Smart Solar Battery Charger by tyndale

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									Smart Solar Battery Charger
       Design Review
         Christine Placek
          Philip Gonski

       TA: Bob Schoonover
            Project #4

             ECE 445
         February 22, 2007
I. Introduction

     1. Title: Smart Solar Battery Charger
          For our project we decided upon building a smart solar battery charger.
          Rechargeable battery usage has dramatically increased over the last few years due
          to devices such as cell phones, PDAs, and mp3 players. Consumers must
          constantly be charging these devices at home to obtain just a few hours of working
          functionality. We mean to design a solar powered system that would enable the
          consumer to charge up virtually all of his small electronic equipment via solar
          power. This project has advantages for both the consumer and the environment.
          We chose to undertake this project because we saw a need in the market for an
          alternative energy device that can charge different types of batteries. Also, this
          project will incorporate both of our specialties in Electrical Engineering: power
          and circuitry.

     2. Objectives
          Our goal is to use solar power to charge nickel and lithium-ion batteries. The user
          will input the battery type. Using the output of the solar panel and the user input,
          a microprocessor will determine the correct voltage and current for charging that
          specific battery. The charging status will be displayed for the user. A temperature
          sensor will be used to stop charging if the battery gets too hot.

           Eliminates need for multiple chargers
           Provides protection of battery from environment
           Efficient use of energy
              o Solar-powered
              o Automatic shut-off
           Convenient – can be used wherever adequate light is present

           Can charge any nickel or lithium-ion battery
           Solar-powered
           Displays charging status
           Temperature monitoring

II. Design
      1. Block Diagram

                       Display               Solar Panel

  User Input       Microprocessor           Charging Unit              Battery


      2. Module Descriptions:
               1. User Input
                  The operator of the device will choose the chemistry of the battery he
                  wishes to charge. This will determine the correct voltage that his device
                  needs as well as any necessary current limits that need to be supplied to
                  compensate for the resistance of the battery.
               2. Microprocessor
                  The microprocessor is the brain of our device. It will manage the user
                  input and also respond to the feedback loop of the sensing unit. Thus, it
                  will be able to decide upon these variables which signals will be sent to the
                  charging unit, as well the LED displays. Most importantly, it will
                  determine the voltage and current requirements that will have to be met for
                  each different type of chemistry that the user can input. In this sense, it will
                  control all of the functionalities of the device.
               3. Display
                  This unit will display the current status of the battery and will inform the
                  user whether or not it is done charging. Also, we may adapt its
                  functionalities to incorporate a display for the user to see which battery is
                  currently selected for charging.
               4. Charging unit

                  The charging unit will react to the input that it receives from the
                  microprocessor. A signal will be sent telling the charging unit exactly what
                  is required for the charging of the battery.
               5. Solar Panel
                  In our design, the solar panels will function as a power supply to our
                  circuit. It will receive light from the sun and convert this to energy and
                  thus voltage. This voltage will then be used to operate the ICs as well as be
                  the source of the current and voltage that the charging unit will receive.
               6. Sensors
                  Temperature will be monitored in our device via the sensors. This is then
                  made into a feedback loop from the batteries to the microprocessor in order
                  to make sure the circuit runs in an efficient manner within allowed limits.
                  We may also add polarity checking to make sure the battery is connected
                  the right way.
               7. Battery
                  The battery in our device is the end byproduct of all of our other devices. It
                  will be a rechargeable nickel-cadmium, nickel metal hydride, or lithium-
                  ion battery.

