Thought I'd post a few answers, more coming when I have time. Apologies for the long section on batteries, that safety information takes up a lot of room!
1) LED Technology - Why are LED flashlights so great?
Why choose an LED flashlight? There are three main reasons: efficiency, durability, and versatility. Compared to other light sources, LEDs use the electricity they consume very effectively, which means that they produce more light using the same amount of electrical power, or can produce the same amount of light for much less electrical power. The difference between an LED and a bulb is staggering, since even the cheapest LEDs produce at least twice as much light as a bulb for every unit of energy they use. All of this means that your LED flashlights will run longer and brighter than a regular bulb, and are available in smaller sizes since they don't need as many batteries!
Have you ever dropped a flashlight and watched in frustration as the light went out on striking the floor? Been in the middle of a task and had the bulb burn out? You're not the only one. Sudden impacts can easily break the filament of a bulb, and everyone knows they can burn out. The durable LEDs used in flashlights don't have either of these problems, though, since they don't have glass globes or filaments to break. Over a very long period of time an LED might get dimmer, but this period of time is usually measured in tens of thousands of hours (that's 10,000 hours), so for most practical purposes this is never an issue - and of course, LEDs never just "go out." The exception to these rules is if the flashlight is poorly designed, in which case the LEDs (also called emitters) can get too hot or are forced to use more power than they are intended to, which can drastically shorten the time it takes for them to dim. This is VERY COMMON in cheap flashlights, since some manufactures assume the lights will not be used very often. Always remember, you get what you pay for!
One of the greatest things about LEDs is that a flashlight manufacturer can arrange it so that they produce different amounts of light. This means that instead of being stuck with one brightness like a normal light bulb, a person with an LED flashlight can choose to use more or less light for any situation, which saves power. For instance, if you need to rummage in your purse or find your way down a dark movie aisle, you don't need much light, no more than 10 lumens at most - which doesn't take much electricity to produce. However, if you went to search for a dog that has gotten loose in the area around your residence you would need much more, usually at least 100 lumens and often many times more than that - which takes much more electricity to make. Since LEDs can produce different amounts of light you can use the same flashlight for many different tasks and only use as much power as you need, reducing the need to replace or recharge your batteries.
2) Lumens vs. Candlepower - Sorting through advertising gimmicks
To understand the advertising about lights which claim "80 lumens" or "1,000,000 candlepower," you need to understand what those things are. To put it simply, lumens are a measurement of how much light a device produces, and candlepower is a measurement of concentration of the beam a device produces. These are not the same thing.
To use an analogy, imagine you have an adjustable water hose that shoots out 80 gallons of water every minute, and that those 80 gallons of water are spread out in a 6-ft. wide cone so that if you sprayed a wall for 1 minute, every square inch of wall would get about 2.5 oz. If that hose were a light, you could think of the 80 gallons as 80 lumens, and the 2.5 oz. of water per square inch as the candlepower. However, let's say that you adjust your hose so that it shoots the same 80 gallons of water per minute, but that it is concentrated into a very thin jet that hits a 3-inch circle on the wall instead of the 6-foot one. The wall would still be hit by 80 gallons of water in the course of 1 minute, but each square-inch of the wall would get almost 1450 oz. (around 11.3 gallons) from the jet instead of 2.5 oz. Again, we can think of the 80 gallons of water as 80 lumens of light, and the 1450 oz. as the candlepower.
The important point here is: Just because a light produces more candlepower, that doesn't mean it produces more light. More candlepower isn't always better, either – would you use a laser-pointer (which has high candlepower for its brightness) to light up a room? In other words, some flashlights produce a very thin, strong beam which travels for great distances, something that is great for spotting objects a good distance away but very poorly suited to lighting up a general area or searching for something up close. In general, lights that have wide beams good for close-range work are poor for long-range work and vice-versa. A light that has an "in the middle" beam is usually the best choice for most people. Thankfully, fewer and fewer flashlight manufacturers are bothering to rate their products with candlepower, and lumens are being listed more often as time continues. A good rule of thumb is, "Lumens are brightness, candlepower is beam shape. Buy the number of lumens you need with the beam shape for the job."
3) The Truth About Runtimes - Quality vs Quantity
Have you ever purchased a flashlight which advertised that it would run for 200 hours, but when you actually went to use it only lasted for 5 hours before it became so dim that you had to change batteries? This is because the 200 hour number usually means 200 hours of "useful light" which is usually defined (somewhere in fine print often not found on the package) as less than 3 lumens or so. Not exactly the same thing as 200 hours of full brightness! The problem is, in a normal circuit, LEDs and bulbs will draw less energy from normal batteries as they get weaker and become dim as a result. This phenomenon explains the fact that cheap flashlights are reasonably bright at first, but steadily dim as less and less energy can be taken from the batteries, which end up feeding a small amount of energy to the emitters for a long time. The good thing is, many of the modern LED flashlights (and a very few incandescent) on the market use regulation circuits to balance the 5-hour and 200-hour numbers out by keeping the energy coming from the batteries at the rate it started with. This means you get more time at full brightness and less time with less brightness.
