Most Li-Poly batteries are grouped cells that have been wired together to achieve a particular voltage or capacity. The key to get the best performance out of your batteries is to understand how they work. There are many types of Li-Poly batteries on the market. You should choose the right one to suits your RC need.

What is Series

An individual Lithium Polymer cell has a nominal voltage of 3.7 volts (vs. 1.2volts per cell for Ni-Cds). When the cells are connected in series, the output voltage multiplies.

1 cell = 3.7 V
2 cells in series = 3.7x2=7.4 V
3 cells in series = 3.7x3=11.1 V
4 cells in series = 3.7x4=14.8V

What is Parallel

When individual cells are connected in parallel, the overall output voltage is not changed but the capacity multiplies. Wiring two Li-Poly cells in parallel doubles the capacity, which means more run time.

What is 4S2P

A battery pack that has four cells wired in series (giving 14.8 volts) and 2 of these 4-cell packs are wired in parallel is commonly referred to as a 4S2P (4 series, 2 parallel). The wiring combination can increase both voltage and capacity. A “4S2P” pack means it is an 8 cells pack: two sets of 4 cells wired in series with the two 4 cell also wired in parallel.

What is C Rating

Li-poly cells are also commonly given a C or current rating that specifies how much current it is capable of supplying. The charge and discharge current of a battery is measured in C-rate. This is also the maximum average recommended discharge current for the cell. For example, the AirThunder 2200mAh packs have a 25C rating. To determine the maximum recommended discharge rate, multiply the capacity times the C rating, i.e. 2200mAh x 25C = 55,000. The maximum recommended discharge rate would be 55,000mA or 55 amps.

To calculate of load current value with charge/discharge rate, it can be done by:

C-rate( C )= Charge or Discharge Current ( A) /Rated Capacity of Battery

Also, a battery’s run time on given discharge capacity can be obtained by:

Used hour of the battery=Discharge capacity(Ah)/Discharge current (A)

Recommended Reading regarding lithium polymer battery:

BackYard Flyer, March, 2007 , http://www.backyardflyer.com/articles/batterybulletin.asp

RC Heli, Feb/March 2007/Issue 9

Fly RC March 2007/Issue 40

Glossary

Amp / Ampere (A): The basic unit of current flow

Ampere hour (Amp hour, Ah): A measurement of the capacity of a storage medium (a single cell or a battery). A cell which can supply 1 Amp for 1 hour before it is discharged to a specified minimum level is said to have a capacity of 1 Amp hour (1Ah)

Balance Connector: A connector specifically designed for lithium polymer battery pack that charges all individual cells at the same time.

Battery: A bank of individual cells connected together

Battery Capacity: The measure of how long the battery will function at maximum current draw. Battery capacity is usually measured in mAh.

Burst: The maximum current that can be pulled from a pack for a very short time-generally only a few seconds (5-10). A pack has to be run under continuous current for a while before running the burst current again.

C Rating: Specifies how much current it is capable of supplying. The charge and discharge current of a battery is measured in C-rate. This is also the maximum average recommended discharge current for the cell.

Continuous: How much current a pack will deliver during continuous operation before it’s depleted.

Cell: One section of a battery.

Current: The flow of electricity through a conductor. Current is expressed as "I" and is measured in amperes or amps (A).

Hertz (Hz): The measurement of frequency. Hertz represents the number of cycles of an electrical signal measured in one second.

Impedance (Z): A resistance which is not just a pure resistance. This is to say that its loading characteristics are frequency dependent. Impedance consists of some value of resistance in conjunction with capacitance and/or inductance.

Max Continuous: The maximum current that can be pulled from a pack for continuous operation or until it has been depleted.

Parallel: It is referred to Li-Poly cells that are hooked up in parallel for charging and discharging purpose without causing any detrimental effect.

Power: The amount of current multiplied by the voltage at a given point. Power is expressed as "P" and is measured in wattage or watts (W).

Resistance: The measurement of the properties of a conductor that limit its ability to allow electricity to flow. Resistance is expressed as "R" and is measured in ohms (Ω).

Resistor (R ): An electrical device which impedes (resists) current flow regardless of frequency. Basic unit of measurement is the Ohm

Series: Most Li-Poly batteries are connected in series to provide high voltage.

Sustained: The maximum current that can be pulled from a pack for brief periods-generally less than 1 minute. The battery pack will have to be run the continuous current for a while before running the sustained current again.

Volt/Voltage (V): The basic unit of "electromotive force". One Volt applied to a resistance of one Ohm will force a current of one Ampere to flow. This is the difference in electrical potential between two points in a circuit. It's the force or pressure that pushes electricity through a conductor. Voltage is expressed as "E" and is measured in volts (V).

Voltage Port: Also known as Wire Gauge.

Watt/Wattage (W): The basic unit of power. 1 Volt across 1 Ohm (giving 1 Amp) dissipates 1 Watt (all as heat with a resistive load)

Watt hour (Wh): A measurement of the capacity of a storage medium (a single cell or a battery). A cell which can supply 1 Watt for 1 hour before it is discharged to a specified minimum level is said to have a capacity of 1 Watt hour (1Wh)

Wire Gauge:

Sources:

Backyard Flyer, March 2007

RC Heli, Feb/Mar 2007/Issue 9

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