11.1v, 1500mAh, 15C Lithium Polymer Battery; the full stock type. A 105 x 35 x 15mm 106g solid brick designed to go in AEG stocks.
NiCad (Nickel-Cadmium) and NiMH (Nickel-Metal Hydride) batteries use electro chemical reactions in metallic assemblies to generate electrical voltage. Lithium Polymer batteries use Lithium ions moving back and for embedded in a polymer membrane to achieve voltage. The difference is that normal batteries are like a turbo charged version of a potato / lemon clock reaction, its pretty stable and gets the job done if a little heavy and large. LiPos are essentially burning / exploding lithium metal ions stuck in slo-mo. For the same applications, LiPos offer a much higher energy density then traditional batteries.
Not only are the same voltages achievable in smaller packages but LiPos have much lower internal resistance allowing electrical motors to draw more current from them. For those of you who are not up on their High School electronics, what that means in reality is that the same voltage of LiPo will run a motor faster then its equivalent equal voltage model NiCad battery. Much, much faster.
Traditional NiCads cells have 1.2v each so when arranged in batteries they are multiples of 1.2v (7.2v, 8.4v, 9.6v, 10.8v etc). NiCads have a higher internal resistance which means your motor pulls less current and the greater options of voltage increments mean you can tune your battery option resulting in a more forgiving setup. LiPo cells are 3.7v each as such they come in two-cell 7.4v or three-cell 11.1 versions.
When batteries of cells discharge, they can do so unevenly which is a problem. If a battery has uneven charges, one cell can ?charge? another cell resulting in a sort of short-circuit which is very bad and can result in severe battery damage. That said, the slow charge rate means that NiCads can ?settle? imbalance more easily. In a LiPo battery the flow of charge is of such a high rate that imbalance in charge can result in catastrophic damage. Over charging a LiPo can, worse case scenario, result in overheating and the release of a flammable gas which can result in fire or an explosion in the most dramatic cases.
As such LiPo batteries minimize the chance of charge imbalance by using fewer cells (reducing the chance of imbalance occurrence) and are better to be charged by balanced chargers. Although technically you can use a normal charger, this is very dangerous, like trying to charge a car battery with a lightning strike.
However, if all is well, you have yourself a great setup. A LiPo battery is small, light and on a balanced intelligent charger (with an auto cutoff) will charge very fast and safely. A 7.4v LiPo will run an AEG like a 9.6v NiCad would and an 11.1v will run an AEG like no NiCad can (short of a 12v anyway); the rates of fire achievable while running an 11.1 make semi-auto cycles very crisp and make automatic fire so ferocious that short bursts are more then enough for an improved hit ratio or suppression fire. If you treat your LiPo properly, it will treat you properly.