Can MAX232 powered at 5V and connect to tx rx(TTL) on ESP8266

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  1. RS232 levels are defined by specification. You if you MPPT controller (what ever that is) expects RS232 voltage levels then it irrelevant what the respective MAX's are power with.

  2. The 5V supplied MAX will output something in the order of 3.5 to 4.5 volts (going to the ESP8266). You can simply put a 10k between the MAX rx and the ESP8266 to limit the "fault" current. Some folks don't like this approach but it has worked for a lot of folks.