Which exact onboard regulator do you have? Many of the small regulators typically used on those breakout boards can't handle >5-6V. Connecting two 3.7V batteries can easily kill them. Do check for the schematics/data sheet of your particular module, and check the limits for the on-board regulator.
What coin cell do you use that produces 3.3V rather than the common 3V?
hardik311095:
Circuit Diagram and HC-05 image attached below.