As with everything, one solution isn't always the best one for every need. Some lights have regulation circuits which will remove practically all the energy from the batteries before allowing the brightness to decrease, which means that the light will usually go from full-power to dead with a short and very-low dim "moon-mode" before the light shuts down completely. For most situations, this is fine, and results in the best brightness for the longest period of time. However, some lights have regulation circuits that will keep the brightness the same for most of the batteries' life, but intentionally dim the light before the batteries are completely dead to give the user a longer and brighter moon-mode. Neither setup is better, it just depends on what you want. Regardless of which you choose, an effective and efficient regulation circuit is a critical part of a high-quality light.
4) Battery Chemistry - Choosing the right battery type
Thousands of pages could be (and have been) written about the pros and cons of different batteries, but thankfully it's not necessary to go into great depth to make good choices in this area. The first step is to understand the severe limitations alkaline batteries place on the performance of a flashlight. No variety of alkaline battery carries very much energy, and the faster you try to extract that energy the less of it you'll be able to get. The bigger the cell is, the faster it is possible to extract energy from it, which means that a AAA or AA cell would quickly fail in most lights designed for D cells, but even D-sized alkaline batteries don't hold up very well for high-power lights. So what are alkaline batteries good for? Low-power lights. The slower you drain energy from an alkaline battery, the more energy you'll be able to get out of it. This means that alkalines are good for very small lights that produce low output for a long time. For instance, there are keychain lights which will run on a single AAA cell for 8-10 hours of almost constant brightness, but they are only around 8-10 lumens. This makes them great for close-up tasks, use with dark-adjusted eyes, and as backup-lights. Another low-power application might be the low and/or medium modes of a multilevel light. The important thing to remember is, the more energy you need, the worse alkalines will perform.
The most popular type of primary cell (that is, non-rechargeable) aside from alkaline batteries are lithium batteries. These cells hold much more energy and are capable of releasing it faster than alkalines. The most popular sizes of lithium primaries for flashlights are AA, CR123A, and CR2. Lithium cells are capable of powering high-performance lights, and the quality brands also have a 10-year shelf-life before they lose some of their charge (alkaline batteries usually lose a lot of their charge within a few years of manufacture). The downsides to lithium cells are worth mentioning though, particularly the safety aspects. Because they store so much energy, the results of a lithium battery failure are far more serious than alkalines. Manufacturers often refer to a lithium battery failure as "vent with flame," which is a nice way of saying "explosion." This is not to say that all lithium batteries are unstable bombs, but particular care must be taken that the batteries are not shorted out by connecting the positive and negative ends together, and that, in flashlights that use more than one cell, all of the cells are in the same state of charge. This means that you cannot safely taken partially used batteries from two different lights and put them into another! As they drain (and in very rare cases as they sit unused) a strong battery can "try to charge" a weak battery, and… boom. That being said, those two rules are very easy to follow, and as of this writing, Surefire, perhaps the best flashlight manufacturer in the world, powers its lights exclusively with CR123A batteries. Despite the high expense, lithium batteries remain the best choice for powering a flashlight with non-rechargeable cells.
There are two popular types of rechargeable batteries used for flashlights: Nickel-Metal Hydride (NiMH) and Lithium-Ion (Li-Ion).
NiMH batteries are available in the same sizes as alkaline batteries (plus some others), so many lights that are running on NiMH may use alkalines if necessary, although the performance penalties for alkalines will still apply. NiMH batteries hold more energy than alkalines and can release it more quickly as well, making them much better suited for high-power lights. However, not all NiMH batteries are created equal. In general it is a bad idea to run to the store and buy any-old NiMH batteries for two reasons: most suffer from rapid self-discharge (that is, they lose their charge quickly after being charged), and they don't release their energy that much better than alkalines do. In general, the only NiMH batteries worth buying for the average person are going to be labeled "pre-charged" or "low self-discharge," which means that instead of going dead in a week to a month, they still have 80-85% of their charge left after a year. A side-effect of the technology used to accomplish this is that they can release their energy much more effectively in high-power lights than normal NiMH cells can. No matter what kind of NiMH you get, however, it is extremely important (and even more so if you don't get "pre-charged" cells) that you not use a cheap charger or one that charges very quickly. In fact, the best way to destroy NiMH cells is to put them in a 15-minute charger. Look for charge times measured in hours, and chargers that can charge one battery at a time – those that require two cells to charge can overcharge one and undercharge the other, resulting in less runtime and decreasing the number of times a battery can be used before it dies.
