Problem with voltage between a DC step up and Raspberry
The minimum voltage you can put onto the XL6009 is 5 volts. If you go lower than this (i.e. 3.8 volts from your cellphone battery) then all bets are off and you can't predict what the output voltage will be under load and no-load conditions.



I would also look at the table above and note that the values are specified with an output load current of 0.5 amps. I'm mentioning this because the XL6009 doesn't appear to specify a minimum load current so if your load is significantly less than 0.5 amps then expect other strange things too.