How to estimate energy of piezo igniter discharge? Question for fun. We all know piezo igniters, these small friends, like following one:

All of them are probably quite similar: same spring, same piezo, same hit force, and so on.
Also I have some 18650 LiIon batteries, rated $3.7 \text{V} \cdot 2.2 \text{Ah} = 8.14 \text{Wh}$.
So for a joke I wondered: How many clicks could be required to charge battery?

I know, that $8.14\text{Wh}$ is roughly $30,000 \text{Ws}$, or joules. And I imagined that my theoretical conversion device has 75% conversion efficiency, so at a total, I need to produce 40,000 joules of energy with clicks. But I don't know how to estimate single click energy.
 A: Here is how to have this fun. 
Using a spring force gauge and a ruler, measure the spring force in the igniter as a function of button push distance for a series of different distances. Take your measurements all the way up to the push distance point where the spring mechanism finally lets go and releases the hammer inside to strike the piezo stack. 
On a piece of graph paper, make a chart with button push distance on the x-axis and measured spring force on the y-axis. It will probably be a straight line, or close to it, which starts at zero on the x-axis and ends at the release point distance. 
Measure the area under that plot line. This is numerically equal to the total amount of mechanical work stored in the internal spring at that instant when the release mechanism lets go. 
Convert that value of mechanical work into electrical work units and assume that all the mechanical work is converted into electrical work by the piezo stack. This gives you the electrical work output of a single button push. 
Now you can solve for an estimate of the number of button push cycles it will take to charge that battery!
