It's come to light that there is just too much to explain every time somebody asks "what battery should I get for x gun?", so here's a whole thread to explain the crap out of the subject!
Gone are the days when airsoft guns either used mini, stick or large battery packs, now there's a different pack for just about every gun out there and half a dozen chemistries. Knowing what shape of battery your gun uses is entirely your responsibility, google is a great place to find that out! As for what chemistry to use, that's at your discretion.
So of the many chemistries I'm only going to cover the most popular two; NiMH and LiPo
And we'll start with some common terms:
Voltage - voltage is determined by how many cells there are in a pack, and determines the top speed of your motor
Capacity or Mah (MilliAmp-hours) - like the physical volume of gas in your tank, this is the amount of energy stored in the battery. A 15C battery that has 1200mah will have the same rate of fire as a 15C 4000mah battery.
Amperage - this is torque, it's what you need to get your motor to move stronger springs and get up to full speed under load
C rating - typically used to state the maximum amperage that can be drawn from a battery. On a LiPo, you multiply the C rating with the Mah/1000 (or the Ah, Amp-hours) to find it's constant safe draw; a 1600mah 20C battery would be able to supply (1.6Ahx20C) 32 amps continuously. On a LiPo, higher C ratings reduce internal resistance on the battery, raising your ROF by a very small amount.
cell size (for NiMH) - typically 2/3A, 4/5A and sub-C, when one talks about mini and large batteries they mean 2/3A and sub-C packs. 4/5A packs are less common, but usually used for crane stock batteries. 2/3A will be 1200-1600mah, 4/5A from 1700-2200mah, and Sub-C typically 3000-5000mah.
Arcing - Basically, an arc is lightning between two contact points. In airsoft we commonly refer to 'trigger arcing': lightning jumps across the trigger contacts before they actually make contact, causing pitting and carbon buildup. Higher voltage causes more damaging arcing.
MOSFET - A small electronic device used primarily to prevent trigger arcing and melting. Has the added benefit of increasing ROF. Some computerized mosfets are available (SW-COMP) which allows neat features like burst, semi only, motor speed control, and active braking. Some of them are "LiPo compatible" which means they have some kind of built in low voltage sensing for LiPos, meaning you don't NEED a low voltage alarm in addition to the mosfet.
Amp draw - the amperage that your motor requires to maintain it's full speed. Say you're running a 9v battery, and your motor tops out at 27000rpm (it may only need 1.5A to maintain that outside the gun), once you plug it into your gun we'll say it requires 8A to operate at full speed. The motor will only ever require 8A, you can use a battery that supplies 300A, the motor will only ever draw 8A. If your battery only supplies 5A, then the motor will just run at a reduced speed.
Nickel metal hydride
The standardized battery for airsoft.
Unlike it's predecessor, NiCd (nickel cadium), it doesn't have a memory effect, so you don't need to worry about discharging the packs, and it's safe to charge them when they're half-full or whatever.
Packs typically come in 8.4v and 9.6v, although larger packs are available it's uncommon to see them used.
Cell size has a great effect on your ROF. An 8.4v large battery has a similar rate of fire to a 9.6v mini battery. This is because a mini NiMH pack never supplies all the amperage your motor needs, and a physically larger cell supplies more amperage.
-You can run them until your gun slows right down and stops working without hurting the battery
-packs that commonly fit in airsoft guns are typically available at, or can be built by, any local battery store
-There are less expensive smart chargers available for these than for LiPo
-The smaller packs don't supply all the amperage your motor needs, resulting in less performance over a LiPo of the same size
-Their capacity is greatly reduced in the cold. A battery that lasts 1600 rounds in summer may only last 500 rounds in winter
-More expensive than LiPo
-Because the smaller packs supply less amperage, one would increase motor speed by increasing voltage, which would in turn cause arcing on your trigger contacts and reduce the life the motor
-If you've used them a few times, you'll notice the ROF starts out high, then it settles after a few mags, and it will drop off slowly once it gets really low until it stops cycling
-If using a 9.6v large battery, it's highly recommended you install a mosfet to prevent the trigger from arcing or melting
-If bare wires touch inside your gun (shorting the battery), NiMH batteries catch on fire just as quickly as LiPos
-More capacity doesn't increase your ROF, but a physically larger battery will.
This is what most people are switching to, just about all the veterans run them.
There's more to know about these, so it's a pain to learn, but they really are better batteries.
So firstly, at 3.7v/cell we get a big voltage gap in the packs, you can get 7.4v or 11.1v LiPos for airsoft. Any stock airsoft gun will safely run off a 7.4v LiPo, and this is REALLY what manufacturers mean when they say "LiPo safe", but you'll understand why that's still a lie by the end of this.
You can't charge them on the same charger as NiMH, this will cause them to burst into flames.
Now when you shop for a lipo, they all come with that mysterious C rating in the description, that's important.
As it says above, your C rating x the capacity of the battery equals the continuous draw. So a 4900mah 40C battery has (4.9Ahx40C) 196A that it can safely, continuously supply. And this is the key factor to LiPo batteries, they always supply all the amperage your motor needs to run at full torque, and therefore full speed under load.