Li-Ion batteries are the same as are used in laptop computer battery-packs. They hold the most energy of any rechargeable battery, and are available in sizes like 14500, 17500, 17670, 18500, 18650, and others. The first two numbers are the diameter of the battery, and the second two are the length of the battery, both in millimeters. Unfortunately, Li-Ion batteries also carry the most safety hazards of any chemistry. Safety margins vary slightly by manufacturer, but in general Li-Ion cells are not suitable for very-high power lights since their capacity can be diminished or they can be rendered unsafe if energy is pulled from them too quickly. They must not be overcharged and they must not be over-discharged during use. Overcharging can result in drastically decreased cell capacity, or even explosion and fire. Over-discharging can also lead to drastically decreased cell capacity on later charges, and also cause fires or explosions during normal charging. That is to say, if a Li-Ion cell gets discharged below it's minimum level, do not use it. Li-Ion batteries require their own charger and CANNOT be used with a NiCad or NiMH charger. Li-Ion batteries are often available in two versions for any given cell size: Protected and Unprotected. Protected cells have little circuit-boards in them to prevent them from being charged or discharged to an unsafe level and to limit the speed with which energy may be drained from them. This does not mean that they will prevent you from abusing the batteries enough to meaningfully decrease their capacity however. It is up to you to make sure you pick a reputable flashlight manufacturer who will respect the practical limitations of its chosen power sources. Unprotected cells are cheaper than Protected cells and can often offer slightly higher capacity, but they have no safeguards at all. It is entirely up to you to make sure the batteries are not pushed beyond their safe limits of charge, discharge, and energy drain rate. In general, if you're not sure Unprotected cells are for you, they aren't. In spite of the safety limitations of Li-Ion batteries, they hold the most energy in the smallest package of any battery chemistry and are extremely popular in enthusiast or professional lights, and if approached with a mature attitude can provide a great battery solution for many flashlights.
5) Flashlight's Role in Self Defense - Efficacy & Limitations
Contrary to what the marketing departments of some flashlight brands would have you believe, a flashlight is not a self-defense tool. If you're interested in taking responsibility for your own safety and live in a free area, the clerk at the local gun-shop will be able to show you many pistols you can carry, preferably concealed (ask your local law enforcement about regulations and requirements), which will intimidate and, if necessary, halt, any aggressors far more effectively than any flashlight. That being said, a flashlight, like any object, may be employed as necessary in a defensive situation, and some are more suited to this use than others. Like a firearm, the best (and most common) way a flashlight is used in a defensive situation is as a de-escalator. That is to say, the presence of loud shout, a bright flashlight, and the will of the user to stand up against an attacker actually prevents the encounter from turning violent because the attacker decides their intended victim will be more trouble than they are worth.
If the aggressor is determined to proceed with a physical assault and retreat is not a viable option for whatever reason, a flashlight can be used to gain the upper-hand in two ways, usually one following the other: first, brightest part of the beam is directed into the aggressor's eyes to temporarily impair their vision. Some lights even include a strobe function which can sometimes enhance this effect. If it is apparent that the aggressor will continue the attack, the flashlight can be used to either reinforce a fist in a manner similar to a kubaton, or as a direct striking weapon. The popularity of the Maglite brand is in no small part based on their utility as a striking weapon because of their size and weight. Other lights are made more suitable impact weapons by the reinforcement of the head of the light and the addition of crenellations (they often look like saw teeth) on the bezel, which focus the force of a blow into smaller areas and can increase soft-tissue damage. Of course, the most effective use of a flashlight in an unavoidable conflict is to pair it with a pistol, since the presence of both objects indicates preparedness and confidence of the user, and the very real threat of personal injury to the assailant should they continue in their attempt. Presenting an aggressor with a handgun and a bright flashlight is one of the strongest de-escalation methods available to almost everyone.
It goes without saying that the presence of any defensive weapon does not guarantee the user the successful defense of their lives in every situation. The best method of self-defense is to simply avoid a situation where it may become necessary. After that, defusing dangerous situations by de-escalation should be tried, and physical response only after other avenues have been attempted. It is extremely important that the efficacy of blinding an assailant (or multiple assailants) not be overstated – the greater the ambient light, the brighter the flashlight has to be in order for it to even briefly disorient them. The absolute minimum brightness for an effective tactical flashlight (that is, for disorienting an opponent) in almost-complete-darkness is around 65 "torch-lumens" or "lumens-out-the-front," combined with an optic or reflector designed to concentrate the light in a tight spot. In general, most lights sold as "tactical" will fulfill this requirement even if they fall short in other ways. However, in most urban areas, the inhabitants never acquire dark-adjusted vision and therefore are largely immune to disorientation by all but the brightest flashlight beams. It is far better to avoid self-defense situations entirely by traveling in pairs or groups, maintaining awareness of your surroundings so you may remove yourself from threatening situations before they happen, or waiting until daylight to conduct your business in a dangerous area.
