Question

What is the Voltage of two Li-on 18650 batteries connected in series

Answer 1

In the series circuit the voltages add up. The voltage of one Li-on 18650 battery is 3.7 V. Therefore, the voltage of two Li-on 18650 batteries connected in series will be:

$$V_{tot}=V_1+V_2=3.7\ V+3.7\ V=7.4\ V.$$