Solar Panel Charge Controller Using PICAXE Microcontroller
by Lewis Loflin
The following is the PICAXE version of the Arduino Solar Panel Charge Controller. While the electronics is identical, the programming was very different. The main problem for the PICAXE was the inability of the software to do compound 'if' statements: if ((x > y) & (y < CP) & (x > CP)) . I had to break that into three different statements then had to add a fourth.
Otherwise it worked just the same. See Arduino Solar Panel Charge Controller for additional schematics.
#rem Picaxe solar panel charge controller Purpose: to cycle to charge voltage on/off and to check other aspects such as solar panel input voltage when charging lead-acid batteries. #picaxe 18m2 ; type chip used B.4 LED1 indicator 'bad' meaning the input voltage below charging voltage when on. B.7 turns on charge switch transistor. Will blink on/off with charge cycle. LED3 will turn on during the charge cycle.(Charge enable DP11) B.5 LED2 indicator fully charged battery. (DP10) A 10-bit analog-to-digital converter (ADC) has a step voltage of about 4.9 mV over a 5-volt range. This relates to the charge point (CP) variable. To measure input voltage from the solar panel and the voltage on the battery we use a voltage divider to drop the voltage below 5-volts. This uses two resistor voltage dividers (15k and 2.2k) which produces a voltage of about 1.7 - 1.9 volts when fully charged. This equates to about decimal 346 - 400 from the ADC and is compared to the charge point variable CP. Note line "chon = CP - y * 1000" when uncommented the charge 'on' time will decrease gradually as battery is more charged. When fully charged the charge voltage is disabled. The variables chon (charge on time) and choff (charge off time) can be preset to any value. One can experiment with this CP value. Too small, battery won't fully charge. Too large, battery will over charge. The voltage input is connected to C.1 (AD0 in schematic) while the voltage on the battery is monitored at C.0 (AD1 in schematic ). This same circuit can be used with a 24-volt system by changing the 15K to 27K, and using a 24-volt relay. The power for the PICAXE itself can be obtained from the battery bank under charge through a 5-volt regulator or separate supply. Note if the battery bank is completely dead the circuit won't function with no power to the Microcontroller. A separate source for the controller is recommended. This circuit will also work using a power supply instead of a solar panel as a simple battery charger. #endrem symbol voltage_in = C.1 symbol voltage_out = C.0 symbol LED1 = B.5 symbol LED2 = B.4 symbol charge_enable = C.7 LOW LED1 LOW LED2 LOW charge_enable symbol x = w0 symbol y = w1 symbol chon = w2 symbol choff = w3 chon = 2000 choff = 6000 symbol CP = w4 ; charge point variable symbol temp = B10 CP = 340 main: temp = 0 readadc10 voltage_in, x ; voltage from solar panel readadc10 voltage_out, y ; voltage on battery if x <= CP then HIGH LED1 else LOW LED1 endif ;LED off indicates good input voltage if y >= CP then HIGH LED2 else LOW LED2 endif ;LED on means battery is charged if x > CP then inc temp endif if y < x then inc temp endif if y < CP then inc temp endif if temp = 3 then gosub charge ;turn on charge cycle if voltage input good AND battery voltage low. goto main charge: ; charge enable routine HIGH charge_enable; // turn on voltage to battery ; chon = CP - y * 1000 ;uncomment this for variable charge rate pause chon; wait LOW charge_enable ;turn off charge enable pause choff; wait for charge to equalize goto main
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