What Exactly are LiPo Battery Packs?

...and what makes them so special?

LiPo battery packs have the highest power to weight ratio than any other battery available. These batteries are being used in just about everything portable these days ranging from hearing aids to laptop computers.

By drastically extending flight times and greatly increasing power, LiPo battery packs are making it possible for electric RC airplanes to compete with gas powered airplanes of similar size.

This was not even conceivable a few years back! So what exactly are Li-Po batteries and what makes them so special?

About Lithium Polymer Batteries

In a nutshell, a battery consists of two metals immersed in an electrolyte solution. A chemical reaction takes place that causes electrons to flow from one metal to the other. This produces a voltage difference between the positive and negative poles. When a load (such as a motor) is placed between the two poles, electrons will flow through the circuit until the metals are chemically neutralized (battery dead).

The voltage produced between the two metals is called the electrochemical potential. This energy is directly related to the types of metal used in the battery.

Lithium is the lightest metal in the universe and has the highest electrochemical potential. Lithium would be perfect for making lightweight powerful batteries right?

Well, almost... Because lithium metal is very unstable, battery manufacturers began using non-metallic lithium-ions.

Lithium Ion vs. Lithium polymer Battery Cells

A lithium-ion battery pack, Li Ion, uses a liquid electrolyte solution just like Ni-Cad and other common batteries. A rigid container is necessary to hold this liquid.

A lithium ion polymer battery back ,LiPo, pack uses a “gel-type” polymer as the electrolyte media. This polymer can take any shape and does not require a rigid enclosure. LiPo battery packs have a much higher energy density than other battery packs because the cells are not bound to cylindrical containers with dead space in between.

Voltage and Capacity of LiPo Battery Packs

Each individual lithium polymer cell has a nominal voltage of 3.7 volts. It is extremely important that a LiPo battery cell is not discharged below 3.0 volts or charged beyond 4.2 volts. Doing so will damage the battery.

Electronic speed controllers (ESC's) designed for LiPo battery packs have a Low Voltage Cutoff (LVC) that will automatically cut the power to the motor when the battery voltage (of an individual cell) reaches 3.0 volts.

To understand how LiPo battery packs are put together, you need to have a basic understanding of voltage and capacity. If you are not familiar with batteries, think of voltage as the "muscle" and capacity as the "energy" of a battery.

LiPo Cells in Series Increases Voltage

To build voltage, cells are added together in series. The voltage (muscle) of the LiPo battery pack will continue to increase as more and more cells are added in series. But the capacity (energy) will NOT increase as cells are added in series.

LiPo Cells in Parallel Increases Capacity

To build capacity, cells are added together in parallel. The capacity (energy) of the LiPo battery pack will continue to increase as more and more cells are added in parallel. But the voltage (muscle) will NOT increase as cells are added in series.

Total Voltage = 3.7 V
Total Capacity =2100mAh + 2100mAh + 2100mAh = 6300mAh

Building a Lithium Polymer Battery Pack

You can build a lithium polymer battery pack to meet the voltage and capacity requirements for any electric RC airplane by mixing a combination of cells in series and in parallel. The specification of each LiPo battery back can be represented by the standard xsyp naming scheme. The “x” represents the number of cells in series and the “y” represents the number of cells wired in parallel. Consider the following examples.

4s2p Lipo Battery Pack

The 4s2p battery pack illustrated above consists of two 4s packs in parallel.

The voltage of a 4s2p battery pack is 4 x 3.7V = 14.8V

The capacity of *this 4s2p battery pack is 2 x 2100mAh = 4200mAh

2s4p Lipo Battery Pack
4s2p LiPo battery packs

A 2s4p Lipo Battery packs consists of four 2s packs in parallel.

The voltage of a 2s4p battery pack is:
2 x 3.7v = 7.4V

The capacity of *this 2s4p battery pack is:
4 x 2100mAh = 8400mAh

*All LiPo cells are 3.7V, but the capacity of LiPo cells vary

Using Lithium Polymer Batteries

Lithium ion polymer batteries harness an enormous amount of energy in a very small space. These batteries will explode if used improperly. I don't say this to scare you. When proper LiPo battery safety precautions are followed, these batteries are completely safe. Lithium polymer battery care can be broke down into three categories, which are discharging, charging and storage.

