Hobbyists who own drones or other RC toys have probably used a LiPo battery at some point. Although LiPo battery technology is not new, it has enjoyed a surge in usage because of the increasing popularity of drones in the past couple of years. This guide was developed to provide all the essential information on LiPo batteries – what they are, how they work, what benefits they give, and how to care for them. If you have a LiPo battery or are thinking of buying a device that uses one, then it’s worth your time to go through this whole guide.
What are LiPo batteries?
“LiPo” is short for lithium polymer, which describes the type of electrolyte used in LiPo batteries. Although the development of LiPo batteries started in the 1970s, it is only in the recent year that they have become widely used in mainstream applications. The compact and lightweight design of LiPo batteries have made them a viable alternative to a fuel power source for unmanned aerial vehicles.
LiPo batteries are also known to have very high output values which have also made them appropriate for battery-operated airsoft guns. Currently, the mobile device and smartphone industry have started the transition from Li-ion, with more and more of the newer models choosing to use LiPo batteries. Among other applications, LiPo batteries have been used for laptops, portable music players, e-cigarettes, power banks, and video game controllers.
How are they different from Li-ion and Ni-Cd batteries?
To understand why LiPo has become the battery technology of choice for RC aircraft, we need to understand how a battery works. Any battery has three main components: a positive end (cathode, a negative end (anode), and an electrolyte solution. The cathode and the anode of the battery are separated by a polymer material which prevents them from coming into contact with each other and causing a short circuit.
It is thru the chemical reactions between the electrolyte and both the cathode and anode that a battery can store and generate energy. Charging the battery forces ions in the electrolyte solution from the cathode to the anode, and using the battery reverses this flow. Older battery types, such as nickel-cadmium (Ni-Cd) and lithium-ion (Li-ion) use a liquid electrolyte solution. For instance, a potassium hydroxide solution is usually used as an electrolyte for Ni-Cd batteries.
LiPo batteries improve on the design of traditional batteries by using its namesake, a lithium polymer, as an electrolyte. Instead of a liquid, the lithium polymer is a gel-like substance that can be configured into a very thin and long semi-porous layer. Most LiPo batteries use any of four polymer types to suspend the lithium salt electrolyte: polyethylene oxide, polyacrylonitrile, polymethyl methacrylate, or polyvinylidene fluoride.
Aside from the difference in electrolyte material, the construction of a LiPo battery is also radically different compared to older battery types. The traditional construction of a battery, as seen in Li-ion or Ni-Cd batteries, use anode or cathode plates layered alternately. Commonly known as a “jelly roll” orientation, each pair of these plates is separated by a polymer material. These layers are then rolled to form the classical cylinder shape of batteries that everyone has become familiar with.
The use of a polymer electrolyte material allows LiPo batteries to have a much more compact build. Anode and cathode layers, typically made from aluminum and copper, are layered alternately. A semi-porous lithium polymer separates each anode-cathode pair. This whole film is then folded accordion-style, which results in alternating layers of cathode and anode, with the continuous layer of semi-porous lithium polymer sandwiched throughout. This matrix is then saturated with an electrolyte solution, filling in the interstitial space between the cathode and anode layers.
Pressure is applied to the folded matrix, helping keep the battery compact and maximizing contact between the anode and cathode layers and the lithium polymer. Application of pressure also removes any excess air that may have been trapped inside the matrix during injection with the electrolyte. The matrix is then wrapped in a plastic pouch before being encased in the battery container.
The accordion folding style allows a very long polymer film to be contained in a very small package. For instance, a 5000 mAh LiPo battery which has a thickness of less than half an inch can contain a film that is about 7 feet long. This unique and innovative construction makes LiPo batteries incredibly compact and gives them a very high energy density. Moreover, the absence of metal plates inside a LiPo battery makes them significantly lighter compared to other battery types.
Understanding the numbers
A LiPo battery, much like any other battery, comes marked with several values that describe its capabilities. Understanding these values may help you decide which battery pack is appropriate for your application. The following are the three main ratings you should be looking out for.
Voltage is the measure of a potential difference between the cathode and the anode of a battery. It is determined by the types of materials used in the cathode and anode, as each chemical reaction exhibits a unique electrochemical potential. As a rule of thumb, you only need to remember that most LiPo cells are rated with a nominal voltage of 3.7V. This means that a battery with a rating of 7.4V is made of two LiPo cells arranged in series.
A battery’s voltage influences how fast a motor can go when it is connected to the battery. For instance, a 3500kV motor will turn at 12,950 rpm when connected to a single-cell LiPo battery. By using higher voltage batteries, you can increase the rpm output of any motor. Take note that, although it’s possible to “overclock” a motor using more powerful batteries, each motor is only designed to run safely at a maximum output before it starts to overheat.
