According to this Wikipedia article:
It can use nearly all of the energy in a single-cell electric
battery, even far below the voltage where other circuits consider
the battery fully discharged (or "dead"); hence the name, which
suggests the notion that the circuit is stealing energy or "joules"
from the source. The term is a pun.
Why is it that having multiple batteries in series would not work?
What would happen if the battery was full? (I would assume the device would not oscillate, therefore doing nothing until it was low enough to trip the transistor?)
Best Answer
A joule thief is nothing but a primitive switching regulator. There is nothing about it's design that restricts it to a single 1.5V cell. Most use a 1.5V cell for size reasons. And because a single 1.5V cell, at 1V, can easily be boosted to 3+ volts to power a white or other 3+ Vf leds.
I've used a joule thief self osscilating circuit with 2 AA to power an LED for months without issues, with a random ferrite ring and Telco wire. Nothing optimized.
If the cell is full, the oscillating circuit gets saturated faster and less energy is needed to boost it to saturation. It will simply last longer.