Indeed it varies between 4.2V and 3.1V, but the value op chose (3.7V) is not an unreasonable average number. But 3.5V probably is a better guesstimate. See http

Indeed it varies between 4.2V and 3.1V, but the value op chose (3.7V) is not an unreasonable average number. But 3.5V probably is a better guesstimate. See https://cdn.sparkfun.com/assets/6/7/4/7/9/5112a224ce395fb479000003.png https://cdn.sparkfun.com/assets/6/7/4/7/9/5112a224ce395fb479...