I have one of those old spring and flywheel electric fencers. They could care less about the input voltage. I ran it off 12 volts with a pair of 8 ohm 20 watt radio shack resistors in parallel. they keep the RF pop down but still make it hurt when you touched it! Ran one like that all summer for several years on an old car battery. just charged it up once or twice a summer. We took that fence out so now its not needed anymore. Its in my antiques collection now.
The old ones put some amps behind that spark, the newer ones put a lot of volts in it to get the exagerated miles of fence thay can run numbers up.
Grab a spark plug wire on your car. Lots of volts less amps! Grab a plug wire off an old magneto driven engine, hurts way more! more amps less volts.
And yes those old fencers run off a battery charger on 6 volts just fine!