According to BritNerf user Hammy:
I am now modifying a RS (commisson) and this one has 4 trustfires with solarbotic motors
I used my fluke multi metre to make the measurement.
When the 4 trust fires are not loaded, then they read 15.86V, however once the flywheels are spinning, this voltage drops to 6.5V. and the current flowing is 1.7A...
Whoa! That is a stupendous amount of voltage sag under a very minor load!
With the variability of Fire brands of cells and the fact that the rest of the system (including the cell holder, which is a VERY significant amount of resistance) was never described, this cannot be a useful numerical data point.
It's still a stark reminder of why these batteries and this practice are not a good idea, though. I will cover and explain the situation with 14500 cells in more detail in one of the upcoming battery guides, but suffice to say, they are a very poor choice, if you didn't already know.
The gist of it is that this type of Li-ion cell on its own, even in a best case (such as cells from the likes of Sanyo) is totally unsuitable for the uses people keep putting them to in nerf! This type of cell is designed for a 1C or 2C discharge rate - 1.6 amps at the very most - and even at that rate, the cells are sagging a good bit and heating up. Its internal resistance is far too high to perform anywhere REMOTELY near how a proper battery choice will on a mod - and it is also being abused quite severely. The quality problems of Fire cells and resistance of cell holders only compound that, perhaps significantly.
Carry on, and seriously, lay off the TrashFires!