uses tlv431 1.25v.
prevents overcharging by switching the left pfet off when battery raises to 4.2V.
disconnects the load through the right pfet when battery falls to 3.2V.
no charge or draw current protection.
no high or low temperature protection.
no reverse protection; body diodes will conduct.
small leakage current through the dividers will discharge the battery if left connected.
from:
https://electronics.stackexchange.com/a/295253
