Li-ion charger circuit with LM339N
You do not use the voltage of a charging lithium battery to see if it is fully charged, instead you detect when the charging current has dropped low.
The voltage shoots up quickly when charging so when the voltage is high then the battery is nowhere near fully charged. You MUST limit the charging voltage to 4.20V or a little less (3.73V is much too low) and limit the charging current to avoid an explosion.