Lithium Polymer Batteries used in RC airplanes

Lithium Polymer Batteries and how to select and care for these RC airplane batteries.

E-Flite Lipo battery

The latest RC battery technology is the Li-Po or Lithium Polymer battery. These radio control batteries have the highest power to weight ratio and are the main reason why electric RC airplanes are so successful and popular.

However they are different  when compared to the Ni-Cd batteries we are all used to.These Lipo batteries CANNOT be charged using a Ni-Cd RC battery charger. Only use RC battery chargers specifically intended for use with the lipo battery. It is possible to start a fire if these RC batteries are mis-used! ALWAYS remove Li-Po from the airplane, prior to charging and place in a fire safe area.


Series or Parallel Connection for Lithium Polymer Batteries?

Series/Parallel battery connection sketch

A single lipo cell is nominally rated at 3.7 volts. The individual cells can be connected together, to form a battery, either in a series configuration or a parallel style. If we say the cells shown in the diagram are 2100mAh, the voltage of the series pack(4S) will be- 3.7Vx 4 cells=14.8 Volts. The capacity of this battery will be 2100mAh.

The lipo battery shown in a parallel configuration (4P)will have a voltage of 3.7V and a capacity of 2100mAhx4 cells=8400 mAh.
 

The series pack is known as a 4S pack and the parallel as a 4Ppack.( the number denotes number of cells and the letter denotes series (S) or parallel (P).

If the series pack is doubled up and connected together in parallel it will be known as a 4S2P pack and will have a voltage of 14.8V and a capacity of 4200mAh. (2100 x 2 = 4200 mAh)

Another number you will come across when discussing lithium polymer batteries will be the "C"rating. This stands for the capacity rate of the RC battery. As an example you may come across 20C. Capacity is normally expressed as mAh (milliamp-hour) and there are 1000 milliamps in one amp. Thus a battery of 1000mAh can deliver 1A for one hour. If this same battery is rated at 20C then it can supply its 1 amp capacity up to 20 times higher, i.e. a maximum of 20amps can flow through it. However there is a price to pay in the form of duration. It can only supply this power for 3 minutes ( 1Ahx60/20=3 minutes).

If the lipo battery was 30C rated then current could be drawn through at up to 30 times higher ( 30A) but duration drops to 2 minutes ( 1Ahx60/30=2 minutes)

It is a good idea to be very conservative when it comes to batteries and use the cells at about half the claimed C rate. The risk of damaging the cells is considerably reduced and the number of re-charge cycles will be increased i.e the life will be longer.

 If not you can make up your own packs. You must be able to solder and please keep the wires separate from each other! A short circuit can be very damaging to the cells and yourself!. A further point when making batteries is-series packs can have cells of different voltage but the capacity must be the same. For parallel packs the voltage must be the same but the capacity can vary.    

 



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