Discharging Lithium Ion Polymer Batteries

It is important to have a properly sized LiPo battery back for the motor being used. An RC LiPo battery pack is designed to delivery a set amount of current to the motor. This value is determined by the C rating of the battery. To determine how much current a battery is rated for, simply multiply the C rating by the capacity of the battery back.

For example, a 5000mah LiPo battery pack with a 20C rating can safely provide 20 x 5000mah = 100,000 mA or 100 amps.
Discharging a LiPo battery pack at higher current than it is rated for is very dangerous.

If you are new to the hobby, don't let all of this technical jargon intimidate you. Ready to fly RC airplanes come with the correctly sized battery for the motor being used, and most all RC airplanes will tell you what size battery to use.

Lithium Polymer Battery Charging

It is absolutely critical that every precaution is taken when charging your LiPo battery back. Lithium polymer chargers use a different algorithm than any other type of battery charger. For this reason, a lithium polymer charger must be used when charging your LiPo battery back. Using a charger that is not specifically designed for LiPo batteries will damage the battery and could result in an explosion.

Lithium polymer battery packs do not develop a "memory" as NiCad batteries do. You don't have to worry about fully discharging LiPo battery packs before charging them. Your RC LiPo battery will be especially hot immediately after a flight. Always wait until the battery has reached ambient temperature before charging.

Current and voltage are the two variables that must be determined when charging LiPo batteries. The voltage should be set to the nominal voltage of the LiPo battery pack. A 5000mAh 11.1V LiPo battery pack would be charged at 11.1V.

The current should never exceed a 1C charge rate. For example, a 5000mAh 11.1V pack should be charged at 5 Amps or less. Charging at lower rates down to .77C could increase the life of the battery, although there is not conclusive evidence that I am aware of to support this. For example, a .77C charge rate of a 5000mAh pack would be 3.85 Amps.

Most LiPo chargers automatically detect the voltage (or cell count) of the LiPo battery pack. It is always a good idea to double check, or confirm that the charger detects correct the voltage pack.

LiPo Balancers

As stated above, lithium polymer cells must stay between 3.0V and 4.2V for optimal performance. A standard LiPo charger looks at voltage of the entire LiPo battery pack when charging. Some times the individual LiPo cells become unbalanced over time. For example, one cell may have a voltage of 3.8V while the rest are at 4.2V.

A LiPo balancer monitors and controls how much each cell in the LiPo pack is charged or discharged. Balancing the pack will allow for the fullest possible safe voltage which results in more power while in flight. A balanced pack also tends to last longer as no individual LiPo cells become over charged.

BizRate

Storing LiPo Battery Packs

LiPo batteries should always be stored in a cool environment. In other words, don't leave them in your car with the windows up on a hot summer's day. Keep them in a cool location out of the direct sunlight. It is highly recommended that Lipo batteries be kept in a charging bag while be charged and while not in used. A charging bag is a very cheap form of insurance!

Advantages of LiPo Batteries

Have nearly double the capacity with half the weight of NiCad or NiMH battery packs

Retain charge significantly longer than NiCad or NiMH batteries when not being used

Do not develop a "memory" as NiCad batteries do

With LiPo cells in combination of series and parallel, LiPo packs can be built to power most all RC aircraft

Disadvantages of LiPo Batteries

Have very short shelf life of two to three years
Very sensitive to high temperatures
Very sensitive to high and low voltages. Exceeding either will damage the battery
Prone to explode if exposed to heat, or charged/discharged improperly

LiPo Battery Safety

LiPo Battery Packs are safe as long as they are used properly. The video below is an example of what will happen when a LiPo battery is used improperly.

If you follow the below LiPo battery safety guidelines you can rest assured that you will avoid what happened in the above video.

Always let your LiPo batteries cool to ambient temperature before charging
Make sure your charger is designed specifically for Lipo battery packs
Double check that your charger has counted the cells correctly and is charging at or below 1C charge rate
Always charge your batteries on a solid non-flammable surface in a well ventilated area

Always check your LiPo batteries for any damage after a crash. If the LiPo back is punctured or damaged in any way, don't use it.

Always charge and store your LiPo batteries in a charging bag! Charging bags are relatively inexpensive and could potentially prevent your housing from burning down.

Never, under any circumstances, leave your batteries charging unattended
Never leave your batteries inside aircraft while charging

Examples of Lipo Battery Packs

ads from Shopzilla

BizRate

This page was provided for informational purposes only. Always consult with the battery manufacturer for specific procedures for charging and handling of your lithium polymer batteries.