Rechargeable

Rechargeable batteries
Rechargeable batteries are popular with torch applications, because of the oftentimes relatively high current draw, compared to electronic goods such as iPods. One set of rechargables can be re-used instead of several sets of alkalines. Not only does this save money over the medium to long term, it also results in less environmental waste.

Cell Sizes
Li-ion and other rechargeables are often referred to by their size code. The size code is easy to decipher: xxyy0 denotes a battery of xx millimeters in diamter by yy millimeters in length. The 0 at the end denotes it is a round cell. Generally when people on the forums talk about using some number cell, they are talking about li-ion cells, but really this is just a size, not a type of battery. Common sizes are:

 10440  AAA size 14500  AA size 15270  CR2 size 16340  CR123A size (also called a RCR123A) 17670  2xCR123A size 18650  Roughly the size of two CR123A cells, but a little bigger in diameter. This is also a standard industry size used in laptop computer battery packs. 

Cell voltages
A lot of confusion exists about battery voltage. Ordinary alkalines are rated to 1.5v, but typically are down to 1.2v when 50% used, and rapidly fall thereafter. This is why devices using alkalines often become less responsive over time, or dimmer. Rechargeable NiCad or NiMH batteries are rated to 1.2v, but will hold that until right at the end. You tend therefore to get a more consistent performance from rechargeable batteries. This improved consistency of voltage, is a major reason to prefer rechargeables for torch applications.

Torch circuitry
Three cells seem to work better in many flashlights. This is because most bulbs, especially LEDs, are designed to operate in the 3-4v range. 3 cells * 1.2 = 3.6v. No conversion circuitry is therefore required in 3 cell torches. The bulb can be powered directly off the battery cells.

Chargers
A good charger will use an appropriate charging rate (15 minutes is not appropriate), charge each battery individually, and will cut off when the battery is full rather than just after some period of time. It is very hard to find such a charger in stores. See Chargers for more information.

Battery types
These are the main types of rechargable batteries sold.

Nickel-Cadmium (NiCad) - 1.2V/cell
The was the first truly mass market rechargeable consumer battery. They retain charge well, but due to the toxicity of cadmium used in the cells, NiCad cells are being phased out of many markets. At this time, however, they are still readily available. NiCad batteries will tolerate a trickle charge or overcharge better than NiMH batteries, but they are also prone to memory problems where they will not fully charge and discharge.

Nickel-Metal Hydride (NiMH) - 1.2V/cell
Currently a popular cell type, with fast charging times, high capacity, and much improved recyclability. However, they do lose charge faster than the old NiCad types, so work less well as, for example, a TV remote battery (economically, you might be better off using alkalines in a TV remote or other very low drain device).

A newer generation of NiMH cells, called Low Self-Discharge (LSD), are able to hold a charge for a much longer time, claiming 75% charge after a year, instead of 50% discharge after 6 months for standard NiMH cells. The trade-off is that these new cells only have 75% capacity of current 'state-of-the-art' products. These batteries are marketed as 'Pre-Charged' because they still retain much of their factory charge after months or years on a store shelf. Brands include Sanyo Eneloop, Duracell Precharged, Uniross Hybrio, Rayovac Hybrid, Powerex Imedion, Sony Cycle Energy, GP Recyko, and others. Because they are NiMH batteries, they can still be charged in any NiMH battery charger.

Eneloops seem to perform better than any other LSD cells (Eneloop Self-Discharge Study Eneloop vs. Rayovac Hybrid Eneloop vs. Imedion). While most LSD cells retain 75-80% of their charge after a year, Eneloops can retain 85%. They are very consistent in their performance, retain their original capacity better after many cycles, and deal well with high drain applications (like a flashlight on "turbo" or a flash for a camera). Therefore if you ask on the forums what kind of battery to buy, you will be told to buy Eneloops. That said, the other brands also perform well, just not quite as well as Eneloops. If money is an issue, try the other brands, which can sometimes be purchased for as little as $1 each on a good sale. The term "duraloop" (a CPF nickname, not an official name) refers to Duracell precharged cells that are made in Japan and are white around the positive button just like Eneloops, and in fact are thought to be Eneloops re-badged as Duracells. Other Duracell precharged cells have a black top and are made in China, and are not called "duraloop."

Larger cells are also available as NiMH cells. A true D-sized NiMH should have a capacity of about 10,000 mAh. Watch out for Energizer, Rayovac, and other brand versions that are 2200-2500 mAh: these are mostly hollow and have no more capacity than AA cells. If lower capacity is not a problem, hollow adapters can be purchased (provided with some chargers like the BC-9009 and Costco Eneloop Pack) that allow you to put a AA in a D-sized shell.

Some batteries, including Eneloops, Duraloops, and Rayovacs, use a battery date codes that can be used to determine the date of manufacture.

