ALWAYS use a balance charger. A balance charger charges each cell individually through the small charging plug. The charging plug is the smaller one and NOT the one that plugs into the gun. When abattery is balance charged, each cells voltage is checked and restored to where it should be. This way, one cell doesn’t die prematurely do to the cells being discharged unbalanced while shooting(not common, but is noticabl slightly over time). So, if you find a charger and it does not say it’s a balancing charger, keep your distance. If it charges through the main connector, keep your distance. Also make sure you lipo is capable of charging you number cells that your lipo has. A 7.4v is 2 cells, 11.1v 3 cells and so on in 3.7v increments.

Which lipo is for me(7.4v or 11.1v)?

Think about your ROF and consider what is a reasonable at your fps. Also consider your motors power. Most guns under 380 fps should not run an 11.1v lipo. No matter what the C rate(will elaborate more on that later), you will most likely have pre-engagement issues. So now that you have figured which voltage your going for, you should consider what “C” rate you should look for. Your “C” rate is your discharge rate. That’s how much current your battery can safely discharge at one time. I’m not an expert on this so if someone could add more to that or fix me that would be great. But, the higher the C rating, the better the trigger response and ROF. People claim 7.4v lipos to be worse than 8.4v NIMH batteries and others claim them to be better than their 9.6v NIMHs. It can be very confusing, but that’s all because of the “C” rating. A 20C 7.4v will be around a 8.4v or less. On the other hand, a 40C 7.4v will be more powerful than a 9.6v in most cases. Same goes with 11.1v. A 12C 11.1v will be considerably less powerful than a 20C 11.1v.

How long do they last?

Lipos last much much longer than NIMH batteries. I’m not sure why, but they just do. I’ve heard accounts from people saying a 11.1v 20C 1100mah batteries lasting a whole 5k round box mag with juice left over. I can’t verify this as most people have never risked running their battery that long. But I wouldn’t doubt that its possible. 7.4v don’t seem to last quite as long as 11.1v, but they still last much longer than normal. I’ve been told by people that own 2000mah 7.4v lipos, that they can get anywhere from 3000-4000 rounds out of it. How long you gun lasts also depends on how much power your motor pulls and how strong your spring is. So a 2000mah battery will last considerably longer at 330fps than at 400fps. There is a reason you don’t see many lipos in high Mah ratings. You don’t need it.

credit goes to G36fanatic of airsoftsociety.com

For instructional purposes only, lets assume a lipo has 6 cells, each cell is like a link in a chain. When the battery discharges to a certain point one of these cells will damage and will not retain a charge or preform as it once did. When one of these links gets to this point it will degrade the entire chain.

There are many tools to help you avoid this situation. Lipo batteries come with smaller connectors, you can plug these connectors into voltage meters which tell you how much charge the entire battery has left, or you can plug a lipo alarm which will make an annoying alarm sound informing you that the battery is close to discharging and is recommended to stop using and recharge.

@ How long do they last?

Theoretically a fully charged NiMH should discharge itself more then 25% if left unattended over one year. A lipo on the other hand should not discharge anywhere close to 2% a year if left unattended.

It's not unheard of for a player to use the same battery without charging it for multiple games. I've had a guy use the same battery every weekend for a month before he needed to recharge.

Recommended things to do before switching over to Lipos:

Upgrade your connections to Deans, there's more connectivity with deans then Tamyia. Ensure the battery end is FEMALE so there's no way to bridge the connection.

Always charge your lipo battery in a lipo safe charging bag, they're designed to contain fires so if something does go wrong the worse thing that can damage your home is a little bit of smoke instead of a huge fire.

Added (2010-05-30, 9:20 PM)
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As for "C ratings", it's honestly a worthless, overly used concept in most cases. It can be useful for charge rates for lipos since a 1C charge is capacity(in miliamps)/1000. ie. 3000mah battery charged at 1C is just means 3000/1000 = 3 amps charge rate.
But for the rest C rating alone can't tell you anything about the actual amps that your battery puts out.
To calculate amps you need that battery's capacity. So a 3000mah battery at 10C can give 30amps via the following calculation: (3000miliamps)*(1amp/1000miliamps)*10C = 30amps
In short, capacity/1000)*rating = amps

To understand how misleading C rating alone can be, lets take a look at the following batteries (voltage is irrelivant in this case).
1) 3000mah, 10C. This one puts out 30 amps
2) 900mah, 25C. This one puts out 22.5 amps
You can clearly see why you need to calculate the amps especially if your motor is upgraded. The max output of your battery is measured in amps.

A motor will only draw a maximum number of amps depending on how it's made. Usually that number is unknown and hard to measure but it can be guessed if you have experience with this. Most stock motors max out around 15-20 amps. Upgraded motors can be anywhere from 25-50+ amps. This is why it's essential that you calculate a batteries output in amps when choosing which one to buy.
If a motor can only take 15 amps, then the batteries output in amps more or less doesn't matter past 15amps. At that point you have to up the voltage to increase your overall power to the motor. I like to call it 'forcing volts' into the motor since it won't take anymore amps. The batteries total power delivered is given by volts*amps = watts. For example the following batteries:
1) 8.4v nimh 15amps
2) 9.6v nimh 15amps
3) 7.4v lipo 30 amps
Case 1: Used on a stock motor that will use a max of 15 amps, the 9.6v would give the highest rof because the motor only accepts 15 amps. SO the 9.6v gives 144 watts (9.6v*15amps), the 8.4v gives 126 watts (8.4v*15amps) and the lipo gives the lowest 111 watts (7.4v*15amps). As you can see the lipo's high output is limitted by the motor.
Case 2: Used on an upgraded motor taking 30amps max. The 7.4 lipo would give the highest rof by far, putting out 222 watts compared to the 9.6v which can still only give 144 watts and the 8.4v which only puts out 126 watts.

Since lipos have so many high discharge options (100+amps) it's perfectly alright to buy something with that rating. It most likely will never get discharged at 100 amps, but the potential is there and it' won't hurt your motor either.

The final word is pretty much that lipos and motors are a game of calculations. Some of the statistics you know, some you don't. Generally you won't know how many amps your motor is capable of drawing. The more time you spend buying and testing lipos, the more you will understand which batteries are most appropriate in which cases. I hope that gives some insight as to how the math aspect of this all works out. It's a pretty simple concept once you get the hang of it.

Most airsoft Lipo owners state to store it fully charged always (for example, charge them to full immediately when you get home)

However, most RC hobbyists state storing a fully charged lipo for 3-6 months increases the resistance in the pack and lowers your battery life. They state that a Lipo staored anywhere longer than 3 weeks should be stored at 40% charge.

So what lvl of charge do you guys leave your lipo's at for long term/ short term storage?

I've never really heard of people storing them fully charged.

Airsoft retailers overcharge on batteries

Tom I don't usually try different battery brands if I can't find any reviews on them. Preferably look to rc forums for reviews, wattflyer ect. Hobbypartz and Hobbyking are probably the more popular HK retailers among airsofters.

Personally I've been ordering from Hobbyking lipos because I actually use their xt60 connectors for a few of my guns. The 4mm bullet connectors from hobbypartz I have no use for so they'd literally be trash in my parts box lol. Plus I constantly buy a multiple meters worth of wiring and battery wrap as well.

I really don't mind paying the shipping prices, but there's a lot of times hobbyking is missing some crucial items I need and I'll either have to wait or put in two orders. I've also seen a few times where hobbypartz gets really low on stock for most of their items.