Lithium Polymer Battery

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Overview

Lithium polymer battery

Lithium polymer (also known as Li-Poly or Lipo) batteries are the most common type of battery used in present-day UAVs. They are commonly used as the main power source for the UAV's propulsion system, and/or as a secondary power source for electronic subsystems. Their high energy density and discharge rates make them ideal for use in flying vehicles. However, they can be easily damaged or even pose fire hazards if not used or maintained properly.


Selection

The main parameters used when selecting a battery are its number of cells (sometimes referred to as "S", for example 3S is a 3-cell battery), its capacity (expressed in milliamp-hours or mAh), and the batteries discharge rating (called the C rating).

The number of cells determines the voltage of the battery pack. Since each cell has a maximum voltage of 4.2 Volts, a 3 cell battery pack would have a maximum Voltage of 12.6 volts. Higher voltages will allow the motor to deliver more power, but care must be taken not to exceed the motor's current rating, or electronics voltage ratings when selecting the voltage of the battery.

The battery's capacity specifies how much energy the battery can store. For example, a 1000 mAh (milliamp-hour) battery can deliver 1000 milliamps for one hour. Similarly, you can calculate the run time in hours for any other current draw by dividing the capacity in milliamp-hours by the current draw in milliamps.

The C rating specifies the maximum current the battery can safely provide. For example for a 40 C, 2000 mAh battery you can draw a maximum of 2000*40 = 80,000 milliamps = 80 Amps of current. Exceeding this value would damage the battery and cause overheating.

Some batteries specify a 'continuous' C rating and a higher 'burst' C rating. While the continuous C rating specifies the maximum current at normal operation, the burst rating gives you a higher value that can be safely achieved for short periods of time (a few seconds).

Another important factor in battery selection is the weight of the battery. The battery capacity is the main driver of the weight, adding roughly 1 gram for each 10 mAh of capacity. A higher cell count will also increase the battery's weight, although the effect is small.


Dynamics

Lithium polymer battery voltage curve

Lithium polymer batteries have a voltage per cell that varies between 3.0 and 4.2 Volts. The voltage of a multi-cell battery pack is the voltage per cell multiplied by the number of cells. The voltage per cell varies as shown in the graph to the right. As the battery is discharged, the voltage drops gradually and constantly until it reaches 15% charge at a voltage of 3.7 Volts. Below this point the voltage drops much more rapidly.

Each lithium polymer battery cell also has an internal resistance of around 5 milliohms for a new cell and 20 milliohms for an old cell. The effect of this internal resistance is to cause a significant instantaneous drop in the battery voltage as a function of the actual current draw. For example, if we are drawing 40 Amps from a brand new 2-cell battery, this means the voltage would drop by 1.6 Volts:


V_{{drop}}=Current*Resistance=40Amps*(0.020Ohms*2Cells)=1.6Volts


If the battery is fully charged, its fully charged voltage is 4.2V*2= 8.4 Volts, but it would drop to 8.4V-1.6V = 6.8 Volts while we are pulling the 40 amps.


Setup

Lithium polymer batteries have two connectors. One set is used for charging (called the balance plug), and another for providing power to the aircraft (the discharge plug). The discharge plug is the one with only two cables colored red and black. The discharge plug is typically connected directly to an UAV's Electronic Speed Controller (ESC), or to a power distribution board which connects to multiple ESCs. Although the balance plug is used mainly for charging the battery, some devices such as LED lights might use this plug to draw low-power current. Voltage meters can also be connected directly to the balance plug to monitor the battery's voltage while in use.

Almost all batteries use a JST-style connector for the balance plug. However, there is no such convention for the discharge plug and there are many connector types on the market. Often times you will need to manually solder your own preferred type of connector to the batteries discharge plug. When this is done, it is important to assure a strong soldering connection and to avoid the red and black cables being crossed.


Usage

There are a three main things to watch when using a lithium polymer battery: not to exceed the maximum current, not to drain the battery below its minimum voltage, and not to overcharge the battery above its maximum voltage. Doing any of these will likely damage the battery and render it useless, or even make it catch fire or explode. The maximum and minimum voltage of a battery is determined by its cell count. The absolute minimum safe voltage per cell is 3 volts, and the maximum is 4.2 volts. Multiply these values by the cell count to get the minimum and maximum voltages for a given battery pack. These values are summarized in the following table, along with the battery nominal voltage which is the voltage commonly used by sellers to specify the cell count of the battery.

Lithium Polymer Battery Voltages
Cell Count Nominal Voltage Minimum Voltage Maximum Voltage
1 3.7 3.0 4.2
2 7.4 6.0 8.4
3 11.1 9.0 12.6
4 14.8 12.0 16.8
5 18.5 15.0 21.0
6 22.2 18.0 25.2

It is often desirable to use a battery voltage alarm to notify you when the battery voltage is getting too low. Another common method is to use a timer and limit the flight time of the UAV based on knowing the approximate time to depletion of the battery. To prevent battery damage it is also important to prevent the battery from overheating by exposing it to the airflow on the UAV. A damaged lithium polymer battery will likely look inflated or bloated, and it will not hold as much charge.

Charging

Lithium polymer batteries require specialized chargers. The battery is charged through its balance plug, which allows the charger to balance the voltage of all the cells. Some chargers also require you to connect the discharge plug during charging. Batteries are usually charged at 1C to 2C, meaning that a 2000 mAh battery would be charged at 2000 mA (2 Amps) to 4000 mA (4 Amps) current. The appropriate charge rate is often times found on the label of the battery, or in the manufacturer's website. It is also important to let the battery cool in between use and charging.

Safety

Lithium polymer batteries can catch fire or explode if not managed properly. Some triggers could be over-charging the batteries, unbalanced cell voltage, or causing a short-circuit. It is therefore important to assure the battery leads never short out during storing, transportation or usage of the battery. Also, the batteries should be charged and stored away from flammable objects, and in cool places. Batteries are sometimes placed in fireproof metal boxes during charging or transportation to improve safety.

Maintenance

Lithium polymer batteries have a limited lifetime. It can vary roughly from 50 charge-discharge cycles at maximum current discharge, to 500 cycles at 1C discharge.

There are some things can be done to maximize its lifetime. Using a charger with a 'balance' feature to always keep the cell voltages equal is helpful. Using a charger to 'cycle' the battery from full charge to discharge every once in a while can also help the battery performance.

Lithium polymer batteries should not be discarded in the trash bin, and should be recycled instead. Many stores such as Best Buy offer free battery recycling.