LiCo02 [Lithium Cobalt Oxide]:
General Information:
3.7V cells ordinarily found in consumer devices like cell phone, laptops, MP3 players, PDAs, etc. These energy dense cells have found their way into flashlights in recent years as the demand for more compact, lightweight, rechargeable power solutions has gone up. When Someone says "lithium-ion" without stating a specific lithium chemistry this is almost exclusively the chemistry being discussed. LiCo02 cells are available in a variety of sizes including AAA and AA and CR123 size, but they use a different naming structure for size labeling. The size of the cell is described in a string of numbers that define the dimensions in millimeters. So a AAA li-ion cell is a 10440. (10mm x 44mm x "cylindrical"). AA = 14500. CR123 = 16340. other common sizes: 17500, 18500, 17670, 18650. These are NOT USUALLY compatible with devices that call for a 1.5V or 1.2V alkaline/NiCD/NiMH, however, there are SOME flashlights on the market that ARE compatible with the higher voltage. Most flashlights that are compatible with them, will indicate so in the product details. These should never be considered direct replacements for 3V CR123 primary cells as they have substantially higher operating voltage. Always check for compatibility with the higher voltage on devices before using them. There are a large number of LED flashlights that are compatible, and a number that are not.
Advantages:
Highest available energy density commonly found in rechargeable cells. Especially in the larger sizes, 17500 and up. Very efficient charging and discharging with the least amount of energy expelled as heat. Higher per-cell voltage means less cells are needed to achieve certain voltage requirements. When treated properly, these have exceptional cycle life (hundreds+). Li-Ion also has exceptionally low self discharge.
Disadvantages:
Smaller size Li-Ion cells, like 14500, RCR123 (16340), and 10440, do not generally live up to their label capacity claims and usually have lower energy density than alternative chemistries in the same size. LiCo02 is not tolerant to abuse, these cells must be used within the bounds as listed by the manufacture. Rapid charging (faster than 1 hour) and rapid discharging (faster than 30 minutes) is not possible with these, so they are not necessarily as flexible as Nickel chemistry cells. In order for loose li-ion cells to be used in devices like flashlights, they need to have protection circuits installed for safety reasons, which adds a layer of potential failure to the device. Li-Ion is more prone to vent-with-flame/explode than Nickel chemistry cells if abused. LiCo02 also suffers from the effects of aging whether it is being used or not, though in recent times, this has becomes less and less of a factor with li-ion cells. Used to be that they would be considered "dead" after a few years from production whether they were used or not. Now they seem to be lasting 7-10 years without much trouble.
Charging:
The proper charging technique for LiCo02 must be followed to tight specification for maximum safety. The cell should be charged at a 1C or slower rate at a constant current until the cell reaches 4.20V, at which point the charger should hold 4.20V (constant voltage) until the charge current drops to some fraction of the original charge current (usually around 0.05C give or take) (varies from charger to charger, but there is probably an ideal termination current based on cell capacity that would be impossible to have perfect on a charger designed for multiple cell sizes). Charging in series packs can only be done properly with balance taps on the pack and a balance charger. Li-Ion cells in a similar state of charge can be charged in parallel as if they were a single cell. Charging above 4.20V will cause increased rate of internal oxidation, reducing effective cycle life and capacity, while simultaneously increasing the risk of explosion/fire. 4.30V will not usually cause an immediate danger, this is where most protection circuits will kick in. Use a high quality charger to perform charging if possible. Most cheap chargers do not follow the proper charging requirements. The Pila ICB is most often recommended and is worth the $40 or so.
Discharging:
LiCo02 cells should not be discharged below ~3.0V under a load, (varies by manufacture). A good rule of thumb is that when the cell reaches ~3.5V open circuit, it is dead and should be recharged. Over-discharging a cell will increase the rate of internal oxidation leading to reduced capacity, reduced cycle life, and increased likelihood of explosion/fire. Different cells are rated for different maximum discharge rates, usually specified between 1.5 and 2C. (C ratings are having to do with time, a 2C rating, means 30 minutes, 1C means 1 hour, 4C means 15 minutes, 0.5C means 2 hours, etc etc, bigger C). Check to see what your cells are rated at and use them in an application that is within the bounds of the maximum discharge rate.
Safety Concerns:
Abusing these cells by overcharging, over-discharging, discharging too quickly or charging too quickly, or causing physical damage of sorts can increase the risk of fire/explosion. These cells need to be treated with a higher level of respect and care than NiMH or NiCD. Use protected cells whenever possible to reduce the risk of an incident. Keep in mind that li-ion is most apt to flame/explode while charging, not while discharging, so to maximize the safety of a questionable cell, charging in a fireproof box in a well ventilated area is recommended. A flaming/exploding LiCo02 cell releases Hydrofluoric acid. Breathing the gas or coming into direct contact with the remnants of a LiCo02 fire can cause severe poisoning that can cause major illness or death.
Myths:
"I have a protection circuit, so don't have to worry about over-charging or over-discharging." This is the most common misunderstanding. The protection circuit is set to prevent dangerous events from occurring, it does not prevent smaller scale overcharging and over-discharging. They are often set at ~2.5V and ~4.3V whcih would not be healthy termination points for normal cycles.
"My cell is rated at 900mAH and 2C, so it can handle a 1.8 amp discharge." (I was guilty!)
The C ratings assigned are based on time, not label capacity. In reality, there are many 900mAH RCR123 size cells out there that are actually only good for 500mAH capacity or less at 2C, which means their maximum discharge rate is only 1 amp.
