Lithium-ion polymer battery

Lithium-ion polymer Batteries (LiPo) are very light weight efficient batteries. they have the largest power to weight Ratio in production. When looking for a LiPo pack there are 3 numbers to consider. First is the Pack Voltage, then its capacity (in Amp-Hours) and its C rating (determines max current and charge time)

Voltage

 * Max: 4.2v (fresh charged)
 * Nominal Voltage: 3.7v
 * Minimum voltage: 3v. Irreversible damage will occur below this voltage
 * Dead Pack voltage: 2.7v. Pack is destroyed.

Current

 * Max: Packs Capacity in Amp-Hours * C rating

Examples:
 * 1) A 11.1v 2000 mAH (2 Amp-Hour) 1C battery pack will supply 2 amps for 1 hour.
 * 2) A 11.1v 2000 mAH 2C battery pack will supply 4 amps (for half the time).

Charging
Use caution and only charge them with LiPo chargers. Multi-cell batteries require periodic (at least once every 10 charge cycles) balancing to ensure each cell does not exceed 4.2v.

If you must charge them manually (not recommended), you must closely monitor the cell voltage and ensure at no point does it exceed 4.2 volts. Limit the current to the packs C rating until the current consumed by the battery drops to 0.1C (10%) or 90 minutes has passed.

Safety
LiPo batteries are known to catch fire (usually violently) if shorted, over charged (above 4.2v per cell), and/or punctured/crushed. This is a dangerous fire as common fire extinguishers will have no effect on the fire or make the fire worse.

Availability

 * Sparkfun
 * Any Hobby store that sells R/C planes/cars