How to Choose Lipo Battery for RC Airplane?

by Zoey Song on February 20, 2022

The Battery is an essential yet frustrating part of RC flying. It is needed to power the motor and other devices on the RC airplanes but adds most of the unwanted weight that hinders flight.

A battery can also cause damage to an electrical system if not properly used. Therefore, finding the right battery that will safely supply the best power with the least amount of weight will make endeavors with RC flying more successful.

That is why it is important to understand how batteries work. In order to understand how they work, it is necessary to learn some basic electrical terms such as voltage, current, resistance, capacity, power, and circuit.

Circuit Illustration

Here is an illustration of a circuit to help clarify the above terms:

The source of the circuit is a battery and the load is an electric motor. In between the battery and the motor is a speed control which is the resistor.

The battery contains nine volts of electrical pressure (voltage), and 2000 milliamp-hours of stored energy (capacity). The speed control (resistance) is in the off position, therefore, zero amps of current are flowing through the wire from the battery to the motor, and zero watts of power is being dissipated.

The speed control is then opened enough to allow 4 amps of current to flow from the battery to the motor. Since the battery has 2000 milliamp-hours of capacity, 36 watts (9 volts x 4 amps) of power will be dissipated (used up) in one-half hour ((2000 milliamp-hours / 1000) / 4 amps).

This circuit illustration roughly depicts the electrical configuration of some of the devices on an RC aircraft. With an understanding of the illustration, it is then easy to make some useful calculations for batteries.

Cell Arrangement

A battery (battery pack) is made up of two or more cells. A cell consists of a hard cylinder casing (Li-Po’s have soft casing) that contains a certain type of chemical. The type of chemical that is in the cell determines its voltage (V). For example, both Ni-Cd and Ni-MH batteries have 1.2 volts per cell, where Li-Po batteries have 3.7 volts per cell.

Cells are connected together either in series to increase the voltage, in parallel to increase capacity, or in any combination of the two. How the cells are connected together is called cell arrangement.

Cell arrangement is often labeled as XSXP which indicates how many cells are in series and how many are in parallel. For example, if a battery pack has six cells – four in series and two in parallel, then it would be labeled 4S2P.

There are a couple of simple calculations that will help explain the concept of cell arrangement further.

Memory Effect

Memory effect is when batteries lose their longevity due to not being fully discharged. The battery “remembers” where it was discharged to.

If a battery is not fully discharged, the next time the battery is charged, it starts from the place where it was last discharged to. Then when it is discharged again, it will only discharge to that remembered place.

This reduces its capacity. Batteries will retain more capacity and last longer if they are fully charged and then fully discharged. This process is called cycling and can be performed in succession with certain chargers.

Ni-Cd and Ni-MH

Two types of batteries have been the norm for electric RC airplanes – Nickel Cadmium (Ni-Cd) and Nickel Metal Hydride (Ni-MH).

Ni-Cd batteries have been around the longest and are quickly becoming obsolete. They suffer badly from memory effect and the cadmium is toxic so the batteries must be recycled properly. They also lose their energy quickly so they must be used fairly soon after they are charged.

Ni-MH batteries are cadmium-free and don’t suffer quite as badly from memory effect. They also have better energy retention.

Factors

There are a few things to notice when purchasing Ni-Cd or Ni-MH batteries for electric RC airplanes:

The batteries (battery packs) consist of cylinder shaped cells.
The cells are arranged in series to determine voltage.
The cells are rarely arranged in parallel. Instead, the capacity is set by the manufacturer for each pack and may vary from pack to pack.
The cells are sometimes grouped all in one row, sometimes grouped in more than one row, and held together with shrink wrap.
The number of cells in the pack are apparent and can be counted and multiplied by 1.2 to figure the voltage.
The type of battery, the voltage, and the capacity is labeled on the front of the pack, for example, Ni-MH BATTERY, 7.2V, 800mAh.
The higher the voltage, the more power the pack will give to the motor.
The higher the capacity, the longer the flight time.
Each battery pack has a wire lead with a connector that extends out from one end of the pack.
Wire leads have different types of connectors that may or may not match your equipment.
It may be necessary to cut off the included connector and solder on an appropriate connector.

Li-Po

Li-Po stands for Lithium Polymer. Each cell of a Li-Po battery produces 3.7 volts so a 2-cell battery produces 7.4 volts and a 3-cell battery produces 11.1 volts.

These batteries have many advantages over NiMH and NiCd batteries.

Lithium Polymer is a gel that can be cased in a thinner container than other batteries, thus making them much lighter.

The Li-Po batteries have greater storage capacity, so they can be charged a week in advance, whereas the Ni-Cd and Ni-MH must be used soon after they are charged.

Even though Li-Po batteries have advantages, they are still somewhat experimental and have some drawbacks.

Li-Po batteries must be slow charged, so it takes longer to charge them.

But more importantly, they still have some inherent dangers associated with them. In fact, all distributers of Li-Po batteries must publish and have consumers agree to the risks involved.

Here is a standard notice that is on all websites that sell Li-Po batteries:

Li-Po Safety Tips and Buyer Agreement

Please note that a Li-Po battery must be charged with a dedicated Lithium Polymer charger such as the Apache, WesTech, ST Model 2S, Triton or Orbit. The buyer must understand that there are risks, known and yet unknown, associated with the use of the Li-Po battery in RC hobby.

Only use chargers designed specifically for Lithium Polymer battery.
Double check the charger, number of cell selection setting, and mAh before every charge.
Only charge the batteries on a non-flammable non-conducting surface, such as bare cement floor.
Do not charge the battery inside the model plane, inside of ones car, home furniture or wood floor/carpet, or anywhere near flammable material.
Monitor the charging of the battery pack at all times and do not leave the battery unattended.
Place a fire alarm above the Li-Poly battery charging location. Follow the fire alarm installation instructions.
After a crash, inspect the battery pack for damage. Discard the pack outdoors if there is any sign of damage.
Store the pack fully charged in a cool and dry location and out of reach of children.
Do not assemble packs in series into parallel packs as mismatch could result in fire.
Do not store Lithium Polymer battery inside a car.
Do not short the battery as it may catch on fire. If accidentally short a battery, place it in open space and observe the battery for 10 minutes. It may swell up and possibly even catch on fire.