hiya! I have 2 mini versions, and I have never found a way of checking on the battery.
from what I have read, "the mini breakout board has no ADC, thus it is impossible to monitor the battery’s voltage level and battery."
however, LED DS3... "When breakout board is connected to the power source with USB or J21, DS3 lights up which means external power source is connected and can charge lithium battery. When the lithium battery is fully charged, DS3 will be turned off."
(I also get blinking DS3, very-rarely-occasionally. some weird state where the LiPo is neither charged nor charging. I don't comprehend.)
so it seems like there may still be some understanding at software level?
plus, there is recognition if J1 is simply shorted or has a therm on it. but this also seems to only be known to the charge-circuit hardware, software only knows open/closed?