     3. Performance Requirements
                   Charge any nickel or lithium-ion battery from 0-4V
                   Operates under normal sunlight conditions
                   Operates under other light conditions (e.g. indoors), perhaps with limited

III. Verification
     1. Testing Procedures

              Charging unit output
                  We will hook up a function generator to simulate the sunlight and test the
                  charging unit (the MAX1501). We will monitor the output of the charging
                  unit with no battery connected to find the maximum no load voltages and
                  currents. After these maximum values are found, we will simulate by
                  using various resistances connected at the 'battery terminals' to simulate
                  the affect of having the battery in place. The voltage and the current going
                  into the circuitry will be varied and measurements will be taken at the
                  BATT output of the MAX1501 in order to determine whether or not the
                  correct values are being outputted. This will also help us to determine
                  whether or not multiple solar cells will be needed in order to achieve
                  functional charging since our cell outputs a typically low current of
                  100mA. Thus, this will help us to determine exactly what size and values
                  of solar cells we need in order to achieve nominal output parameters of the

        Solar panel output
            We will measure the current/voltage output of the solar panel for various
            light intensities, using different types of artificial light, as well as sunlight
            at various hours of the day using our oscilloscope. By doing this, we can
            obtain average values for the current and voltage that our solar panel will
            output to the chips. Thus, we will be able to see how the chip itself will
            respond to this variance and if we need a different rating of solar panel.
            We also will find a way to measure the light intensity, either with a light-
            intensity meter or an electronic component that responds to light. Snell's
            Law will also be applied by varying the angle of incidence between the
            panel and the sun. Thus, we will compute the critical angle at which light
            is reflected instead of absorbed by the solar cells.

        Charge cycle
            We will monitor a battery throughout a charge cycle, making sure the
            voltage/current is always correct and that charging is within our desired
            parameters. For Li+ and Ni batteries we must ensure that the current is
            around the 500mA our chip will be programmed to output. The voltage
            being outputted for Li+ batteries mode should be around 4.2 when it is
            charged, while for Ni battery mode, it should be around 4.95 when

        Temperature Cycle
            We will connect batteries of various temperatures and make sure the
            charging unit outputs the correct current in response to the battery’s
            resistance change. The LM35 Temperature sensor will be used to ensure
            that the temperature does not reach a critical point where the circuit would
            be damaged. Thus, our complete circuit will be subjected to different
            temperatures to ensure that the current and voltage being output by our
            MAX1501 chip is within charging parameters of the batteries.

        Special battery cases
            We will connect batteries at various states of being charged, and dead
            batteries, to make sure the charger responds correctly and is still able to
            reach desired maximum charging efficiencies.

2. Tolerance Analysis
     The module that most affects the operation of the device is the solar panel. If the
     solar panel does not work, the device will not be able to turn on or charge batteries
     at all. The device should be able to work over a large range of light levels, so we
     will test to find nominal light conditions for effective charging, and verify that it
     charges under a variety of light conditions. This will be done by monitoring the
     charger's current and voltage output under different measured light intensities.
     Angle will also be of critical importance since via Snell's Law there will be an
     angle between the cells and the sun at which no power can be outputted.

IV. Schematics/Design Details

    1. Charging Unit

                                                  INP (1)                 (12) BATT
                                                 IN (2)
                                           1uF                                          10uF
                                                                          (14) SELV
                                                                               (5) VL
                                                 RLED (15)
                                                 GLED (16)
                                                                           (6) TMAX
                                                                           (7) FULLI
                                                 ACOK (11)
                                                                           (8) TEMP
                      Micro                      MODE (9)

                                                 CHGEN (10)                 (4) SETI

                                                             GND (3) GND (13)

         To perform our charging, we have selected a chip, the MAX1501. The MAX
         1501 is an intelligent, constant-current, constant-voltage, temperature-regulated
         battery charger. It can charge Li, NiMH, and NiCd batteries all within the
         regulated charging parameters specified by the different chemistries of the battery.
         It accepts a 4.5-13Volt supply with typical values being around 5v. This design
         constraint works well with our design since we will get approximately 4.5-7V out
         of our solar cells. This cell predominately works by using adjustable fast
         charging methods, which achieves lowest time to charge, but also decreases the
         charge level of the battery. Most references state that fast-charging only charges
         batteries up to 70% of capacity. However, this is not that important should the
         user desire to use his device while it is also charging. The cell also uses topping
         off techniques which increases the battery life-span. For clarity, the pins are
         listed below with descriptions in order to fully understand the inner workings of
         the chip and our design.

