control(BQ20Z70)

Feedback Voltage/Current

Sense

Possible Loads

CONCLUSIONOur team designed and created the MPS, which effectively tracks the MPP of a connected solar array while charging and balancing a Li-ion battery pack. The MPS will provide an efficient, lightweight, and affordable source of energy for users in remote locations, with the adaptability of charging from a non-solar source.

SCHEDULE & BUDGET

18%

21%12%

38%

11% ResearchDesignImplementationTestingDocumentation

Costs Per Unit (when ordered in units of 100) MPS components $ 21.25 Battery protection circuitry $ 9.55 Battery (3x 3.7V 8Ah Li-ion) $ 97.25 PCB $ 5.00 AC/DC input & output adapters $ 12.75 Total $ 145.80

TESTING & FABRICATIONThe MPS was rapidly developed and tested on a breadboard for functionality. Next, a PCB was designed, populated, and tested with great improvements in voltage and current sensing. The MPS was first tested with a constant power supply, then a solar array.

Team:Brad Jensen (EE)Will Klema (EE)

Nate Schares (EE)

Advisor:Dr. Ayman Fayed

Client:PowerFilm Inc.

Project ID:dec1013

GUIA GUI was developed to display and plot critical data (via USB) from the MPS including voltage, current, and duty cycle.