Electronic – Switching Electric Fence Live Wire

This is actually quite a simple circuit which works by stepping up the collapse of a magnetic field in a small audio transformer.

The schematic looks like this:

I think something like this part should work for the transformer.

If you touch the two output wires, you'll get a very small electric shock as you release the push button.

I'd try connecting a small transformer (one designed for a flyback power-supply) with a high step-down ratio between the energized cable and ground, you could then rectify the output and do whatever with it. The HV coil impedance should be high enough to avoid a tremendous draw on the cattle killer, but I don't know much about them. It may take a long while to charge a battery, but possible. If you just want to power a sensor, I'd use a capacitor.