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.
LiFeP04 [Lithium Iron Phosphate]:
General Information:
Often sold as 3.0V rechargeable cells, these are technically 3.2V li-ion cells based on a new cathode material that is inherently safe. These cells can *often* be used as a direct replacement for CR123 primary cells in lights that can tolerate the slight voltage difference compared with primary cells. (keep in mind, that primary CR123s actually operate below 3V when under a load, more like 2.5V). For the most part, LED lights that normally run CR123s can run these no problem, incans usually can not unless regulated (rare). LiFeP04 cells are currently available in only a few sizes, including "RCR123" (16340) and 18650 and few others we won't discuss at this time. I lean towards recommending these over the 3.0V regulated cells discussed below.
Advantages:
Safe chemistry won't explode or flame, can tolerate some abuse without too much issue, does not need protection circuit like LiCo02 to be used in consumer devices, so less failure points. Higher voltage per cell than NiMH/NiCD means less cells are required to achieve voltage desired, can often be used where 3.7V cell is not advisable. Offers a safer more reliable alternative to 3.0V voltage regulated LiCoO2 cells.
Disadvantages:
Much lower energy density compared to LiCo02, generally speaking, around 50% less stored energy per volume. Needs special LiFeP04 charger, one more device to have floating around. Label capacities are generally way overstated on smaller cells. Expect 200-400mAH from 16340 size cells depending on load. For comparison purposes, a CR123 primary has between 1200 and 1500mAH capacity. So these will really hurt runtime.
Charging:
Charging rate is fairly flexible on these, most small RCR123s in this chemistry are sold with matching charger that charges in an hour or a few hours. Charge is usually just constant current until voltage reaches about 3.6-3.8V (varies by manufacture) followed by some constant voltage until the current drops to around 0.05C give or take. (when charged CC to 3.8V it's probably pretty close to full, when terminating at 3.6V, some CV is probably required to finish the charge) overcharging won't cause too much damage provided it isn't done too rapidly or for too long. A LiFeP04 cell can be charged in a "4.2V" LiCo02 charger in a pinch, but you would want to pull the cell manually sometime around 3.8V if possible(use volt-meter to check). As far as I understand, these can be charged in series or parallel most of the time, but should be isolated on occasional charges to balance them out. (I could be wrong on this)
Discharging:
Discharge capabilities vary by cell manufacture and size. Larger scale LiFeP04 cells were originally invented for use in high drain applications like power tools and electric cars. Small scale LiFeP04 cells aren't quite as tolerant to high discharge rates and tend to "fall on their face" at discharge rates higher than 2C. But Discharging even the small cells at higher than recommended rates is still not really dangerous, just wears out the cells more quickly. Discharge should be terminated at 2.0V whenever possible. Discharging below 2V will degrade the cell more rapidly, some cells seem to be more tolerant to over-discharging than others.
Safety Concerns:
Very few issues of safety, I would classify them as similar in safety to NiCD/NiMH cells, major heating from constant abuse might cause a hot gaseous venting or leak, but this chemistry does not typically ignite.
Myths:
"It's a 3V cell so will work in any device designed for CR123 primaries."
They will work in most devices, but any direct drive incandescent will likely blow it's bulb on these cells.
3.0V RCR123s not labeled LiFeP04:
General Information:
These are usually 3.7V LiCo02 RCR123 cells that have a voltage regulator installed to shunt the operating voltage down to around 3.0V to make them more compatible with voltage sensitive devices. These are often the alternative to the LiFeP04 cell, or you could say, that the LiFeP04 cell is the alternative to this. Most of these cells are sold as protected cells, but I just found one the other day online that is voltage regulated but NOT protected. I highly recomend picking protected versions of this type of cell if you decide to use them. Overall I lean towards recommending the LiFeP04 cells for applications where these are often specified.
Advantages:
Can often work where 3.7V cells would not. Usually has slightly better capacity compared to LiFeP04 RCR123s.
Disadvantages:
More components to fail. The voltage regulating component of these generates heat right next to the cell, which is less than desirable for cell longevity. The cell itself has to be smaller to make room for additional components, or the cell ends up being too long for some devices. Accidentally putting a 3.7V cell into a charger designed for these would probably cause an explosion. Not a good charger to have floating around in a collection of various cells and chargers and devices.
Charging:
Charging must be done on the charger that is sold with the cells or recommended for the cells as these things vary from one manufacture to the next on their recommended charge voltage termination from 4.4-4.5V. The cell itself still needs to be terminated at 4.20V, but the charger has to overcome the voltage regulation device "backwards" through the circuit, so to speak, (if that makes any sense). Do NOT use one of these chargers on any cell other than the cells it is sold with!!!
Discharging:
Often limited by the voltage regulating device to around 1-2C, discharging continuously above 1C IMO could cause overheating of the cell or failure of the voltage regulator. Discharge should be terminated at around 2.0-2.5V give or take (follow manufacture recommendations).
Safety Concerns:
Same as LiCo02 cells above. Abuse can lead to vent with flame, these are IMO more susceptible because of that heat making deice attached to the cell.
Myths:
"It's a 3V cell so will work in any device designed for CR123 primaries."
They will work in most devices, but any direct drive incandescent will likely blow it's bulb on these cells.
---------------------
I have highlighted in blue the more important points.
I should also point out, that there is now a 4th chemistry of RCR123 available, from AW, a Lithium Manganese Oxide cell, (LiMn), which shares the same charging requirements as a LiCo cell, but has higher maximum safe discharge and charge rates, and is made from a safe chemistry that is not likely to vent with flame if abused. They are sold as "IMR16340" cells.
