ESP32: Disable Brownout Detector
For me brownout issue was solved by removing the esp32 from Arduino IDE board manager + removing files from disk and use install procedure "Using Arduino IDE with the development repository" - https://github.com/espressif/arduino-esp32/

I guess it will use newer version of arduino-esp32 that has some brownout protection due to CONFIG_REDUCE_PHY_TX_POWER being activated by default. Also you should probably learn how to decrease wifi power + deep sleep for longer battery usage.