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.

It is worth mentioning that a "battery" consists of multiple "cells" therefore most "batteries" mentioned here are actually "cells". The lead acid batteries mentioned later are true batteries, being comprised of several individual cells.

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:

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 with flashlights). Rechargeable NiCad or NiMH batteries are rated to 1.2v (starting at 1.4v at first) but will hold 1.2v 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 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 or if there is a circuit, it will work more efficiently. 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 rechargeable 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. 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 typically 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 LSD cells usually have lower capacity (75%) than some non-LSD NiMH cells. These batteries are marketed as 'Pre-Charged' because they still retain much of their factory charge after months or years on a store shelf. Among LSD brands, Sanyo's Eneloop batteries have the best reputation and performance (retaining 85% of their charge after a year). Other brands include Duracell Precharged, Uniross Hybrio, Rayovac Hybrid, Powerex Imedion, Sony Cycle Energy, GP Recyko, and others. Because LSD cells are NiMH batteries, they can still be charged in any NiMH battery charger.

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
Introduced in 2008, Nickel-Zinc cells have already disappeared from the market. They require 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 and Quantaray made the batteries and chargers, claiming 2500 mWh of capacity (not mAh, see battery capacity). 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. Some people found the performance of the cells to be inconsistent, while others saw good performance. In February of 2011, Powergenix said they would no longer make AA batteries and chargers and instead focus on industrial uses of larger NiZn batteries.

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).***

Though there are other lithium ion chemistries (listed below), but usually when people talk about lithium ion batteries they are talking about lithium cobalt, LiCoO2, after the chemistry of the battery cathode (these batteries are also sometimes called LiCo, LCR, or ICR). They are available in a wide variety of sizes including CR123A, AA, and AAA. However, 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 and 18650's will not fit in some SureFire and Quark lights) which offers substantially more capacity than the two smaller cells together.

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 and/or lights. Just because a cell is sold as an 18650 does not mean it is exactly 65 mm long: some are 68 mm long.

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 twice the nominal milliamp-hour capacity in milliamps - so a 2000mAh cell should not be asked to produce more than 4000mA (4 amps)), 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. A battery at 3.6 volts measured at rest should be reacharged. However, the low-voltage protection often kicks in only when the battery voltage goes below 3 volts, which can be low. 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.

Li-ion cells have fairly low self-discharge, but in order to prolong the life of the battery, should be stored partially discharged.

Recommended batteries: Many CPF members recommend AW (available through some online dealers and directly from AW via CPF Marketplace/Dealers under the latest AW's LiIon Batteries Sales Thread) or Redilast cells (also on CPF) which use high quality cells and have their own protection circuits that may perform better than cheaper cells, but are very expensive ($12 each and up). Both of those brands use Panasonic NCR18650A 3100mAh (and NCR18650 2900mAh) cells which can also be purchased from other providers (but are still expensive). These are the highest capacity cells available in this size (in 2012, Panasonic introduced the NCR18650B with 3400mAh capacity). Trustfire has a line of li-ion batteries with flames on the label that are a decent budget alternative in nominal capacities of 2400 or 3000 mAh though actual capacity will be less (roughly 2100 and 2600 mAh), however it is hard to find genuine ones even at DealExtreme which has been a reliable source in the past. eBay is a notoriously bad place to buy lithium ion batteries, with fake brand name batteries and false claims regarding capacity and battery protection. XTAR (using 2600mAh Sanyo cells) and Hi Max (using 2600 mAh Samsung cells) brands are better than Trustfire, but cost a little more ($14.50 per pair for XTARs via this BLF thread).

Reviews: Here is HKJ's comparison tool of a large number of li-ion 18650 battery brands, mostly higher end, plus this older comparison by DrJones of budget brands. Here's Mitro's group of discharge graphs of different brands at different currents.

Lithium Iron Phosphate (LiFePO4) - 3.3V/cell
New LiFePO4 batteries have about 20% lower energy density than Li-ion, but also have much longer life (typically about 10 years, as opposed to 2-3 years for Li-ion) and can endure almost double the number of charge/discharge cycles of Li-ion. They also have lower self-discharge rate, 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 (LiMn2O4) 3.7V/cell
These cells allow higher drain (5C, up to 8C) than LiCo cells without some of the dangers. AW sells these as IMR (supposedly based on "LMR" for lithium manganese rechargeable, but the lower case "L" was mistaken for an "I" and never corrected), and Sony as Konion. 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. While they are safer than lithium-ion cells (therefore they do not come with protection circuits), they can still be made to explode if provoked sufficiently, unlike the LiFePO4 cells which cannot. As a budget alternative to AW or Sony cells, Best In One has generic LiMn cells in a number of sizes. These cells are also sold in the US by Shaotech at his store and on CPF Marketplace.

Lithium Manganese Nickel (LiNiCoMn) 3.7/cell
Similar to IMR cells above, and sometimes called INR, these batteries fall somewhere between LiCo and LiMn cells in safety. They do not usually have protection circuits, are not subject to thermal runaway, and are capable of high discharge rates of up to 10 amps. Panasonic makes 2250mAh cells that have the product number CGR18650CH and are available from Callie's Kustoms or International Outdoor.

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. They can be made in almost any shape if required, instead of a cylinder shape typical of lithium ion cells, they consist of a bunch of chemicals in a plastic bag usually with a rigid case around them.

Sealed Lead-Acid (SLA) - 2.0V/cell
A classic rechargeable battery where cells are hooked up in series to generate higher voltages. A 6V battery would have 3 cells and a 12V would have 6. They are sometimes used in HID lights, but not for more portable lights due to poor size/weight characteristics. However, SLA batteries are very reliable if kept topped up, cheap, and as such, often still used in cars, if not torches.