Solar Powered Battery Charger

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Solar Powered Battery Charger

• Christine Placek
• Philip Gonski
• Group 4
• ECE 445 Spring 2007

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Objectives

• Operate in various sunlight conditions
• Sunny and cloudy days
• Operate for a wide variety of input voltages
• 6.8V to 13.5V
• Charge both lithium and nickel chemistries

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Lithium Ion Battery
Common Usage Cell phones, MP3 players, other
portable electronics Energy/weight 160
Wh/kg Energy/size 270 Wh/L Power/weight 1800
W/kg Charge/discharge efficiency
99.9 Self-discharge rate 5-10/month Cycle
durability 1200 cycles Nominal Cell Voltage 3.6
V Li1/2CoO2 Li1/2C6 ? C6 LiCoO2
Source Wikipedia
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Nickel-Cadmium Batteries
Common Usage widely used in small electronic
devices Energy/weight 40-60 Wh/kg Energy/size
50-150 Wh/L Power/weight
150W/kg Charge/discharge efficiency
70-90 Self-discharge rate 10/month Cycle
durability 2000 cycles Nominal Cell Voltage 1.2
V
2 NiO(OH) Cd 2 H2O ? 2 Ni(OH)2 Cd(OH)2
Source Wikipedia
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Charging Lithium Batteries

• Very Strict Regulation of Current vs. Voltage
• Charge Voltage at 4.2V or 4.1V most common
• Our batteries charge at 4.1V to prolong lifetime
• Many Built-in safety precautions
• Temperature, low voltage
• Charge time 3 hours at 500ma
• Fast chargers skip stage 2 in next slide
• 70 charged

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GLED ON
http//www.batteryuniversity.com/partone-12.htm
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Charging NiCd Batteries

• Three types of charging
• Slow(.1C-gt14 to 16hrs)
• 70 efficient
• Quick(.5C-gt3 to 6hrs)
• Fast(Full charge with topping-gt1hr)
• 90 efficient
• Pressure and Temperature increase
• Full Charge detected by
• Voltage Drop
• Rate of temperature increase
• Timeout timers

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http//www.batteryuniversity.com/partone-11.htm
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• Circuitry

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Block Diagram
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Solar Cell
Our solar cell 8in x 12in
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Buck Converter

• Accepts input between 6.8V to 48V
• Low Current Drain 2.5mA
• Regulates voltage at 5V and 500mA to MAX1501 as
  determined by main inductor

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Charging Unit

• MAX1501 linear charger
• Charges both Ni and Li chemistries
• Low minimum input voltage
• 4.5 Volts Li
• 5.25 Volts NiCd
• High maximum input voltage
• 13.5 Volts
• Low current drain (5-8 mA)
• Safety Features

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Charger Specs

• Battery mode can be selected (done w/ PIC)
• Can choose charge voltage
• We use 4.1V for Li
• Current set by external circuitry
• Our circuit set for 467mA

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Charging Unit
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Voltage Regulator

• MAX6129
• Can receive 5.2V to 12.6V from solar cell
• Draws only 5.25uA from supply
• 200mV Output voltage dropout
• Outputs to the PIC which needs low current and
  around 5V to remain in operation

http//datasheets.maxim-ic.com/en/ds/MAX6129.pdf
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PIC

• Battery type is selected by switch, PIC tells
  charging chip which battery has been selected

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Testing/Results
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Circuit Performance

• GLED on for fully charged battery
• Vbatt4.1 V
• RLED on for discharged battery
• Steady voltage increase across battery
• Drew about 450mA from supply at 7V
• Within boundary of solar cell operation
• Low output ripple voltage20mVpp

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Output Ripple
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Charging Time
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Power Considerations
Our solar panel gives 8.06W (peak sunlight)
Charging efficiency 23.7
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Solar Panel Conclusions

• Although only 2.27W are used, we need a solar
  panel that gives
• 6.8V (min to turn buck converter on)
• 450mA (max current draw)
• -gt 3.06W
• Most smaller solar panels do not give enough
  current

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Challenges

• Inductor coupling
• The LT1777 buck converter sometimes produced
  sporadic voltage/current
• Appeared to be load-dependent
• Unable to charge NiCd batteries buck converter
  does not give high enough output

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Future Recommendations

• Both Voltage Reference and Buck Converter are
  temperature sensitive!
• Only in very limited proportions, but heat
  sinking should be considered
• Choose different buck converter
• -Need output more reliable, higher than 4.5V

http//datasheets.maxim-ic.com/en/ds/MAX6129.pdf
www.linear.com
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Power Recommendations

• Use buck converter that needs less current
• Then, smaller solar panel can be used
• Decrease power used by LEDs
• Find lower-power PIC, or implement in different
  fashion

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Questions?
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