How do you change 3.7 V to 12V?

I have 2 cellphone battery brick chargers. I took one apart and found it to have 6, 18650 LiPo batteries connected in parallel to a circuit board to charge it. I have a project that requires 12 V and I would like to use these in some fashion. I was thinking put the two parallel battery bricks in series to get 7.4 V and step up with a DC to DC converter. Which one would be appropriate? 
To charge the LiPo's I was thinking of parallel connecting the two circuit boards from the 2 cellphone battery chargers to a single wire connecting to a 5V cellphone wall usb charger. The usb charger supplies 5V at max 2.4A. Any help would be appreciated! 

iceng7 days ago

You only say 12 volt but no indication of the intended current ability needed ?

Well, some might have already noticed that I am not a big fan of parallel batteries but anyway:
To increase the voltage you can use a step up converter or boost converter.
Be aware though that you will have a quite reduced current going out of it.
If 10W go in then 10W minus some losses come out, with 12V instead of 5V the current will be less then half.
And unless your bricks have a build in charge controller you will have little luck with the phone charger as there is nothing that would regulate the charging process.