A. Pin descriptions of 1501
      INP (pin1)
       High input charger input. This will accept the input from the solar cells
       in terms of voltage and current. The voltage level must be between 4.5-
       13Volts in order to power the logic inside the chip. The input current that
       the chip needs is around 4.5mA which is likewise received from the solar
      IN (pin2)
       Low current charger input. This is connected to INP and thus the solar
       cells and is used to power the internal LDO and references. IN also will
       draw current when device is in shutdown
      GND (pin3,13)
      SETI (pin4)
       The charging voltage of the chemistry of the battery is regulated via
       modifying the resistance Rseti which comes out of the SETI pin(4). This
       is governed via the equation :
       Thus, if we choose to charge the battery around 500mA or 0.5C according
       to some references we would thus need a resistance value of around 2.8k.
       This is listed as the optimal current that can be used to charge batteries,
       since the lower charge current reduces the time at which the cell will
       reside at 4.20volts. This was also chosen since our solar cells input very
       low current into the chip. The lower charge current reduces the time in
       which the cell resides at 4.20V By using this low current, we will also
       lose less power to dissipation. Perhaps the greatest reason for choosing
       this low charging current would be the fact that solar cells do not output
       adequate current that would enable us to charge at any faster rate than
      VL (pin5) and SELV (pin14)
       Together these pins operate to select the battery voltage that will be used.
       They function together according to the following table:
                                             SELV Connection
             Charge Mode                  GND                     VL
                    Li                    4.1V                   4.2V
              NiMH/NiCd                   4.5V                  4.95V
       From this chart, the user is given different options of voltages that can be
       used to charge up the specified batteries. For our design, we will be using
       the option of connecting the SELV pin to the VL pin. From research,
       most Li batteries are charged to 4.2 volts with a tolerance of +/- .05V/cell.

    Had we connected this pin to GND, the charge would have been reduced
    by 10% total, yet the total lifespan of the battery would increase. This is
    mostly related to the fact that most of our references citing using around
    4.2 volts to charge up lithium ion batteries. Likewise, this should also
    help decrease the total charging time by allowing the battery to reach
    charged levels quicker. Full charge is attained after the voltage threshold
    has been reached and the current has dropped to 3% of the rated current or
    has leveled off. Refer to appendix for charge cycle graph.
   TMAX (pin6)
    This is the time limit placed on the chip to ensure maximum safety from
    overcharging the battery, as well as improving efficiency. If TMAX is
    connected to ground, the charge time is 3hrs, if floating, 4.5hrs, and if
    connected to VL, 6hrs. For our design, we will leave TMAX floating to
    achieve a median value of charging to time to leave plenty of room for
    error in our design.
   FULLI (pin7)
    This selects the value of the top-off current for the battery charging as a
    percentage of fast-charge current. In our design we will design for 30%
    top-off of maximum current since this will quicken the time. To achieve
    this value, the connection is left floating.
   TEMP (pin8)
    The operation of this pin functions to regulate the maximum die-
    temperature regulation point for thermal control. We chose to leave this
    connection floating, which, according to the data sheets, will set the
    regulation to 115C which should be more than enough room for our
    design to operate in standard conditions.
   MODE (pin9), CHGEN (pin10)
    MODE and CHGEN are used to select which type of battery is being
    charged. The microprocessor will send the correct signals to these pins
    depending on what type of battery the user has selected.
                  MODE                 (CHGEN)’            (MODE)’
                 Li Charge                0                   0
                NiMH/NiCD                 0                   1
                  Disable                    1                  0
              No battery mode                1                  1

