I've just started out but it looks like the following shows the voltage and mine maxed out at 4.178V, this gradually decreases and I think below 3.6V isn't really usable. You can use this and some testing to determine the percentage of battery.

#include "M5StickCPlus.h"

TFT_eSprite Disbuff = TFT_eSprite(&M5.Lcd);

void setup() {
    M5.begin();
    M5.Axp.ScreenBreath(8);
    M5.Lcd.setRotation(1);

    Disbuff.createSprite(240, 135);
    Disbuff.setTextColor(TFT_WHITE);
}

void loop() {
    Disbuff.fillRect(0, 0, 240, 135, TFT_BLACK);
    Disbuff.setCursor(10, 10);
    Disbuff.printf("BAT_V: %.3fV",M5.Axp.GetBatVoltage());
    Disbuff.pushSprite(0,0);
}

There are other options power functions that may help in this doc:
AXP192 (Power management)

Good luck.