bought these AA 1200 mah 14500 3.2 v lifepo4 batteries, should fit in the on board holder.
then remove some components, like the voltage regulator circuit and feed direct (and only) from the battery… aiming for very low power (hard coded) because they will be very close stationairy to my gateway.
disclaimer : I will have to test these extensively, because I don’t know (China you know) if they really are 1200 mAh what the label and shop tells me. 
someone did a review
https://lygte-info.dk/review/batteries2012/Litelong%2014500%20LiFePO4%201200mAh%20(White)%20UK.html