The nominal voltage of a battery will also determine the maximum voltage it can hold and its minimum safe charge. If you have a voltmeter, then you can check the battery’s voltage while it is plugged in. A 3.7V battery holds a maximum of 4.0V when fully charged. It is generally not recommended to run the battery below 3.0V.
The capacity of a battery, typically expressed in milli-ampere hours (mAh), is a measure of how much charge the battery can hold. The mAh rating of a battery describes how much load it will take to completely discharge a battery in an hour. This can be easily correlated to the rated current draw of a motor to estimate just how long the battery can power the motor before it needs to be recharged.
The size of the anode and cathode layers influences the number of sites where charged particles can bond to, determining the charge capacity of the battery. This means that increasing a battery’s capacity normally comes at the price of increased size and weight. This is a critical balance that battery manufacturers need to make, especially for batteries designed for use in unmanned aerial vehicles.
For reference, the DJI Spark uses a 3-cell 1480 mAh LiPo battery that weighs around 0.2 lbs. and provides a maximum flight time of only 16 minutes. On the other hand, the DJI Mavic 2 drones use a 4-cell 3850 mAh battery. Although it offers up to 31 minutes of flight time, it is also much heavier at around 0.64 lbs.
Using a higher capacity battery is not always an option to extend the operations of a motor. Most motors are only rated to operate for a specific length of time before it starts to get overheat. Using a battery with a higher capacity than the one recommended by the motor manufacturer may result in the motor operating beyond its limitations.
3. Discharge Rating
The Discharge Rating of a battery is a measure of the maximum rate at which current can be drawn from the battery. Although motors have a rated average current draw rate, there may be certain instances where this its draw rate can spike, such as when it is moving at maximum speed or when it is lifting a heavy payload.
A battery’s discharge rating is typically expressed as a multiple of its capacity. For example, a battery with a discharge rating of 50C can safely supply a current draw that is 50 times its capacity. For a 5000mAh battery, current can be safely drawn up to 250A. Beyond this draw rate, the battery runs the risk of suffering permanent damage.
Modern LiPo batteries usually have two discharge ratings: the Continuous Rating (or C Rating) and the Burst Rating. The C Rating, as its name implies, applies to a continuous current draw and is always lower than the Burst Rating. On the other hand, the Burst Rating is only acceptable for 10-second bursts of extra-heavy motor operations.
To check if your battery’s discharge rating is compatible with the current draw of your motor, you will need to check the motor’s rated maximum continuous draw and its burst draw. It is generally recommended to select a battery that has ratings a little higher than the requirements of your motor. You don’t have to go overboard, though – a 10% to 20% “over-rating” should suffice so that you don’t end up buying an unnecessarily powerful and expensive battery.
What are the advantages of using LiPo batteries?
1. Compact and lightweight
The accordion-style folding of the lithium polymer film and the lack of metal plates make LiPo batteries exceptionally small and lightweight. They also do not come in rigid plastic cases, making it possible to construct LiPo batteries in a variety of shapes and sizes. These characteristics have made LiPo batteries the power supply of choice for RC toys and drone, where minimizing weight is a priority.
2. High discharge rate
Li-ion batteries, currently the most commonly used battery for mobile devices, are notoriously known to have very low discharge rates. This is fine for tablets and smartphones who have somewhat constant current discharge rates and have a smart mechanism to regulate current draw. However, the same cannot be said for motors that support a mechanical load, such as those that power a drone’s propellers or the engine of an RC car.
LiPo batteries have a higher discharge rate, allowing motors to operate in a much wider safety margin. This means that LiPo batteries are better equipped to support changing mechanical loads, such as when an RC car has to climb uphill or traverse rough terrain.
What are the disadvantages?
1. More expensive
One of the biggest hurdles to the wider adoption of LiPo batteries is the higher cost compared to their Li-ion or Ni-Cd counterparts. The manufacturing cost of Li-Po batteries is around 10% to 30% higher, and this directly translates to a higher price tag. The overall cost of exclusively using LiPo batteries does not end there, as the following item emphasizes.
2. Shorter lifespan
In addition to being more expensive, LiPo batteries also tend to die earlier compares to Li-ion batteries. A LiPo battery typically starts to show a decline in performance after about 200 to 300 full recharge cycles. This is a massive downgrade considering that standard rechargeable Li-ion batteries available today can survive up to 1000 cycles.