   ACOK (pin11)
    ACOK outputs low if the voltage on IN is between 4.2 and 6.25V and
    greater than the battery voltage by at least 0.1V. Thus, ACOK checks to
    makes sure the input voltage is within an acceptable range to charge the
    battery. ACOK is connected to the microprocessor, which will give an

              error message if the input voltage (i.e., the solar panel output) is not high
              enough. The spec requires that ACOK has a 100k pullup resistor.
             BATT (pin12)
              Output of chip to be connected to battery being charged. Required by
              spec to have a 10uF capacitor for output voltage stability. Outputs 4.148-
              4.252V for Li+ battery, 4.85-5.05V for Ni. Charge current varies, but is
              roughly 525mA for both battery types during fast charging mode.
             RLED (pin15)
              RLED is connected to the anode of an LED whose cathode is pulled high.
              When the battery is charging, RLED goes low, turning on the LED.
              RLED outputs 10mA, taken into consideration in the calculations for
              power/current needed. RLED may be connected to the microprocessor if
              we decide to indicate charging status in some way besides an LED.
             GLED (pin16)
              Same as RLED, but turns LED on when battery is done charging.
              Current not taken into consideration in calculations because GLED will
              only sink current when no charge current is being outputted. Thus, chip
              output current when battery is done charging is trivial.
     B.   Calculations for 1501 (values from spec)

                                         Battery                     RLED needs
               1501 needs
                                     charging needs          (battery charging indicator)
               4.5-6.25V              4.2V for Li+
               (assume 5)             4.95V for Ni
                  4.6mA                  525mA                           10mA
                 0.023W                   2.6W                           0.01W
              Total power usage: 2.633W
              Charger efficiency: 2.1W/2.633W = 80%

2. Solar Panel
     For our design we chose to use OEM 'Flexible Solar Cell' model number MPT6-
     150. This appeared to the cheapest model we can find that produce the adequate
     voltage we need. It has the properties:
              Voltage: 6.0 V
              Current: 100mA
              Voltage (oc): 8.0 V
              Current (sc): 120mA
              Thickness: 0.2mm (8 mil)
              Total Size: 114mm x 150mm (4.5 x 5.9 inches)

                Aperture Size: 100mm x 150mm (3.9 x 5.9 inches)
                Weight: 4.6g (0.1oz)

                Thus, the 8V maximum voltage should be more than enough to power a
                chip that requires greater than 4.5V to operate its internal logic. Also, the
                solar cell is very small for comparable outputs, being only 4.5” by 5.9”

                Assuming outdoor average solar flux is 1000W/m2, we can calculate an
                estimated output voltage that our solar cell will provide since it is listed as

                Thus we have 1000W/m2 x (.1m x .150) x .05= .75W in conditions of
                best operating sunlight

                Assuming indoor average solar flux would be approximately 10% of the
                maximum, we can thus calculate similar data assuming a solar flux of

                Thus we have 100W/m2 x (.1m x .150) x .05= .075W indoors

3. Temperature Sensor


              0.45V        -
  LM35                         MAX9019           To
                      R2        - Vsolar

         Nickel and lithium batteries are sensitive to temperature and should not be
         charged if their temperature is over 45°C. To safeguard against this, we will use
         a temperature sensor (placed next to the battery) that will shut off the charger if
         the temperature is too high. We are using the LM35 temperature sensor which
         outputs a voltage proportional to temperature. Vout = 0V + 10mV/°C. The
         critical temperature is 45°C, when the sensor outputs 10mV/°C * 45°C = 0.45V.
         An op-amp comparator (MAX9019) will be used to detect this voltage, and will
         output a high if the temperature is above 45°C. The microprocessor will detect
         the op-amp output and shut down the charger if the temperature is too high. The
         values of R1 and R2 will depend on the precise voltage outputted by the solar
         panel, which will be determined from testing:

               (Vsolar – 0.45V)/R1 = 0.45V/R2
               (Vsolar – 0.45)/0.45 = R1/R2
       Power used by temp sensor:
               6uW (comparator chip) + 47uW (LM35) = 53uW

