I've been getting a lot of questions about the new rechargeable 123A lithium-ion cells. These are often called RCR123A cells

123A rechargeables look like regular 123A cells in size but there are some very big differences. First, they come in two voltages, and two varieties in each voltage. Some are 3.6V (sometimes designated as 3.7V), some are 3.0V. Each of these can be either "protected" or "unprotected".

Unprotected cells have no special protection circuit to prevent over-discharge or over-charge. As a result they should only be used with lights and chargers that are designed to detect the voltage in the battery and shut down when the voltage goes outside of acceptable limits. Using these cells in a light or charger without this safety system could result in damage to the device, propery, and personnel. Lithium-Ion batteries don't react well (as in potential "kaboom") when overcharged or over-discharged.

Protected cells have a special protection circuit built directly into the cell. As a result they can be used in a greater variety of chargers and devices, but they shut down by themselves without warning when the voltage drops below the minimum acceptable limit. This can leave you in the dark instantly with no way to turn the light back on unless you have spare batteries. The circuits reset themselves only after being placed in a charger. These circuits also can trip spontaneously when two or more cells are used in series, so they are best used in single-cell applications.

3.0V batteries should be fine, from a voltage perspective, in most applications where normal 123A cells are used. Protected 3.0V cells are probably the safest choice as a direct 123A cell replacement.

3.6V can be a big problem. Normal 123A cells work at about 3.0V. The 3.6V cells obviously produce more voltage than the normal cells, but they also pack about 4.2V right off the charger. This means that they can permanently damage many lights designed for 3.0V. Put 2 regular 123A cells in a light and they run at about 6V. Put 2 of these 3.6V 123A rechargeables into a light and you could be pumping 8.4V though the light. This can be fatal to LED lights as well as incandescent lights.

Some folks like to use the 3.6V rechargeables in single cell 123A LED lights to coax a bit more brightness out of the light. If the flashlight is not specifically designed to remove the excess heat from driving the LED beyond the norm, the LED will be damaged over time, sometimes within just a few hours of use, resulting in an LED which produces little/no light or discolored light. This type of damage is usually NOT covered by manufacturers' warranties unless they specifically state that it is OK to use 3.6V rechargeable 123A cells.

Please check with the manufacturer of the device before trying to put these rechargeable 123A cells into ANY device at all.

In fact, if a manufacturer doesn't EXPLICITLY state that a particular battery type should be used in their device, assume that it can not. Instead of listing all batteries that can not be used in their devices, manufacturers list only the batteries that can/should be used. If it's not listed, it should be avoided.

PILA rechargeable Lithium-Ion 123A replacement cells a little different and are usually safe to put into almost any light because their voltage is a bit lower than comparable 123A cells. However, they are only designed for 2 or 3 cell applications, not single cell applications. The Pila cells shouldn't fry your light since they work at lower voltages, but your light may not be as bright as with normal 123A cells either.