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I would add a Buck Boost Converter to expand battery range. otherwise, looks good
I would add a buck boost converter to drop the voltage when to high, and boost it when to low. it extends battery lifehttps://www.banggood.com/5pcs-DC-DC-Boost-Buck-Adjustable-Step-Up-Step-Down-Automatic-Converter-XL6009-Module-p-1087606.html?rmmds=myorder
You need to set the serial port monitor the same rate as in the code. the monitor is set to 9600 baud, while the Arduino is transmitting at 115200 baud
The serial monitor is set to 9600b/s while the code is transmitting 115200b/s. On the right of the serial monitor is is a drop down, choose 115200 baud.