Hi Rob

if you look into Power.cpp you'll see that the current already is set to about 400 mA:
https://github.com/m5stack/M5Stack/blob/master/src/utility/Power.cpp

void POWER::begin() { uint8_t data; //Initial I2C Wire.begin(21, 22); // 450ma setVinMaxCurrent(CURRENT_400MA);

are you calling M5.Power.begin() in your setup code?
Or maybe the current has been reduced due the the battery being close to full?

Cheers
Felix