By The Numbers: Batteries

LiPo Batteries.

• Configuration: How many cells the battery has. (1S, 2S, 3S)
  
• Capacity : How much electricity the battery holds. (500 mAh, 1000 mAh, 2200 mAh)
• Constant discharge: How fast the electricity can flow from the battery. (15C, 25C, 35-45C)
• Burst rate: A peak value that the battery can discharge for a short period of time.  (37C ,15sec)
• Pack size: three dimensions, HxWxL
  
• Weight: in these modern times, always expressed in grams.
• Charge Rate: how high of a current at which the battery can be charged (1C, 5C)

Notes.

"C" rate: This is the amount of power that can be pushed out of the battery at any particular instant in time. It is a ratio of the Capacity.  a 500 mAh battery discharging at 15C will be discharging at 750 mA. (500 * 15 = 750)
Lots of details here.

Voltage:  A single lipo cell will have a voltage range of 3.0V - 4.2V. The "nominal" voltage (how it's typically referred to) is 3.7V. If the cell goes below 3.0V, it won't be able to be recharged.  Most people recommend not letting a cell go below 3.2 volts so that you'll have a bit of safety margin.

So, a 3S battery will have 3 cells, a nominal voltage of 11.1V, and a voltage range of 9V - 12.6V (the nominal values multiplied by 3.

Your discharge rate requirements will be defined by your motor and ESC combination.  Models with high instantaneous power demands (helis, stunt 3D flying) will have higher discharge requirements.  Models with low instantaneous power demands (gliders, trainers) will have lower requirements.  Higher discharge rates usually mean the batteries will be heavier and more expensive.

Example: http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9312

• Configuration : 2S 7.4v
• Capacity : 2150mAh
• Constant discharge: 25C
• Burst rate: 37C (15sec)
• Pack size: 113x33x16mm
• Weight : 129g
• Charge rate not specified, so we assume 1C