The fact that you will need to buy new LiPo batteries more frequently, on top of them being more expensive in the first place, makes the use of LiPo batteries very expensive.
3. Less durable
In contrast to the rigid battery casing of traditional “jelly-roll” batteries, LiPo batteries are encased in thin and flexible material. This gives LiPo batteries more flexibility in terms of application but also makes them more vulnerable to mechanical damage such as getting pierced or punctured. Applying excess pressure on LiPo batteries also has a chance of damaging the polymer separator, which can result in the battery forming a short circuit. The lack of mechanical durability of LiPo batteries is one of the main reasons why there have been so many instances of these batteries spontaneously exploding or catching fire.
What causes swelling or puffing of LiPo batteries?
If you’ve ever held on to a smartphone or any mobile device for too long, then you have probably seen how a battery can puff up or swell after some time. Although this phenomenon is common to all batteries, the lack of a rigid casing in LiPo batteries makes swelling up even more prominent.
The key process behind this phenomenon is called electrolytic decomposition. As mentioned earlier, all batteries have an electrolyte component which allows for the passage of ions from cathode to anode and vice-versa. The problem is that this electrolyte solution naturally degrades over time, releasing a suite of gases that include oxygen, carbon dioxide, and carbon monoxide. As these gases continue to expand, they exert more and more pressure on the battery’s casing, causing them to puff or swell.
There are two major consequences when a battery starts to swell. The first comes from the fact that the electrolyte solution plays a crucial role in the performance of the battery. The decomposition of the electrolyte means that there are a fewer number of ions that can be exchanged by the cathode and the anode. This means that the battery has a decreased capacity to hold a charge, although this is not the only phenomenon that contributes to a battery’s declining performance (as we shall see later).
The second and more important consequence of a battery swelling up comes from the fact that oxygen is a highly flammable substance. This means that a battery that gets overheated, or a single spark caused by the contact of the cathode and the anode, is enough for the battery to catch on fire. As a battery continues to swell, the buildup of flammable gases makes it more likely that the battery will explore or catch fire eventually.
For both these reasons, it is recommended to stop using a swollen LiPo battery. Although some users have told accounts of continuing to use swollen batteries without any heavy consequences (a swollen battery will probably not fail right away), this should not be an acceptable practice. Even if the battery does not catch fire, the electrolyte solution is highly corrosive and will probably damage your device permanently should it leak. When your LiPo battery starts to swell, we recommend looking for a replacement battery right away.
What causes the aging of LiPo batteries?
As with other batteries, LiPo batteries start to deteriorate over time. In fact, the process of a LiPo battery declining in performance over time starts the very first time that it is activated. To understand this phenomenon, we merely have to go back to the process of how the different components of the battery interact to store and generate power.
As mentioned earlier, charging the battery forces ions to move from the cathode to the anode. This cycle reverses when the battery produces power. However, the process of ion exchange between the electrolyte solution and either one of the two terminals of the battery is not perfect. For instance, as the ions migrate from the cathode to the electrolyte when the battery is in use, some of the ions become permanently stuck to the anode. This thin film made of lithium is formed on both the anode and the cathode, reducing the number of bonding sites where lithium ions can be exchanged with the electrolyte. As these restrictive layers continue to build, a battery’s capacity to hold a charge continues to decrease.
The formation of restrictive layers on the cathode and anode, in tandem with electrolytic decomposition, cause the performance decline of a battery over time. Both these processes occur naturally, are inevitable, and are not reversible.
Best tips for caring and using LiPo batteries
Although all LiPo batteries only have a limited time in which they can function normally, you can make a few measures to extend their useful life. Most of these battery care tips are also applicable for Li-ion or Ni-Cd rechargeable batteries.
1. Don’t leave them plugged in unattended
When left plugged in for a long time, and after it has been fully charged, LiPo batteries tend to build up heat. Not only will the high temperature accelerate the deterioration of the battery but can also result in the battery catching fire or exploding. Based on reports of LiPo battery-related fires, most of them happen while the battery is plugged in.
As a precautionary measure, we also recommend charging your LiPo battery away from flammable materials. You can also have a bucket of sand on hand nearby just in case the battery causes a fire. Better yet, you can charge the battery inside a LiPo-safe bag which will prevent a fire from spreading.
2. Don’t let LiPo batteries run dead
For each 3.7V (nominal) cell of a LiPo battery, we do not recommend letting them run until they are below 3.0V. Letting the battery go into “deep discharge” runs the risk of allowing some of the anode material to dissolve into the electrolyte solution. This will permanently reduce the ability of the battery to hold a charge. Although most LiPo batteries have a built-in voltage threshold that will prevent deep discharge, they can still slowly discharge over time while in storage.
