Q: Can I use (rechargeable) (lithium) batteries in my normally alkaline battery light?
A: The short answer is "no", unless recommended by the manufacturer.

The long answer:

Lots of people would like to use more "environmentally-" and "wallet-" friendly NiMH cells whenever they can. Others would like to take advantage of the better performance and cold resistance of new lithium AA and AAA cells that are now on the market. However, there are some serious issues you need to consider before trying either.

NiMH in an Alkaline Cell Designed light:

Alkaline cells don't like to give up too much energy at one time. This is due to the internal design and chemicals used. You see, the chemical reaction that generates electricity in an alkaline battery can only happen just so fast. As a result they are great for low-energy-draw devices, but poor in high energy draw devices. Some manufacturers take advantage of the slow chemical reaction in alkaline cells and use the slow reaction to limit how much energy is available to the device. NiMH cells work at a lower voltage than alkaline cells (1.2 vs 1.5 Volts) and can supply energy at a much, much faster rate. So what happens when you put NiMH cells in a light designed for alkaline cells? Generally one of two things: Either the light works, but at a lower output level, or it becomes VERY bright and either burns itself out or runs for a very short time.

For example, I tested a light with 28 LEDs in it with alkaline cells - it ran for about an hour to 50% starting output. When I put in NiMH cells it ran for 8 MINUTES! It was much brighter, and it started to melt the reflector. This light was designed to take advantage of the fact that the chemical reaction in alkaline cells is slow and the therefore the alkaline batteries were actually regulating the current that the LEDs could get. Once the wide-open NiMH cells were put in the light, the LEDs drew as much as they could, started to overheat, and drained the batteries very quickly.

NiMH cells also have a problem with something called "self discharge". Over time they lose a portion (or all!) of their charge just by sitting around. I've had high capacity NiMH cells register as dead only two months after a full charge with no use. This makes them inappropriate for use in flashlights that just lie around. In late 2006 several companies started coming out with Low Self Discharge NiMH cells that can still hold 85% of their charge one year after charging. This is a huge breakthrough and I strongly recommend looking into these cells for those not-so-commonly used devices. I never use my old regular high capacity NiMH cells any more since they were often dead when I finally got around to using them. I get more usable power from the L.S.D.cells over time.

Lithium cells in an alkaline cell designed light:

What about lithium batteries? Well, they are rated at 1.7 Volts, not the 1.5 Volts of Alkaline cells. The higher voltage can damage some devices, especially ones that take several batteries in series. A light that takes 4 AA alkaline cells is considered to operate at 6 Volts. Put in four lithium AA cells and you now have 6.8 Volts. Also, the chemical reaction that generates electricity inside a lithium cell happens very fast, just like a NiMH cell. So putting lithium batteries in a device designed for alkaline batteries results in more voltage and the ability to provide energy very quickly. Double trouble! This could easily fry a light designed for alkaline cells. A number of folks discovered this when they tried to use lithium AA cells in the Princeton Tec Surge. The result was a toasted light.

What about the new Lithium Ion 123A rechargeables in a 123A light?

The problem with these is a little different. Some of these cells are at 4.2Volts (instead of the normal 123A 3.0Volts) when they come off the charger and the result is an instantly fried light. Some have low voltage protection inside the battery which shuts off the cell when it gets drained to a certain point. Some don't, which can cause damage to the cell and the device it is in, if the cell is not used properly. Be extra careful using these cells and only use them if the manufacturer recommends them.

Conclusion:

ALWAYS check with the manufacturer and see if they recommend using rechargeable or lithium batteries in the device. If not, DON'T. If you ignore their recommendation and fry the light, don't go crying back to them about it - you voided the warranty by using a non-recommended battery formulation.

Here's a quick chart of information on some common batteries (and some not-so-common):

Confused by Rechargeable Lithium Ion cells? I don't blame you. Here's a handy table of different cells.

The 5 digit number deciphered: 18650 = 18mm ( diameter ) x 65mm ( length ) where the last ' 0 ' stands for cylindrical cell