I'm understanding your struggle.
I'm hawing the same issue with my 24V solar sistem and looking for the solution. I found of the shelf devices that do exactly what I need but they are so expensive that you can build your one for the 1/10 of the price.
I'm also understand that my batteries are on the end of there life, but this is learning proces for me and I want to convince myself that I'm wrong.
So this is what I did:
List of parts
1x Arduino Nano V3.0 - https://goo.gl/NkqSv8
1x Step Down Buck Converter - https://goo.gl/J2JNVa
1x 2-Channel Relay Module Shield - https://goo.gl/oPUoL1
1x 0.96 inch 128X64 OLED Display Module - https://goo.gl/Hunx2E
4x Halogen G4 12V - https://goo.gl/qwpbem
-- I'm using pre made voltage sensors but you can build then with these resistors;
2x 30k ohm ressistor
1x 10k ohm ressistor
2x 7.5k ohm ressistor
I'm not going to post the schematics at the time because I don't have time to do them now.
This is the code and picture of my device.
Here is the schematics , and I updated the code.
#include <Adafruit_SSD1306.h> // Display Libary
int Rl_1 = 5, Rl_2 = 6;
float dif = 0.5; // The difference between voltages as a condition for switching on the load
float v_min = 10.2;
int input_1 = A1, input_2 = A2;
int n = 20; // How many times is the average voltage measured
float v1 = 0.00, v2 = 0.00; // volts input
float R1_1 = 29900.0, R1_2 = 40700.0; // calibrate the sensors by changing this values
float R2_1 = 7500.0, R2_2 = 7500.0;
Adafruit_SSD1306 display(4);
void setup(){
pinMode(input_1, INPUT);
pinMode(input_2, INPUT);
pinMode(Rl_1, OUTPUT);
pinMode(Rl_2, OUTPUT);
Serial.begin(9600);
Serial.print("DC VOLTMETER");
display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
display.display();
}
void display_init (float vi1, float vi2){
// Display
display.clearDisplay();
display.setTextColor(WHITE);
display.setTextSize(2);
display.setCursor(0,0);
display.print("B1: ");
display.print(vi1);
display.print("V");
display.setCursor(0,17);
display.print("B2: ");
display.print(vi2);
display.print("V");
display.display();
Serial.print("INPUT 1 V= ");
Serial.println(vi1,2);
Serial.print("INPUT 2 V= ");
Serial.println(vi2,2);
}
// read the value at analog inputs
float volt_1(){
float vout_1 = 0.00, vin_1 = 0.00;
int average_1 = 0, value_1 = 0;
for (int i=0; i < n; i++) {
average_1 = average_1 + analogRead(input_1);
delay(50);
}
average_1 = average_1/n;
value_1 = average_1;
vout_1 = (value_1 * 5.0) / 1024.0; // see text
vin_1 = vout_1 / (R2_1/(R1_1+R2_1));
return vin_1;
}
float volt_2 (float in){
float vout_2 = 0.00, vin_2 = 0.00;
int average_2 = 0, value_2 = 0;
for (int i=0; i < n; i++) {
average_2 = average_2 + analogRead(input_2);
delay(50);
}
average_2 = average_2/n;
value_2 = average_2;
vout_2 = (value_2 * 5.00) / 1024.0; // see text
vin_2 = vout_2 / (R2_2/(R1_2+R2_2));
vin_2 = vin_2 - in;
return vin_2;
}
void loop(){
v1 = volt_1();
v2 = volt_2(v1);
display_init(v1, v2);
// Load #1
if (v1 - v2 > 0 & abs(v1 - v2) > dif & v1 > v_min ) {
while (v1 >= v2 & v1 > v_min){
v1 = volt_1();
v2 = volt_2(v1);
display_init(v1, v2);
digitalWrite(Rl_1, HIGH);
delay(1000);
}
digitalWrite(Rl_1, LOW);
}
// Load #2
if (v1 - v2 < 0 & abs(v1 - v2) > dif & v2 > v_min ) {
while (v1 <= v2 & v2 > v_min){
v1 = volt_1();
v2 = volt_2(v1);
display_init(v1, v2);
digitalWrite(Rl_2, HIGH);
delay(1000);
}
digitalWrite(Rl_2, LOW);
}
delay(600);
}
The device draws 20mA of current when idelig and about 80mA when relay kick in.
