There are a few different types of batteries being used in electric airplanes today. Some are used more than others as technology advances and better batteries are made but here are the basic types:
NiCad (Nickel Cadmium):
these are of the first rechargeable batteries that were used in electric airplanes and are not the most popular primary source of power anymore.
NiMH (Nickel-Metal Hydride):
These batteries were an improvement on the older NiCad batteries and are still often used as receiver batteries and to power a transmitter but even these are beginning to get used less and less.
LiPo (Lithium Polymer):
These batteries have been around for a few years now and are probably the most popular choice for a primary power source in electric flight. While they can prove to be dangerous if not treated with the proper respect they are constantly being improved and larger capacities with greater discharge abilities are being made available quite frequently. They are also lighter for the same capacity as the older NiCad and NiMh batteries and as weight is a large factor to take into consideration in electric planes this is why they are favored over the older types of battery.
A123 (LiFe, Lithium Ferrite):
These batteries have most of the benefits of the LiPo type batteries except that they are a little heavier for the same capacity as a LiPo battery. However, they are also safer and can take a lot more abuse than a LiPo battery. Due to their weight they are usually preferred for larger electric airplanes.
Some Terms Related to Batteries:
MAh: Capacity of the battery in milli Ampere hour (sometimes Ampere hour, Ah). Theoretically this specifies how much current (measured in mili-ampere or Ampere) the battery can supply for a period of one hour.
C rating: Maximum continuous discharge rating of the battery. Theoretically this number when multiplied with the capacity of the battery in mAh (or Ah) gives the maximum current that the battery can supply for an extended period without overheating. It is also used to specify at what multiple of the battery capacity the battery can be safely recharged (the recharge C rating is generally much smaller than the discha.
S: This specifies how many battery cells are connected in series to make up the battery pack.
P: : This specifies how many battery cells or even separate smaller battery packs are connected in parallel to make up the battery pack.