There are a few types of chargers available. There are trickle chargers, which plug it into the wall, and then into the battery. This type of charger charges the battery at a very slow rate (typically under a few hundred mA, so charging usually takes several hours or overnight). They look like a regular wall mounted transformer (or a "wall-wort") with a wire plug that connects to the battery. There are different kinds, one for each different voltage of battery. They can and do overcharge the battery (since there is no way to tell if the battery is full and stop the charger); but there should be minimal damage to the battery since the current is so low.
Another kind of charger is the timer-based "fast charger". These chargers can charge up a battery in an hour or so. They typically feed it with a few amps of charging current. Most of these units (around $20-$30) are a small box with a few switches/dials for setting the battery voltage and/or the charge time or battery capacity. There is one wire that goes to a wall plug, and another that goes to the battery. They charge at a fast rate, and use a timer (that you either manually set for a certain amount of time, or calculated based upon the capacity that you told it your battery had). The problem with this is that the charger has no idea when your battery is really full. These can undercharge the battery (resulting in less battery time for your AEG, and possible memory effect if your battery capacity is more than the charger was set for. If there is still some power left in your battery, the charger will most likely overcharge it, which can cause it serious damage. It is important that you use a battery discharger to make sure that the battery is empty before you charge it back up if you are using with one of these chargers.
Peak Chargers (also called an "intelligent charger" or a "smart charger") can be acquired as well. This is the best type of charger you can have. These chargers can detect the slight drop in voltage that signals that the battery is full and automatically switch themselves off. These chargers also usually incorporate a discharger as well. These chargers typically cost $40 and up, and look similar to the timer based chargers, except that they are usually bigger and have more switches/dials/indicators on them. One very popular charger in the world of airsoft is the $50 TLP Intellicharger, that can run off 110 volt AC, 220 volt AC, and 12 volts DC. A peak detecting charger can help you ensure that your batteries are in tip-top condition and they charge batteries quickly. You can save a little bit of money by getting a cheap timer-based charger, but your batteries will thank you if you buy Peak instead. Before charging any NiMH batteries on your Peak Charger, ensure that it is designed for that or you may risk damaging the battery.
* mAh and Volts are what is important: how much juice you've got and how much it puts out * Peak Chargers are best for conditioning your batteries * NiCD Inexpensive and generally lower MAH, NiMH: Expensive but excellent performance * While people 'know' there is some sort of 'memory effect' no one can explain how it works
mAh: MilliAmp Hours. A 1 mAh battery could last ~1 hour at at one milliAmp load. Amps: Amperes, Current Volts: Voltage Ohms: Resistance NiCd: Nickel Cadmium. A rechargeable battery chemistry. NiMH: Nickel Metal-Hydride. A rechargeable battery chemistry. LiIon: Lithium-Ion rechargeable chemisty (rare) LiPoly: Lithium-polymer recheargeable chemsitry (expensive)
Battery Chemistries There are 4 basic rechargeable batteries and 3 basic kinds of non-rechargeable batteries. The non-rechargeables are carbon-zinc ("Heavy-Duty" or "cheap-o Non-alkaline"), alkaline, and Lithium.
Disposable Batteries Carbon-zinc batteries are the cheapest cells you can get, they have a fairly low capacity, but have a lower internal resistance than Alkaline (still not as low as NiCd or NiMh), so they can deliver a higher current than alkaline (hence the name "heavy duty").
Alkalines have a higher capacity than the carbon-zincs, but they have a very very high internal resistance. These cells do not deliver high currents well. Their high internal resistance causes them to "droop down" under load. The nominal voltage for alkalines is 1.5 volts, but the high internal resistance will cause them to droop down to less than 1.2 volts when heavily loaded. This is why higher powered devices (like Electric Blow Backs and cheap R/C cars) will work better with NiCds or NiMHs than with Alkalines. They are getting a higher voltage from the rechargeable batteries since the alkalines (that are supposed to be 0.3 volts more per cell) are actually providing less voltage. Alkalines also have a much different discharge graph than do the NiCds and NiMH. Imagine a graph with cell voltage as the vertical axis and the percent of charge remaining in the cell along the horizontal axis. Alkalines have a steadily downward sloping curve. This means that alkalines put out more voltage when they are fresh then when they are used up. They will only provide their best performance when new, and lose some of their power as they are used up.
