What is the lowest voltage that can kill you?

Voltage doesn't harm you. Current does. If you can sufficiently reduce the resistance

at the points where the power supply connects to your body, you can do yourself

a lot of harm with a very small voltage.

I am an electrical engineer. We were discussing this exact question in the engineering

dorm, on a night more than 50 years ago that I can never forget. A debate got going

over whether it's possible to kill someone with 12 volts. Some said yes, some said no.

A very stupid proof of concept was proposed, and played out to its unfortunate

conclusion. A method of low-resistance connection was proposed, and one of those

who said it couldn't be done bravely demanded to be the one to demonstrate it,

and show everyone else that it couldn't be done. The power source selected was

a 12-volt car battery, the means of connection are better left not described, and

for the rest of the night, each of us in the group was interviewed privately, one

at a time, by the police, after his body was removed from the dorm.

So I'll say: less than 12 volts, if you know how to do it.