Nickel-Zinc (NiZn) - 1.6V/cell
This is a new (2009) battery chemistry. It requires a special charger due to the higher nominal voltage of NiZn cells. They are 1.6V nominal, but are charged to 1.8V to 1.9V and are recharged at 1.5V. PowerGenix makes the batteries and chargers, claiming 2500 mWh of capacity (see Measures of Capacity). Quantaray is another company that makes NiZn AA cells. The higher voltage may make some flashlights brighter than on NiMH or Alkaline cells, but could also damage devices, especially if they use a number of AA cells in series. NiZn cells are advertised as capable of 200 full charging cycles, which is less than LSD NiMH cells.

Lithium Ion (Li-ion) - 3.7V/cell
Important: ***These are not the same as disposable lithium batteries sold by Energizer (Energizer e2 or Ultimate Lithium), Saft (who make it more confusing by selling 3.6V lithium cells that are not rechargeable), and other companies since those can not be recharged and will explode if you try it (example).***

Lithium ion batteries are typical in mobile phones, where they offer higher density of charge storage. In other words, more amps can be stored in less size and weight. They can also be referred to by the chemistry of the anode (positive) which is lithium cobalt oxide (LiCoO2). They do not suffer from the "memory effect." However, they cost more, and are not as long established as the older AA, C, D, types typical in torches.

Because of the higher voltage, Li-ion cells can only be swapped for NiMH or alkalines if the device manufacturer recommends it (Be careful! A fully charged li-ion battery has 3 times the voltage of a NiMH cell). Some flashlights that use two CR123A batteries can take a single 18650 battery (though this battery is wider than CR123A cells) which offers substantially more capacity than the two smaller cells together.

Li-ion cells must be monitored more closely than other chemistries. If they are charged too high (above 4.2 volts) or discharged too low, they can overheat or cause a fire. Some good advice is that if you don't have a volt meter and aren't willing to monitor the cells during use and won't be able to watch the cells the entire time they are in a charger, then you shouldn't be using them. See this thread for safe use of li-ion batteries. Cells are categorized as being "unprotected" or "protected." Protected cells include circuitry that will essentially turn the battery off if the voltage goes too high, too low, the drain is too high (the current drawn from a li-ion cell should not exceed 2C), or the polarity is reversed (not all protected cells protect for every one of those cases, and the voltage cutoffs vary as well). For this reason many people recommend protected cells. However, the low-voltage protection often kicks in only when the battery voltage goes below 3 volts, but 3.6 volts seems to be the point where most people recommend recharging the battery. The protection is there to prevent a fire, not to tell you when your battery should be recharged. See this CPF Thread for info about protection circuits. In the past, 10440 cells did not come with protection circuits because they were too small. However, some have recently (late 2009) come on the market (they may be too long to use in most AAA lights).

Li-ion cells are available from DealExtreme or KaiDomain under suspiciously similar brandnames like Ultrafire, Trustfire, etc., but most CPF members recommend AW cells (available through some online dealers and directly from AW via CPF Marketplace/Dealers under the latest AW's LiIon Batteries Sales Thread) which are of consistently higher quality and have better protection than the cheaper cells. Some brands of 10440, 14500, and 16340 cells are longer than their AAA, AA, and CR123 counterparts, so make sure you are getting cells that are not too long and that your light can deal with the size variance by reading the forums and reviews of the batteries. Just because a cell is sold as an 18650 does not mean it is exactly 65 mm long: some are 68 mm long.

Lithium-Polymer (Li-Poly) - 3.7V/cell
Offers about a 20% increase in charge density over Li-ion, but again, offset by cost. These batteries are usually rectangular in shape and are used more often in MP3 players and cell phones than flashlights.

Lithium Iron Phosphate (LiFePo4) - 3.3V/cell
New LiFePo4 batteries have about 20% lower energy density than Li-ion, but they become even after about a year of use. LiFePo4 also have much longer life (typically about 10 years, as opposed to 2-3 years for Li-ion) and can endure almost double charge/discharge cycles than Li-ion. They also have lower self-discharge rate, and are able to deliver higher current, and are more resistant to thermal runaway. However, as they are not as popular as Li-ion, they are still somewhat more expensive. Because they can not be charged to 4.2V, they require a special charger (or hobby charger that can be set to 3.6V max). Walmart sells 3.2V AA-sized LiFePo4 cells by Westinghouse for solar lighting (so no charger included).

Lithium Manganese (LiMn) 3.7V/cell
These cells allow higher drain (5C) than li-ion cells without some of the dangers. AW sells these as IMR. They can be charged in most li-ion chargers, but some li-ion chargers will overcharge LiMn cells. LiMn cells should be charged at 1C to 2C. Due to lower overall capacity, these batteries fill a niche with flashlights demanding very high current where runtime isn't that important.

Sealed Lead-Acid (SLA) - 2.0V/cell
A classic rechargeable battery. Never much used for mobile application due to poor size / weight characteristics. However, very reliable if kept topped up, cheap, and as such, often still used in cars, if not torches.