The last type of non-rechargeable battery technology is the Lithium cell. These batteries have the most capacity, and are the most expensive. They are also lighter in weight and provide 1.7 volts per cell. They can also provide a higher current than alkalines, but not as much as the NiCd, NiMH or lead-acid. They are also very poor value for your money.
There are 4 principle chemistries of rechargeable batteries: NiCd, NiMH, Li-ion (Lithium Ion), Li-Poly, and lead-acid.
Li-ion (not to be confused with non-rechargeable Lithium cells) are the newest type of rechargeable battery, and are the lightest that also have the highest capacity. They are designed primarily for capacity. These cells are very fragile. They cannot take the rough treatment of an AEG (high currents). They also tend to explode if they are not charged correctly. These cells are intended for use in high tech mobile stuff, like laptops and cell phones.
LiPoly batteries are also coming into usage for airsoft -- while voltage options remain limited, they are about the right size for airsoft, and if you take care to not heat up the gel-packs, are an extremely good high-ROF battery.
There is also the lead-acid type. This battery has two lead electrodes, with sulfuric acid as an electrolyte. They come in two varieties, "wet" cells (car batteries) that have liquid sulfuric acid (which can spill all over the place...not good at all) and SLA (Sealed Lead Acid, or "gel-cells") these are typically used in Uninterruptable Power Supplies for computers. SLAs use sulfuric acid jelly and are sealed so they cannot spill. These batteries are the toughest kind, readily providing huge currents. They are also really cheap. The one main problem is that they have lousy energy density and weight. Their energy density is about on par with NiCd: a gel-cell and a NiCd of the same physical size hold about the same mAh, but the LEAD-acid battery will be twice as heavy. lead-acid batteries are 2 volts per cell, and typically come in 2 volt, 6 volt, 12 volt, and 24 volt sizes.
The 2 remaining rechargeable types of batteries are the ones usually used in Airsoft: NiCd batteries, and NiMh batteries. Ni-Cd stands for nickel cadmium, and NiMh stands for nickel metal-hydride. NiMH batteries are a bit more expensive, but they have a higher capacity. NiMH are a bit more difficult to charge than NiCd, so make sure your charger is specified to handle NiMH if you use those type of cells. NiMH also have a higher rate of self discharge. These type of batteries will drain all by themselves if you just let them sit there. NiMH self discharges by about 2-3% a day, while NiCd discharges by about 1% a day. A full battery will be practically empty after sitting on the shelf for about a month.
The common perception is that a NiMh battery will last longer, but wont have the voltage of a NiCd with the equivalent number of cells. This is not always true. Older NiMh batteries usually were at a lower voltage per cell compared to Ni-Cds. The more recently produced Panasonic NiMh cells actually have an equal, if not higher voltage than that of NiCd cells. The main perk of using a NiMh battery is the higher capacity, up to 3300 MAH in the Sub-C sized cells.
Each cell of a battery has internal resistance, which limits the power it can put out. A cell with the lower Internal Resistance (IR) will always be better. Early NiMh cells had a high IR making them not perform as well as NiCd or recent NiMh cells. Also, the connectors, wire, and battery bars in a pack all have resistance. Resistance is what is stopping the flow of electrons. The lower resistance in everything, the better. The stock connectors on pack are horrible. They have high resistance, and over time will wear out and get even worse. Getting some low loss connectors such as Deans Power Plugs will help to cut down on the resistance. The wire is also a place where you can cut down on the resistance. You can use some 12 gage wire in place of the thin wire that some pack have. Also, if you look at regular battery pack the cells are connected with thik, foil like strips of tin. These are horrible compared to solid copper, or silver and gold plated battery bars you can get. Each of these things alone may not seem like a big deal, but by replacing the connectors, wire, and battery bars you can reduce the resistance in the circuit.
