Solar Battery Charger/Tester Project (errors, plz help)

Projects Discussion and Showcase Science and Measurement
#1

Hey, for my science fair project I am building (well, attempting to build) a solar battery charger with Arduino Uno testing the charge of the batteries. The original plan was to use an LCD screen, but my LCD (brand new) is defective, so I tried printing values to my laptop. I am still trying to figure out the ratio of the analog input to the actual battery charge level. The circuit goes as follows: Two solar panels in series are wired (positive to positive, negative to negative) to a battery pack (with rechargeable batteries in it). There are two diodes coming off of the battery pack that come together into one wire that goes to the A0 pin of the Arduino. Here is the code:

void setup() {
  Serial.begin(9600);
  pinMode(A0, INPUT);
}

void loop() {
  int analogValue = analogRead(A0);
  Serial.println(analogValue);
  delay(1000);
}

The problem is that I get random values when I run the code. Please help me if you can. The project's due MONDAY!

#2

There are two diodes coming off of the battery pack that come together into one wire that goes to the A0 pin of the Arduino.

If the battery pack voltage is higher than 5V that configuration will fry the Arduino. You need to use a voltage divider using 2 resistors; but you need to tell us what the batteries voltage is, so we can calculate the resistors. Please tell us what the nominal battery voltage is and how much you expect it to swing from discharged to fully charged. If you don't know then you can mention which batteries you are using and they most likely will tell you here what are those. The diodes are probably not needed there. You may need a diode though to protect Arduino's input from voltages higher than its power supply; but we need to know first the batteries voltage.

#3