4. Microprocessor
The microprocessor will perform the following functions:
       Select battery type: check the battery type that was selected from the switch and
        give the correct output to pins MODE and CHGEN of the 1501.
       Temperature shutoff: when the temperature sensor gives a high out (temperature
        out of range), send signal to MODE and CHGEN to stop charging.
       Solar panel check: pin ACOK of the 1501 gives a low when the 1501 has proper
        input voltage for charging, high otherwise. The microprocessor will shut off the
        charger if ACOK is high. We will also possibly interface more I/O with the
        ACOK signal and the microprocessor, for example signaling to the user if the
        input voltage is not within the correct range (perhaps meaning the solar panel
        needs more light).

5. I/O

               off    debouncer
                      MAX6818           To
               Ni                 microprocessor


       In order for the correct battery mode be sent to the microprocessor to operate on,
       the user must flip a single pull-double throw switch to turn the device on or off,
       and then another single-pull double throw switch that will select either lithium or
       nickel battery chemistry. Thus, the correct signal will be sent to the
       microprocessor which will communicate to the MAX1501 to produce the correct
       In order to solve the problem of debouncing, which is when the output of a switch
       ‘bounces’ before reaching a steady state high, we will use the MAX6818 dual
       switch debouncer. This device accepts eight inputs from the switches, and outputs
       two debounced outputs. These signals will thus be sent to our microprocessor for
       analysis and then it will output the correct signals to the MAX1501 charging IC.

          Our design will also have two LEDs to inform the user of charging status: a green
          LED to indicate charging finished and a red LED to indicate charge in progress.
          See RLED and GLED pins on 1501 for more information.

     6. Power Considerations
           Total power needed = 1501 power (2.633W) + temp sensor power (53uW) + uP
           power (1W) + switch power (30uW) = 3.63308W

V. Cost and Schedule
     1. Cost Analysis
           a. Labor
                  ($35/hour) * 2.5 * (12 hours/week) * (12 weeks) * 2 people = $25,200

           b. Parts

                Part                 Quantity                  Price                   Status
  PowerFilm MPT6-150 Solar Panel        1                       $30                  In Route
   Microcontroller(PIC16F877A)          1                       $10                  On Hand
         MAX 1501 IC                    1                      $3.51                 In Route
     LM35 Temperature Sensor            1                      $0.69                 On Hand
   MAX9019 (voltage comparator)         1                      $0.58                 In Route
      MAX 6817 (debouncer)              1                      $2.05                 In Route
        Resistors, capacitors          10                      $1.00                 On Hand
                LED                     2                      $0.30                 On Hand
           SPDT switch                  2                      $1.00                 On Hand
            Protoboard                  1                       $20                  On Hand
              Housing                   1                       $15                  Designing
      LT 1767 Charging Cable             1                   $5.60                   Decision
                                                     Total (Parts) = $89.73

    2. Schedule

              Week            Task
               2/4 – 2/10     Write proposal, research
              2/11 – 2/17     Work on schematics, research parts
                              Finalize schematic, calculations, order
               2/18 – 2/24    parts
                              Build charging unit, test with function
                              Christine: learn PIC programming
                2/25 – 3/3    Phil: order add'l parts if needed
                              Phil: test solar panel & sensors
                3/4 – 3/10    Christine: program PIC
                              Phil: add solar panel and sensors to
                              charging unit
               3/11 – 3/17    Christine: interface all parts with PIC
               3/18 – 3/24    (spring break) Prepare for mock demo
                              Phil: tolerance analysis
               3/25 – 3/31    Christine: design PCB
                              Troubleshooting, testing
                              Phil: build housing
                 4/1 – 4/7    Christine: solder parts to PCB
                4/8 – 4/14    Work on demo, presentation
               4/15 – 4/21    Work on presentation, final paper
               4/22 – 4/28    Work on final paper

    General division of labor:
          Christine: programming, layout, soldering
          Phil: solar panel, sensors
          Both: charging unit, I/O

VI. Appendix


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