So you'll hear us say a 9.6v mini battery has comparable ROF to a 7.4v LiPo. The main reason for that is the 9.6v mini never reaches it's full speed under load, but the 7.4 does. And what's more is the 7.4 has enough amperage to easily move high speed gears, whereas a 9.6v mini would struggle.
-2S, 3S, what?
S stands for 'Series connection' and P standard for parallel, but I haven't seen any parallel LiPo's for airsoft yet.
So 2S 15C 1100mah, that's a 7.4v battery because there are two cells wired in Series
3S is 11.1v
-over current draw
LiPo's cannot operate under the amp draw requirements like NiMH batteries can, it will damage the cells, they will swell, and if you ignore the warning signs, they may eventually flame up. Make sure the battery you choose can supply at least 15A for stock guns, and at least 20A for upgraded guns.
Unfortunately these packs are more sensitive to how much voltage is in each cell. The limit is 3.0 to 4.2v in each cell. If a cell goes over 4.2v it may swell or flame up, and if a cell goes under 3.0v it becomes damaged and may swell or flame up once charging. Because drain is not constant on all 3 cells (usually due to different wire lengths), you always charge LiPo's on a balance charger. And that's what that extra little plug is for on the pack.
-Much higher energy density than NiMH. For the same volume as a 9.6v 1500mah NiMh pack, you could have a 7.4v 20C 2400mah pack with comparable ROF
-Much less effected by cold temperatures, the capacity loss in winter is only around 10-15%
-Supplies 100% amperage to the motor, meaning you're able to get more torque from the motor, and run high speed gear ratios off little stick batteries
-inexpensive. Usually around half the cost of NiMH batteries due to ease of production.
-They come in just about any size you can imagine, but limited to being rectangular
-Much flatter voltage curve as the battery drains. The ROF stays pretty constant until the voltage drop, which is usually like a cliff. It's really noticeable around 3.3v/cell which is about halfway off that voltage drop cliff.
-They require more knowledge of operation
-They require a smart/balance charger
-If using an 11.1v LiPo, you need some kind of mosfet to prevent trigger arcing
-If using a high amp draw/full auto setup on a 7.4v LiPo, you may need a mosfet to prevent your trigger block from melting due to heat buildup
-You can't run a LiPo until you gun stops cycling, this will cause damage to the battery. You typically need some kind of low voltage alarm to signal when it's time to change batteries, unless you're using a really high capacity battery like a 4900mah. In which case, good luck trying to drain that thing completely in one game lol
-They do not explode. I don't care what you've seen on youtube. They don't explode. However, they do vent.
-Unless you short the battery, over-current draw severely, or puncture the battery, they will only flame up during charging, which is why it's recommended to use a lipo charging bag.
-Unlike NiMH batteries, LiPo's will typically give you a lot of warning before smoking or flaming up. They will either swell (it's noticeable) or it will be hot to the touch.
-The C rating by itself means absolutely nothing. And I'd be amazed if you could even measure the difference in performance between a 20C and 25C pack.
-C rating does not significantly affect ROF. A 20C battery will sound just as fast as a 40C battery. However much larger C ratings lower internal resistance on the battery increasing ROF by small amounts.
-You can run any stock AEG off a 7.4v 90C 6000mah battery. The battery still only supplies a maximum of 8.4v, and the motor still only draws 4-12A.
-Despite all these fire and damage warning, they've actually got a REALLY safe track record. Personally I've seen more people light NiMH batteries on fire than LiPo's, and I've yet to find a defective LiPo, whereas I've seen no less than a dozen defective NiMH batteries.
-When the charger tops it's voltage out at 4.20v/cell, the battery is only 70% full. The cells can't go over 4.20v, so the charger can't put more than that into them. The last 30% of the charge is done at 4.20v, the charger will let you know when it's done.
Some safety stuff
-Always balance charge a LiPo pack. There is absolutely no reason not to, and it increases charging safety significantly.
-Never charge a LiPo you suspect of being damaged, that's where most fires start.
-Don't try to resurrect a LiPo that's gone under 3.0v/cell, just discharge it and throw it out.
-Don't mount LiPo's (even the armored ones) outside your gun in stock pouches or whatever, getting hit by a BB may cause trauma to the pack.
-Smoke or fire, this is a sign that you LiPo is on fire. You should probably take it outside/out of your gun and bury it or something.
-Swelling, this shows internal damage. Some LiPo's can take some abuse and be used even after swelling, but this is dangerous.
-LiPo is hot to the touch. Usually due to over-current draw. During use or charging a LiPo should never be hot. If it is, you should dispose of that pack and find out why your gun is damaging your LiPo.
-Low voltage. If you go under the magic 3.0v/cell on any cell, that cell may be damaged, and it may not be a good idea to charge it back up to full capacity.
-Over voltage. If a cell shows something over 4.20v, chances are the pack's already swelled, might not be a good idea to keep it in your pants.