@incmve and @Gleno0h
Thanks for your help. With the two hints I managed to get the sketch ready to work. It looks like this now:

#include <ESP8266WiFi.h> #include <ESP8266WebServer.h> #include <Base64.h> // wird für die HTTP Passwortverschlüsselung benötigt ADC_MODE (ADC_VCC); // ADC an das Stromversorgung WiFiClient client; // wird für client.println() benötigt const char* ssid = "abc"; // SSID const char* password = "def"; // Passwort für WLAN char uname; String PiUser = "user"; // Benutzername pimatic String PiPass = "pass"; // Passwort pimatic ESP8266WebServer server; EspClass ESPm; void setup() { Serial.begin(115200); delay(100); uint ADCValue = 0; // Batteriespannung messen ADCValue = ESPm.getVcc() - 164; // hier Korrektur Wert eintragen float ADCfloat = float(ADCValue); String Ubatt = ""; Ubatt = String(ADCfloat / 1000, 2); #define BASE64_LEN 40 // folgende Funktionen sind für die Authentifikation nötig char unameenc[BASE64_LEN]; String yourdata; char uname[BASE64_LEN]; String auth = String(PiUser) + ":" + String(PiPass); auth.toCharArray(uname, BASE64_LEN); memset(unameenc, 0, sizeof(unameenc)); base64 encoder; // Los gehts mit der Verbindung zum WiFi Netzwerk Serial.println(); Serial.println(); Serial.print("Connecting to "); Serial.println(ssid); WiFi.begin(ssid, password); while (WiFi.status() != WL_CONNECTED) { delay(100); Serial.print("."); } Serial.println(""); Serial.println("WiFi connected"); Serial.print("IP address: "); Serial.println(WiFi.localIP()); PAddress server(192, 168, 69, 15); //die IP des Servers wo die Variable geändert wird Serial.print("IP des EmpfängerPC: "); Serial.println(server); Serial.print("Batterie: "); Serial.println(Ubatt); if (client.connect(server, 80)) { Serial.println("connected!!"); String content = "{\"type\": \"value\", \"valueOrExpression\": " + String(Ubatt) + ", \"unit\": \" Volt\"}"; client.println("PATCH /api/variables/batt-06 HTTP/1.1"); client.println("Host: 192.168.69.15"); client.print("Authorization: Basic "); client.println(encoder.encode(auth)); client.println("User-Agent: ESP8266"); client.println("Accept: */*"); client.println("Content-Type: application/json"); client.println("Content-Length: " + String(content.length())); client.println(); // funktioniert nur mit dieser Leerzeile ?! client.println(content); delay(100); client.stop(); } Serial.println("gehe schlafen"); ESP.deepSleep(0); } void loop() { // nothing here }

In pimatic I created the variable $batt-06. This is changed by the ESP8266 with the battery status.

With the rule:
when $batt-06 gets updated then <do something>
I will check that the ESP is reset to GND via the button. You could also generate a warning if the battery voltage becomes too low.

Set expression for Variable via rule