Combining 3 cells added up to about 12.5V with no load. The voltage drop was quite significant while preheating the extruder and also at the end of the print it dropped to 9.5 V. After printing it came back to 11.2 V but maybe combining 4 cells in series would have been a better solution with a starting 16 V.

This is no science, but just some thoughts on what to do next time because the cells were already half dead, scrapped from an old thinkpad T60 or T400 i think. So it is possible that this could work better with shiny new batteries.