3. Store them in a cool place
As you might have noticed by now, LiPo batteries don’t like high temperatures. A study that compared the rate of performance decline of LiPo batteries during storage showed that a battery stored at 25 degrees Celsius retained 96% of its capacity while another battery stored at 60 degrees Celsius deteriorated to around 75% of its capacity.
4. Store them at 40% to 50% capacity
A scenario that commonly happens is that a drone pilot (or any user of a LiPo battery) charges their LiPo batteries to full capacity, anticipating a full of drone flight. However, plans fall through, and the fully-charged batteries end up in storage for a few months. To the owner’s surprise, the battery has lost up to 20% of its capacity the moment that it’s used again.
Keeping a battery fully charged while in storage speeds up the formation of the restrictive layer in the battery’s anode, resulting in decreased capacity. On the other hand, storing it near 0% runs the risk of allowing to go into deep discharge while in storage. A happy medium is at around 40% to 50%. This should correspond to a voltage reading between 3.6V to 3.8V.
5. Invest in a fire-proof LiPo bag
LiPo bags are fire-proof and explosion-proof bags that are specialty designed for safe storage and transportation of LiPo batteries. Insulated travel packs are highly discouraged for use in storing LiPo batteries, as these are typically made from flammable foam and plastic.
Based on recent IATA regulations, all portable batteries need to be included in the carry-on baggage when traveling by air. If you are carrying LiPo batteries, you may be required to carry them in a fire-proof LiPo bag. You can also take extra precautions to avoid short circuit of the batteries, such as storing several batteries separately and taping the positive and negative terminals of each battery.
How to dispose of a LiPo battery
Alas, even when you take good care of your LiPo battery, it will eventually succumb to old age. Just like any other battery, disposing of a LiPo battery is not as simple as throwing it in with the rest of your rubbish. Batteries disposed of haphazardly can cause fires and can leak toxic and hazardous chemicals. Before disposing of a battery, the owner is responsible for ensuring that is has been completely discharged.
In the past, submerging a LiPo battery in saltwater was an acceptable way of discharging it. More recent wisdom has disproved the safety of this method, as saltwater is very corrosive and can compromise the integrity of the battery’s terminals. Besides, discharging a battery in saltwater is a very slow process that can take several days to weeks to complete. During this time, corrosion can take its course and can result in the toxic electrolyte solution leaking to the water. Disposing of this solution is decidedly more problematic.
The more accepted way of discharging a LiPo battery is by hooking to a load, such as a lightbulb. A high-wattage halogen bulb is best for this application, as it should drain the LiPo battery in just a few hours. You can even speed up the process by hooking up several lightbulbs in parallel.
After the LiPo battery has been discharged, you need to bring it to a proper battery recycling facility. Most waste facilities accept hazardous waste materials, including spent batteries. If you can’t find a waste facility near you, you can also check if large electronic retailers such as Target or Best Buy will accept them. However, you also need to check if these retailers forward these spent batteries to the proper facilities.
Advances in LiPo battery technology
Nowadays, battery technology is no longer exclusive to small devices such as drones and RC cars. Electric cars have started a revolution towards more environmentally friendly transportation solutions. At the very core of the development of electric cars is the use of batteries with long lifespans, high energy density, and high charging efficiency. Naturally, lithium-ion and lithium-polymer batteries are included in the mix of batteries currently being used in electric vehicles.
Lithium-ion batteries are still considered superior to older battery technology such as nickel cadmium and lead acid. This means that research efforts are mostly focused on improving lithium-ion battery design to provide better longevity and to eliminate the risks of fires and explosions. So far, there have been a few innovations proposed to address these problems, such as the use of a lithium metal electrode and the addition of a phosphorous and sulfur compound to the electrolyte solution.
While it remains to be seen how these recent advances will move lithium-ion battery technology forward, there is no doubt that the thriving electric vehicle industry will continue to foster these research efforts.
While the development of LiPo batteries did not exactly revolutionize the battery industry as a whole, LiPo batteries have seemingly found their true calling in drones and RC cars. With more recent models addressing problems on safety and reliability, LiPo batteries have started to be used in more common devices such as tablets and video game controllers.
LiPo batteries may require special care so they can be used safely and preserved for a long time, but the same can be said for other batteries. By following the tips we have listed above on caring for your LiPo battery, there is no reason for you not to get the best out of your LiPo battery.
As with any other battery, your responsibility as its owner does not end when the battery is no longer usable. Proper battery disposal procedures should still be followed to avoid battery-related fires and leakage of toxic substances to the environment.