Batteries, Airsoft and You
Batteries for airsoft have two general characteristics, voltage and capacity. Capacity is measured in mAh. Voltage is measured in volts. Amps is a measure of how strong a current is. Capacity is just how much power the battery can hold; it is basically how long you can use the battery before it stops being able to power the system. Amps measures how much power a battery can push out. Voltage is how fast it pushes.
Think of a battery as a water balloon. The mAh measures how much water is in the balloon. Voltage tells you how big a hole you have in neck of the balloon. Amps tells you how much pressure is being exerted on the balloon. Resistance measures how much fabric you have blocking the water from coming out.
So...more or less mAh is more or less Water. Basically what volume of water you have to work with. Voltage tells you how much water you can let out of the balloon at one time. A bigger neck on a balloon will let more water out at one instant. More or less amps measures how fast the water is moving out of the balloon. More pressure on the balloon makes the water come out faster. The Resistance over the neck slows the water from flowing. The less resistance, the more efficiently the water will be delivered.
The main things Airsofters are concerned with are how much water they have and how much they can deliver at one time. mAh and Volts. For advanced users, the discharge rate (measure in Amperes, varies from 20A ro 35A -- 30A is ideal)
You should usually aim to get the battery with the highest mAh rating you can because it will last longer. This is why it is preferable to use a "large" type battery (generally between 1200mAH and 3000mAH) as opposed to a "small" type battery (generally 600 mAh). There will be no adverse side effects to using a battery with a higher capacity; it will simply last longer before it runs out of juice. A battery with a higher mAH rating will not damage a gun.
The capacity rating of milliAmp hours means that if you drew the rated amount of milliamps from the battery, that the battery will last for 1 hour before "dieing". For example, a 3000 mAH battery can deliver 3 amps (1 Amp = 1000 milliamps) for one hour continuously, 6 amps for 30 minutes, or 1.5 amps for 2 hours. A battery's rating is usually "\20H", meaning that the rating is for 20 hours. For example, a 3000 mAH battery is specified to deliver 150 mA for 20 hours. The batteries will have a slightly less capacity in an AEG application (where they will be providing a high current for a short time) since the higher load is more stressful for the batteries, but you can still compare mAH ratings between batteries.
The battery's voltage determines how much power the motor will have. Stock AEGs take an 8.4 volt battery. Upgraded AEGs sometimes need higher voltage battery packs, such as 9.6, 10.8, and the insane 12 volt pack. The higher voltage will make the motor turn faster (higher ROF) and/or harder (to drive those bigger springs). Please also note that a higher voltage battery will put much more stress on your motor and the mechbox internals. You can easily tell what voltage a battery has by counting the number of cells; each NiCd or NiMH cell is nominally 1.2 volts. A 10 cell pack has 12 volts. A 7 cell pack is a standard 8.4 volt pack. The power of the motor (measured in either watts(W) or Horsepower(HP), 746 W = 1 HP) is a function of the voltage it is being fed and the Current it is drawing. Watts equals Current (in Amps, 1000 Milliamps = 1 Amp) times voltage. Please note that the motor will draw more amps running at a higher voltage since it acts somewhat (not quite, but good enough for a simple comparison) like a resistor. The motor follows Ohm's law: Current(in amps) = Voltage divided by Resistance (in ohms). The resistance of the motor stays somewhat constant. So, the amount of current the motor draws will increase linearly with the voltage. Since the power of the motor is volts multiplied by current (which also increases with the voltage), the motor's power increases with the square of the voltage, while the current increases linearly with the voltage increase.