12V - How to activate an electric item if battery is full
So here is what I did :

1 - I connected an Arduino to collect analog outputs from a voltage sensor PCB. (connected on 5v pin, ground and Analog 0 or A0)

2 - The analog data is sent through the usb cable to my raspberry pi. (You have to install on the raspberry pi : sudo apt-get install arduino arduino-mk)

ON RASPBERRY :

1 - Create a file named "Makefile" in the project's folder.

ARDUINO_DIR = /usr/share/arduino
ARDUINO_PORT = /dev/ttyACM*

USER_LIB_PATH = /home/pi/ARDUINO/librairies
BOARD_TAG = uno

include /usr/share/arduino/Arduino.mk


2 - Create the arduino code

void setup(){
  Serial.begin(9600);
}

void loop(){
    /* Voltage sensor on A0 */
    int volt = analogRead(A0);
    double voltage_a0 = map(volt, 0, 1023, 0, 2500);
    voltage_a0 /= 100;
    Serial.println(voltage_a0,1);
    delay(10000);
}


3 - Run the code on the arduino and check Serial output

make upload monitor clean


3 - Leave the monitoring system with CTRL + A and then CTRL + D

4 - Close all screen with screen -X quit (otherwise it will not be available)

5 - You can now read the arduino Serial manually with cat /dev/ttyACM0 but I created a script for reading it every 45 seconds, and write it in a file in a "logs" folder with the date as filename (keep running in the background with screen) :

#!/bin/bash
while [ 1 ]; do
        TODAY=$(date +%Y_%m_%d)
        read DUMP &lt; /dev/ttyACM0 #DUMP
        sleep 2
        read ARDUINO_LINE &lt; /dev/ttyACM0 #(arduino every 10s) Example : 14.7
        TIME=$(date +%R)
        if [[ "$TIME" != "$OLD_TIME" ]]; then
                echo "${TIME}=${ARDUINO_LINE}"
                echo "${TIME}=${ARDUINO_LINE}" &gt;&gt; "logs/$TODAY"
        fi
        OLD_TIME=$TIME
        sleep 45
done


6 - Now i have every day a file in logs called for example 2019_07_30 and it contains 1440 lines like this one : 14:40=14.2.
I can use this file to create a graph with for example libre office.

7 - NOT DONE YET. I just need to add some relays that will be controlled by the raspberry pi. (I will edit this post as soon as it's done.)