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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?)

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  • \$\begingroup\$ If you have multiple cells, it will still work, but you can then use other simpler methods to light LEDs. With a single cell, this is much the only thing that will work. \$\endgroup\$ – Neil_UK Jul 22 '17 at 3:19
  • \$\begingroup\$ A joule thief's claim to fame is that it can use a single, "used" alkaline cell (which wouldn't work on much else anymore and probably should be discarded) and boost up the voltage enough to run a blue (or 'white') LED. This is probably why it was written that way in wiki. It also can be easily hand-built and is quite tolerant of low skill level construction and part details. If you have available more batteries to place in series, they probably can drive the LED by themselves. Besides the joule thief is arranged to simply drive the LED directly if there's enough battery voltage present. \$\endgroup\$ – jonk Jul 22 '17 at 4:33
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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.

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