I thought I would organize what I know about batteries. It's not everything about Batteries, but I hope a lot of people new to AEG would find it useful.
This is "how strong" electricity is. The higher voltage, the faster motor turns. So 9.6volt would make your AEG motor go faster than 8.4v. The result is faster rate of fire. [Of course higher ROF would wear out parts faster, but that's for other thread]
To get higher voltage, the battery pack must have more cells. Which means higher voltage, the pack gets bigger. So, before you get 9.6 volt pack, make sure your AEG has space to take a bigger pack.
Amp means "how much." You can have a 8.4v 700mAh pack. If you have 8.4v 1400mAh pack, it will last twice as long. But there is no physical difference between two packs. They should look exactly the same. Unless you go from Mini to Large, or large to mini. As long as you get the same size pack, you don't have to worry about whether more capacity pack would fit your AEG or not.
How many shots? Each mAh allows you to shoot roughly about 2 rounds. So 700mAh pack would let you shoot 1400 rounds, 1400mAh pack would let you shoot about 2800 rounds. 4000mAh pack would let you fire 8000 rounds per charge. For most airsofters, they use 500 -1000 rounds per skirmish. So, even 700mAh (1400 rounds) would not be too bad. But if you end up spending 4000 rounds per skirmish, get a pack that's above 2000mAh.
There are three types of rechargeable packs.
1] NiCD: It uses Nickel and Cadmium. Cadmium is heavy metal. So don't eat it and don't toss it in the garbage. It will leak into ground water source and cause environmental damage. Radio shack or Circuit city, etc have battery recyling bin. Recycle.
NiCD has higher voltage peak and drop. As you use your battery, the 8.4v pack would seem like 9.6 volt when it's fully charged. Then gradually, it comes down.
"Memory effect." NiCD has memory effect. If you have 700mAh pack, and use shoot 200 rounds (100mAh) and recharge it, without discharging, your pack will remember that after draining 100mAh, it's time to feed. So it plays dead. Not really dead, just playing dead. So discharge is important. If you have not drained the battery playing airsoft, drain it down to about 6 volt. Too much discharge can kill the pack.
NiCD is relatively carefree. You can leave it unplugged for years, and you will find that you can still use it. I have a 20 year old 1200mAh pack. After years of neglect, it still holds about 300mAh.
2] NiMH: It uses Nickel and other non-toxic metal parts. That does not mean you can eat it.
NiMH has a nice and stable discharge platoe. If you use 8.4v, it will pretty much stay at around 8v, until it's drained. NiMH does not have pronounced memory effect. So there is no need to discharge. One problem with NiMH is that if you leave it discharged for several months, it will be completely dead. So do not discharge NiMH. All batteries diecharge on its own, slowly over time. Worse if you keep it plugged. Always keep it fully charged. Keep it unplugged, and every month, even if you don't use it, recharge it again.
3] LiPO: It's lithium polymer, a new technology developed for cell phones and lap-tops.
It's much like NiMH in that it should not be kept discharged, which is why your cell phone just shuts down below certain voltage to protect the battery. But automatic discharge rate is low. Usually, it has twice more capacity per volume than NiMH. What makes it impractical for airsoft is the price. Too expensive to be practical yet. The price is always coming down, so in a few years we might have LiPo all over the place. Special chargers are needed to charge LiPO.
CHARGING A PACK
NiCD and NiMH and LiPOs all use different chargers. Actually, NiCd and NiMH are not much different. Most chargers can charge both NiCd and NiMH. The difference between NiCd and NiMH is the way they act when they are fully charged. NiCD has a big voltage jump, and NiMH has small voltage jump when it's fully charged. So if you have an automatic charger that charges NiCD only, it will be looking for higher voltage peak, and keep on charging NiMH packs, destroying it. But like I said, most automatic chargers these days can detect small peak, and charge both NiCD and NiMH.
If you use wall charger, charge 1.2 times the capacity. Some math is involved. If you have a 1100mAh pack, and you have a wall charger that has the output of 250mAh. How many hourse do you charge? Little over 5 hours. How do I know? 1100 x 1.2 = 1320. Divide that with 250. You should get little over 5. That's how long you should charge. If you have 700mAh and the wall charger's output is 350mAh? Then you charge 2.4 hours. If you have 3500mAh pack? Charing with 350mAh wall charger, it would take 12 hours.
I recommend Superbrain 959. It's about $50, but it's relatively new and it can handle many types of batteries. And it's automatic. It detects what kind of packs you have, 7.2volt to 9.6 volt. And it charges both NiMH and NiCd. It cuts off automatically. I can charge upto 4 amp per hour, I think, which means even 3500mAh pack can be charged in one hour. It can use car battery to charge packs too, so you can use it on the field. Also resale value is around $30. So when you want to get rid of it, you won't lose much money. For Mini packs, you need an adopter.
As already mentioned, discharge only if you have NiCd. Do not discharge NiMH or LiPo. There is too little gain, and too much to lose. Discharge only if you have NiCd, and do not go down below 6 volt or about 0.9volt per cell. That's about when you can no longer fire your AEG. Too much discharge will force one of the cells to switch polarity. Then the whole pack is as good as dead.
Mini vs Large
Practically speaking, only the size is different. So if you have a P90, it simply does not have enough room to take a large pack. Get the pack that fits in your AEG.
Large packs use SubC cells. They are big fat cells. Mini uses 2/3A cells. They are small cells. If you have a large 8.4v 1100mAh pack, and also a mini 8.4v 1100mAh pack, they should work exactly the same. Same rate of fire, and they should last about the same.
Well, actually, there is a difference: max amp. Large SubC cells does allow more amp to be drawn than 2/3A cells. But since AEG motors simply do not draw that much amp, practically there is no difference. If you have a RC race car, that uses a tirpple wound 7 turn motor, you would definately want a SubC large pack. Those hot motors can draw out 35A. But for smaller AEG motors that are oriented for torque rather than speed, amp draw is limited. There is no need to modify your stock to upgrade to large pack, you won't notice much difference unless your previous mini pack was really old.
Going below Mini(2/3A cells) would be bad. If you solder 8 AAA cells 700mAh, you can make a 9.6volt 700mAh pack. But AAA cells have max amp draw of about 2-3A. That's too small for AEG motors.