Odds of having an explosion with LiFePO4 are pretty low...
Something to consider: Since LiFePO4 RCR123 cells are unprotected, when they are used in series, they have the propensity to be reversed charged toward the end of the discharge. If you make a habit of running them down until the vaping performance drops way off then reverse charging of the lower capacity cell in the group is more likely. Reverse charging is one of the best ways to destabilize lithium chemistry cells. Reverse charging in a series chain of cells is only possible when cells in that chain have different true capacities. It is normal for all cells from a batch/size/type to have some variance of a couple-few percent, however, cheaper brands with lower quality control often have much wider variances in capacity from cell to cell. Tenergy has been a known offender of poor consistency from cell to cell for as long as they have had a name. Labeling cells into groups is only beneficial if you test them for capacity and group them based on those tests. Maximum safety will come from pairing cells with the closest possible true capacity into groups and keeping them together. Most people don't have any way of doing a fancy charted discharge test, but you can gather enough useful information with a $5 multimeter from harbor freight by comparing the voltage of each cell at the end of a discharge and using a few brain cells to organize them from highest to lowest capacity in a series of discharges. Then pair the closest capacity cells up.
LiFePO4 is often considered a "high drain" chemistry, however, that capability really only applies to specialty LiFePO4 chemistry cells that use nano-carbon tech to maximize the surface area of the iron cathode (effectively reducing cell resistance). See A123 brand LiFePO4 cells for an example.... I have not seen any study to prove or disprove the safety of a LiFePO4 cell that does not have this tech used in a high drain rate application.
The true capacity of an RCR123 LiFePO4 cell is about 450mAH. Under the load of a typical atty, more like 400mAH or less. Total available energy for your ecig will be in the neighborhood of 2WH per charge.
An 18650 is easily 8-10WH. With an LR atty, the performance can be about the same, and the runtime quadruple or better... Oh, and no battery stacking/matching to deal with.
Eric
