write a string into ESP8266 eeprom

Other Hardware Storage
9

I need to convert characters to integer,so I can save them into eeprom and then reconvert integer to character.

No, you do not. An int can contain a value in the range -32768 to 32767. A char can contain a value in the range -128 to 127. All ASCII characters are in the range 0 to 127. You can only store bytes in EEPROM, which means values in the range 0 to 255.

  String qsid="Global";
  int charLength=qsid.length();
 
  Serial.println("writing eeprom ssid:");
          for (int i = 0; i < qsid.length(); ++i)
            {
              EEPROM.write(i, qsid[i]);

What is the type returned by the [] operator in the String class? Why didn't you f**k around trying to convert that char to an int? Oh,yeah, because you don't need to.