Hi Rjen,
Thanks a lot
ALAMSCN 5PCS XL6009 DC to DC Boost Converter 3.0-30V to 5-35V Adjustable Step-up Down Converter Module DC Power Supply Voltage Regulator : Amazon.co.uk: Business, Industry & Science
www.amazon.co.uk
It says on that booster that
The input voltage should be within 5V to 30V, other input voltage may cause damage or failure with a high probability.
It also says the optimum input voltage range is from 5V to 30V.
Doesn't sound so promising for a 3.7V input.
I'd prefer one that is adequate for 3.7V and perhaps have charger onboard.