[Solved]weird thing happen: analogRead problem

Using Arduino Programming Questions
1

I got A1 connected to a battery, and have a LCD print out the voltage, but the voltage keep going up and down, down and up, from 4.2 - 3.4 -4.2 - 3.4, etc. why? this is totally not comprehensible by myself.

here is the part of coding for reading a voltage:

  int batteryVolt = analogRead(A1); 
  battery = batteryVolt*(5.0/1024.0)*0.9;  

  nowVolt=battery;

here are all the code:

#include <Wire.h> 
#include "ClickButton.h"
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address/ pins


ClickButton ba(9, LOW, CLICKBTN_PULLUP);
ClickButton bb(8, LOW, CLICKBTN_PULLUP);

int sec=0;
int Min=0;
int hr=0;
int Switch=1;
int pwm=0;
int Pwm=255;
float shunt=0;
int mah=0;
int ncc=0;
float battery=0;

int realMa=0;

float firstVolt=0.0;
float nowVolt=0.0;

unsigned long shuntTotal=0;
unsigned long nowtimer=0;
unsigned long pretimer=0;

unsigned long nptimer=0;
unsigned long pptimer=0;
//==========================

void setup()
{  
  lcd.begin(16,2);               // initialize the lcd   
  ba.multiclickTime = 50;
  bb.multiclickTime = 50;    
  pinMode(7, OUTPUT);
  pinMode(A0, INPUT);  //Shunt
  pinMode(A1, INPUT);  //battery
}

//=====================
void loop()
{
  ba.Update();
  bb.Update();  

  pwm=constrain(pwm, 0, 9); 
  Pwm=map(pwm, 0,9, 255,0); 

  nowtimer=millis();
  if(nowtimer-pretimer>=1000) {
    sec++;
    shuntTotal+=realMa;
    pretimer=nowtimer;    
  }



  if(sec==60){ 
    Min++; 
    sec=0; 
  }
  if(Min==60){ 
    hr++; 
    Min=0; 
  }  


  //=================================

  char buffer[8];
  sprintf(buffer, "%01d:%02d:%02d", hr, Min, sec);
  lcd.setCursor (0,0); 
  lcd.print (buffer);   

  lcd.setCursor (8,0); 
  lcd.print (firstVolt);     

  lcd.setCursor (11,0); 
  lcd.print (">"); 


  lcd.setCursor (15,0); 
  lcd.print ("V");       

  nptimer=millis();
  if(nptimer-pptimer>=300) {

    char bufferx[17];
    sprintf(bufferx, "%04dC %04dmA %01d", mah,realMa, pwm);
    lcd.setCursor (2,1); 
    lcd.print (bufferx);

    lcd.setCursor (12,0); 
    lcd.print (nowVolt);   

    pptimer=nptimer;    

  }



  if(Switch==0){
    lcd.setCursor (0,1); 
    lcd.print ("P:"); 
  }

  if(Switch==1){
    lcd.setCursor (0,1); 
    lcd.print ("S:"); 
  } 

  //================================  

  int shuntVolt= analogRead(A0);   
  shunt = shuntVolt*5.0/1024.0*1000.0*0.89; 
  mah=shuntTotal/3600; 

  int batteryVolt = analogRead(A1); 
  battery = batteryVolt*(5.0/1024.0)*0.9;  

  nowVolt=battery;  

  if(shunt>50) {
    realMa=shunt*pwm/9.0; 

  }

  if(pwm==0){
    realMa=0;  
    digitalWrite(7, LOW);
  }
  if (shunt<50)ncc++;
  if(ncc>20){
    realMa=0; 
    ncc=0; 
    digitalWrite(7, LOW);
  }    


  if(realMa>50 && firstVolt==0.0) firstVolt=battery;

  if(nowVolt < (firstVolt*0.7) && realMa<50) Switch=0;

  if(realMa>50) digitalWrite(7, HIGH);




  //================================    

  if(Switch==1) analogWrite(3, Pwm);
  else {
    analogWrite(3, 255);
    digitalWrite(7, LOW);   
    realMa=0; 
  }

  //================================  

  if(ba.clicks==1) pwm++;
  if(bb.clicks==1) pwm--;  

  if(ba.clicks==-1) Switch=!Switch;
  if(bb.clicks==-1) {
    sec=0;
    Min=0;
    hr=0;
    shuntTotal=0;
    mah=0;
    Switch=0;

  }

}
2

from 4.2 - 3.4 -4.2 - 3.4, etc. why?

Are you saying you're getting negative results?

3

AWOL:

from 4.2 - 3.4 -4.2 - 3.4, etc. why?

Are you saying you're getting negative results?

No, all positive, from 4.2 to 3.4 to 4.2 to 3.4. No other number but these two. And the reading is fine before I increase pwm. And first volt reading sometimes is incorrect too.

4 
battery = batteryVolt*(5.0/1024.0)*0.9;

Why the 0.9?
You haven't got resistors in there have you?
If so what value?

There in no need to set a pin as an input of you are using analogue read.

How is it wired? You might be getting ground bounce.

5

Grumpy_Mike:

battery = batteryVolt*(5.0/1024.0)*0.9;

Why the 0.9?
You haven't got resistors in there have you?
If so what value?

There in no need to set a pin as an input of you are using analogue read.

How is it wired? You might be getting ground bounce.

0.9 to make the reading accurate, because my 5 volt pin is less than 5 volt. I know, analog input doesn't need setup. I put them there to see if it could solve the problem.

6

How is it wired? You might be getting ground bounce.

Schematic and photo please.

7

For debugging just print the actual values returned by analogRead().

...R

8

Robin2:
For debugging just print the actual values returned by analogRead().

...R

actually, the lcd is printing the actual values returned.

9

Grumpy_Mike:

How is it wired? You might be getting ground bounce.

Schematic and photo please.

thank you for remaining me. I think the problem is something you mentioned: ground bounce; I am switching the battery using PWM very fast, and the voltage got ground bounce very fast, thus I got 3.4 to 4.2 to 3.4 to 4.2 reading. it makes sense now.