solarmonk

energy powered by the sun ☀️

safely store energy in a battery 🔋

safely use excess load energy to power 5v devices ⚡

have fun! 🎉

TODO

• ☑ get solar panel
• ☑ get battery
• ☐ 12v -> 5v converter
• ☐ read battery charge
• ☐ display battery charge
• ☐ control discharge

components

solar panel (with built-in charge controller)

120w maxray folding solar panel

• Max Power: 120W
• Maximum Power Tolerance: ±3%
• Open-Circuit Voltage/Voc（V): 17.5V
• Short-Circuit Current/lsc（A): 9.42
• Max Power Voltage/Vmp（V): 14.0
• Max Power Current/lmp（A): 8.57
• Power Spectications at STC: 1000w/㎡，AM1.5,CELL25℃
• Max System Voltage（V) 1000
• Max Over Current Protecting Rating（A): 15
• Weight: 12.6kg

battery

12v - 85Ah deep cycle flooded lead acid battery

voltage must not drop below 11v

guide to lead-acid batteries

fully charged is 12.6v to 12.8v

| State of Charge (approx.) | 12 Volt Battery | Volts per Cell |

| 100% | 12.70 | 2.12 | | 90% | 12.50 | 2.08 | | 80% | 12.42 | 2.07 | | 70% | 12.32 | 2.05 | | 60% | 12.20 | 2.03 | | 50% | 12.06 | 2.01 | | 40% | 11.90 | 1.98 | | 30% | 11.75 | 1.96 | | 20% | 11.58 | 1.93 | | 10% | 11.31 | 1.89 |

| 0 | 10.50 | 1.75 |

load (discharge) controller

• BTE14-04 betamcu.cn Arduino Uno R3 Compatible
• "DF Robot" LCD Keypad Shield
• R1 resistor = 10k ohms
• R2 resistor = 820 ohms
• ceramic capacitor = 100nF
• schottkey diode

12v   5v      5v
 |     |       |
R1   diode  |-------|
 |     |    |       |
 +-----+----|Ain    |
 |     |    |       |
R2    cap   |arduino|
 |     |    |-------|
GND   GND

TODO

12v to 5v step-down (buck) converter

step down dc-dc 9A converter (constant volts or constant amps), 5~40v input & 1.5~35v out

calculations

resistive divider

to measure the battery voltage on the microcontroller, we divide the voltage using resistors:

given

• adc input impedence = 10k ohms
  • https://electronics.stackexchange.com/a/67172
• adc reference voltage = 1.1V
  • https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/
• max battery voltage = 14V + wiggle room = ~15V

so R1 = adc input impedence = 10k ohms

(want to be as high as possible to reduce current)

calculate

R2 = R1 * (1 / ((Vin / Vout) - 1))
   = Rinput_impedence * (1 / ((Vmax_bat / Vref) - 1))
   = 10e3 * (1 / ((15 / 1.1) - 1))
   = 791.37
  ~= 820 ohms

smoothing capacitor

TODO

to stabilize the measurements, we use a capacitor to smooth the values:

(1 / (PI * R1 * C)) = ~100 hz or ~10hz (?)

C = 100nF