Problem with battery level value

[Mattia Reggiani]

Good morning,
Congratulations for this fantastic project ; are my first steps with Arduino and I have a litle problem with battery monitor on Arduino Pro Mini 5V, I use the sketch and instructions find in this page http://www.mysensors.org/build/battery , but the value read in pin A0 is always 1023, i have try othe analog pin with the same result
In touch point however the value is 1,4 /1,5 measured with digital multimeter, in pin a0 the value is the same.
The sensor is powered from 4 AA battery 1,5V .
I do not understand where wrong.

Sorry for the English is not perfect

Thanks
Mattia

[boozz]

@Mattia-Reggiani

In the sketch a internal voltage reference of 1.1 V is used. This means that if a value of 1.1V or higher is measured at the A0 input, you'll read the max int value, which is 1023.

Two options :

1. Change the voltage divider to get maximum of 1.1 V if the batteries supply app. 6 V (4x1.5 V). instead of the 470k you could use 230k or instead of the 1M you could use 2M.

or
2. don't use the internal reference of 1.1V.

I would go for the first option.

some additional info on the internal voltage reference of the arduino to be found here

Note that 4x1.5 VDC is 6 VDC, with a fresh set of batteries possibly higher. you probably connect it to the raw input, so that the step down converter build on the pro-mini will do its work.

Succes!

Boozz

[Mattia Reggiani]

@boozz
Many Thanks, now I understand but I still have doubts, in the skecth i have change this
float batteryV = sensorValue * 0.003363075;
in
float batteryV = sensorValue * 0.00575035;

because if I understand it ((1e6+470e3)/470e3)*1.1 = Vmax = 3.44 Volts
in my case is ((1+0,23)/0,23)*1,1 = 5,88 and after 5,88/1023 = 0,0057478
Now, however with new battery the output is

99
Battery Voltage: 0.57 V
Battery percent: 9 %
:(

The code i used is
// get the battery Voltage
int sensorValue = analogRead(BATTERY_SENSE_PIN);
Serial.println(sensorValue);
// 1M, 470K divider across battery and using internal ADC ref of 1.1V
// Sense point is bypassed with 0.1 uF cap to reduce noise at that point
// ((1e6+470e3)/470e3)*1.1 = Vmax = 3.44 Volts
// 3.44/1023 = Volts per bit = 0.003363075
float batteryV = sensorValue * 0.00575035;
int batteryPcnt = sensorValue / 10;

Thanks
Mattia

[TimO]

Did you change the divider?

In my temperature sensor i measure the battery level with:

R1: 1 MOhm
R2: 220 kOHM

float batteryV = sensorValue * 6.1 / 1023;

With an input voltage of 7.2 V the dividers gives 1.1 V (1023) on the analog input.

Don't forget to place a capacitor between AREF & GND to stabilize the measurement.

[boozz]

@Mattia-Reggiani
As TimO already asked: did you change the divider? (replace the resistor 470k into a resistor 220k or the 1M into 2M)? it's essential.

Boozz

[Mattia Reggiani]

I have change R1 to 2M and R2 470k , I think to have some problem with resistors, tomorrow will buy new resistors and try again from scratch.

Thanks for your help
Mattia

[Mattia Reggiani]

I have rebuild the dividers with R1 1M ohm and R2 220k ohm, i have try with pin A1 and A0 and have strange value

990
Battery Voltage: 5.59 V
Battery percent: 99 %

1023
Battery Voltage: 5.78 V
Battery percent: 102 %

First read is correct, but after this all other value have value 1023, if use multimeter on R1 and R2 have a value of 0,93 on touch point have 0,93 and on the Pin A0 or A1 have the same value.
It is possible that Arduino read the wrong value ?

[TimO]

Did you place a capacitor between AREF and GND?

[Mattia Reggiani]

@TimO
Sorry, but Whatsapp is AREF? Is the Pin thai I use to measure the battery voltage (A0) ? Bcause on Google i find some Arduino Board with AREF pin but my Arduino mini pro has not that pin.

Thanks
Mattia

[TimO]

Oh, I'm sorry, at the Arduino pro mini there already is a capacitor between AREF & GND, so don't worry about it.

Here is my working example, a sensor for humidity, temperature and status of the battery:

link text

It's basically:

line 30: with "analogReference("INTERNAL)", did you call that?

The function in line 70 measures the battery status.

[Mattia Reggiani]

@TimO @boozz
Finally there I did, the problem was due by the resistances; I bought the new resistors
2M Ohm and 470k Ohm and now works correctly.

Thank you all for your support and for your patience

(Now I have a little more knowledge on electronics :) )

Mattia

[boozz]

@Mattia-Reggiani
Good to see that you solved it and learned by doing. Congrats!

Boozz