I measured the voltage of an unused Energizer Lithium AA cell, and found that it was 1.806 VDC, and another one measured 1.807V. Wow! Looked like the Energizer Bunny was running on jet fuel! These are very potent batteries. Their voltage is 20 percent higher than the normal AA cell. These lithium cells are supposed to have a very long 15 year shelf life, so they make good choices for emergency flashlights. I got these with some bargain bin stuff, so they didn’t cost very much, but normally they’re very expensive – over 3 dollars for a single cell, I started to think about the ramifications of using these in certain circuits, especially Joule Thiefs.
There are certain circuits that are sensitive to excessive supply voltage. Take for instance the flasher at Bill Bowden’s Hobby Circuits website. The red LED is in series with a 1k resistor, and both are connected directly across the battery. If the forward voltage of the red LED is 1.8V, it will light up constantly when connected to a 1.8V lithium cell, and soon deplete the cell. The 1k resistor will limit the current, so nothing will happen to the circuit, but there could be a n expensive but dead lithium cell very soon. Red LEDs vary in their forward voltage; some of the ones I have need closer to 2V to be at full brightness, but might light up at 1.8V.
This also applies to the Joule Thief, but to a greater degree. The LED is connected directly across the battery – through the coil winding, but that is very low resistance. If the LED is red, it could light up without the transistor’s help. Most JTs use an LED with a voltage higher than 1.8V, so this shouldn’t be a problem.
But due to the Joule Thief’s high sensitivity to voltage variations, the 1.8V cell will cause the LED current to be much higher than usual. This could be excessive for the LED, but the biggest problem will be the short life of the 1.8V cell. The JT will suck it dry much sooner than an alkaline cell.
Save your money and get the cheaper batteries.
