Thank you for your answer, indeed there's a mistake in the comment, I'll edit that right away!
Yes for AA i mean Alkaline, I implemented the "battery type" feature even if I'm not yet using it, as I currently only power my gadget with LiPo cells. But as it is explained well enough (I hope), anybody can add his battery type in the code! I guess for NiMH boundaries would be 0.9v to 1.3v

@Ben-Andrewes My domoticz instance behaves weirdly with relay sketches too. Maybe you can find some help here: https://forum.mysensors.org/topic/7101/domoticz-relay-node-doesn-t-show-up-in-devices/21

Hello,
A modification I implemented starting from the "battery sensor" sketch, in order to improve battery % reporting; wrapped up in a small library. Please be kind with my code, I'm merely a hobbyist.
Why:
1- LiPo cells can't get under 3.3v without damage risk, AA cells are leak proof down to 0.7v, I wanted my battery reporting to implement this: LiPo: 100% = 4.15v, 0% = 3.3v // AA: 100% = 1.5v, 0% = 0.8v
2 - Voltage divider precision relies on resistors accuracy, I wanted a way to "calibrate" my divider in order to get proper battery reporting
3 - For fun
Please note that the voltage divider is not the same for LiPo and AA battery (explained in cpp's comments)

Feel free to copy, paste, modify, criticize!

Test sketch

#include <BatteryManager.h>

const int BatteryType = 1; // 1 for 1S LiPo, 2 for 2xAA
const uint8_t BatterySensorPin = A0;
const float VoltageDividerCalibration = 1; // default 1, can be calibrated by measurement

BatteryManager bms;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  bms.begin(BatteryType, BatterySensorPin,VoltageDividerCalibration);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.print("Voltage: ");
  Serial.println(bms.reportVoltage());
  Serial.print("Percent: ");
  Serial.println(bms.reportPercent());
  delay(3000);
}

header:

/*
* Battery Manager
*/

#ifndef BatteryManager_h
#define BatteryManager_h

#include <Arduino.h>

class BatteryManager {
	public:
        BatteryManager();
                void begin(int BatteryType, uint8_t SensePin, float Compensation);
		int reportPercent();
		float reportVoltage();
	private:
		int _BatteryType; //1 for 1S LiPO, 2 for 2x AA
		uint8_t _SensePin;
                float _Compensation;
		float Battery_Min_V;
		float Battery_Max_V;
		float Battery_Sense_Pin_Resolution;
};
#endif

cpp

#include <Arduino.h>
#include <BatteryManager.h>

BatteryManager::BatteryManager() { }

void BatteryManager::begin(int BatteryType, uint8_t SensePin, float Compensation)
{
    _SensePin = SensePin;
    _Compensation = Compensation;
    pinMode(_SensePin, INPUT);
    // Battery use the 1.1 V internal reference
    #if defined(__AVR_ATmega2560__)
        analogReference(INTERNAL1V1);
    #else
        analogReference(INTERNAL);
    #endif
    int firstRead = analogRead(_SensePin);
    // check battery type and feed related variables
    if (BatteryType == 1) {
            /* LiPo voltage need to be measured up to 4.2V hence using internal ADC ref of 1.1V,
            *  divider across battery needs to be 1M, 330K, 
            *  which allows a range of 0 to ((1e6+330e3)/330e3)*1.1 = 4.433 Volts
            *  resolution: 4.4333/1023 = Volts per bit = 0.004333659
            *  sense point can be bypassed with 0.1 uF cap to reduce noise at that point
            *  LiPo is full @ 4.2V and risks damage below 3.3v
            *  => LiPo usable range is 3.3 to 4.2 volts so the goal is to report
            *  100% @ 4.2 volts
            *  0% @ 3.3 volts
            */
            Battery_Min_V = 3.3;
            Battery_Max_V = 4.15;
            Battery_Sense_Pin_Resolution = 4.333659; //mv per bit
    }
     if (BatteryType == 2) {
            /* 2 x AA Alkaline voltage need to be measured up to 3V hence using internal ADC ref of 1.1V,
            *  divider across battery needs to be 1M, 470K, 
            *  which allows a range of 0 to ((1e6+470e3)/470e3)*1.1 = 3.44 Volts
            *  resolution: 3.44/1023 = Volts per bit = 0.003363075
            *  sense point can be bypassed with 0.1 uF cap to reduce noise at that point
            *  AA is normally leak proof down to 0.7V, I add an extra 0.1V to avoid any issue
            *  => AA usable range is 1.5 to 0.8 volts so the goal is to report
            *  100% @ 2x1.5 = 3 volts
            *  0% @ 2x0.8 = 1.6 volts
            */
            Battery_Min_V = 1.6;
            Battery_Max_V = 3;
            Battery_Sense_Pin_Resolution = 3.363075; //mv per bit
    }
}

