Only to complement @ballscrewbob. If you try to read the battery from the positive terminal while it is charging over the micro usb, the charging procedure can vary the actual level of the battery, to get a nice reading from the voltage divisor you would turn down the ENABLE pin from the charger IC (this isolates the battery from the charger, then you read just battery) and then turn on again, it can be done from a MKR1000 digital GPIO.
nice hack: actually you can do this every four hours in order to avoid the time limit in the charger.