int BatteryManager::reportPercent() {
    // get the battery Voltage
    int sensorValue = analogRead(_SensePin);
    Serial.println(sensorValue);
	float batteryV  = sensorValue * Battery_Sense_Pin_Resolution * _Compensation / 1000;
	// map to 0 -- 100 according to boundaries
	// need to multiply voltages by 10 as map function processes long variables
	return map(10*batteryV,10*Battery_Min_V, 10*Battery_Max_V, 0, 100);;
}

float BatteryManager::reportVoltage() {
    // get the battery Voltage
    int sensorValue = analogRead(_SensePin);
    Serial.println(sensorValue);
	return sensorValue * Battery_Sense_Pin_Resolution * _Compensation / 1000;
}

Guys I'm confused as @gohan suggested I tried again with a "blank" node (eeprom clearded, etc...) with 2 relays: worked. Then I cleared it again and tried with one: worked also. It's like Domoticz's bouncer now thinks relays are nice lads, everybody gets accepted!
So thank you all, and for people having the issue, there's some stuff:
1- clear node ID, and try again
2- Add a button (or simulate one with a wire, just for the sake of pairing)
3- Add a piece of code to send data to domoticz, as per @mfalkvidd 's code.
All those worked a some point
Thank you again guys, I promise my next post will not be a cry for help (more likely improvement for battery percentage reporting - LiPo -, and/or improvement of BME280 handling - avoid sending data every minute -)

Thank you all for your answers. @gohan : I'll try changing node ID tomorrow, it's bed time in France

Apologies, I posted in troubleshooting as I didn't get hits on domoticz section. I will pay attention not to cross post again.

Ok I did try something else: in void presentation i replaced S_BINARY by pin (of course I deleted the piece of code in the loop()

I deleted the device and the node from domotics, cleared eeprom and tried. It works, the relay gets paired and is presented in devices... and works. does it make sense?

void presentation()
{
// Send the sketch version information to the gateway and Controller
sendSketchInfo("Relay", "1.0");
for (int sensor=1, pin=RELAY_1; sensor<=NUMBER_OF_RELAYS; sensor++, pin++) {
// Register all sensors to gw (they will be created as child devices)
present(sensor, pin);
}
}

Hello, Tried presenting as S_Light, doesn't work, but the piece of code in the loop() work great!
Nice workaround!
Thank you very much!

So I repeated the whole procedure:
--> Relay sketch: never appears in devices
--> Relay with button. pairs fine with the GW, appear in GW setup page, but not in devices. UNTIL... button is pressed. (i waited a few minutes to confirm). Afterwards the relay works as expected.
I'm 100% sure of the behavior, I reproduced the scenario several times.
So there's my way of getting a relay to show up in devices, even if it's not straightforward.
So I guess the "simple relay" sketch needs to send something after it is paired with the GW in order to appear in "Devices". Any piece of code I should try?
Topic half solved?

@gohan Thank for taking some time. I did that already, and the output seemed ok, but I'll try again when I get back home. But I guess the presentation is well received, as the node appears properly in the gateway setup page (see screenshot a few posts above).
Just to specify my setup: RPI running Domoticz, with RFLink GW & devices, some wifi devices (ESP8266 based) & now MySensors V2.1.1 Serial GW & nodes. At the moment I didn't yet tinker with the MySensors sketches: I'm running everything with sample sketches