Lithium Polymer batteries (henceforth referred to as “LiPo” batteries), are a newer type of battery now used in many consumer electronics devices. They have been gaining in popularity in the radio control industry over the last few years, and are now the most popular choice for anyone looking for long run times and high power.
LiPo batteries offer a wide array of benefits. But each user must decide if the benefits outweigh the drawbacks. For more and more people, they do. In my personal opinion, there is nothing to fear from LiPo batteries, so long as you follow the rules and treat the batteries with the respect they deserve.
Let’s first talk about the differences between LiPo batteries and their Nickel-Cadmium and Nickel-Metal Hydride counterparts.
LiPo Packs versus NiMH Batteries
LiPo batteries offer three main advantages over the common Nickel-Metal Hydride (NiMH) or Nickel Cadmium (NiCd) batteries:
- LiPo batteries are much lighter weight, and can be made in almost any size or shape.
- LiPo batteries offer much higher capacities, allowing them to hold much more power.
- LiPo batteries offer much higher discharge rates, meaning they pack more punch.
But, just as a coin has two sides, there are some drawbacks to LiPo batteries as well.
- LiPo batteries have a shorter lifespan than NiMH/NiCd batteries. LiPos average only 300–400 cycles.
- The sensitive chemistry of the batteries can lead to fire if the battery gets punctured and vents into the air.
- LiPo batteries need special care in the way they are charged, discharged, and stored. The required equipment can be expensive.
What Do All the Numbers Mean?
They way we define any battery is through a ratings system. This allows us to compare the properties of a battery and help us determine which battery pack is suitable for the need at hand. There are three main ratings that you need to be aware of on a LiPo battery.
So what does it all mean? Let’s break it down and explain each one.
Battery Voltage / Cell Configuration
A LiPo cell has a nominal voltage of 3.7V. For the 7.4V battery above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a “2S” battery pack – it means that there are 2 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.
NOTE: Nominal voltage is the default, resting voltage of a battery pack. This is how the battery industry has decided to discuss and compare batteries. It is not, however, the full charge voltage of the cell. LiPo batteries are fully charged when they reach 4.2v/cell, and their minimum safe charge, as we will discuss in detail later, is 3.0v/cell. 3.7v is pretty much in the middle, and that is the nominal charge of the cell.
The capacity of a battery is basically a measure of how much power the battery can hold. Think of it as the size of your water tank. The unit of measure here is milliamp hours (mAh). This is saying how much drain can be put on the battery to discharge it in one hour. Since we usually discuss the drain of a motor system in amps (A), here is the conversion:
I said that the capacity of the battery is like the water tank – which means the capacity determines how long you can run before you have to recharge. The higher the number, the longer the run time. Airplanes and helicopters don’t really have a standard capacity, because they come in many different sizes, but for R/C cars and trucks, the average is 5000mAh . But there are companies that make batteries with larger capacities. Traxxas even has one that is over 12000mAh! That’s huge, but there is a downside to large capacities as well. The bigger the capacity, the bigger the physical size and weight of the battery. Another consideration is heat build up in the motor and speed control over such a long run. Unless periodically checked, you can easily burn up a motor if it isn’t given enough time to cool down, and most people don’t stop during a run to check their motor temps. Keep that in mind when picking up a battery with a large capacity.
Discharge Rating (“C” Rating)
Voltage and Capacity had a direct impact on certain aspects of the vehicle, whether it’s speed or run time. This makes them easy to understand. The Discharge Rating (I’ll be referring to it as the C Rating from now on) is a bit harder to understand, and this has lead to it being the most over-hyped and misunderstood aspects of LiPo batteries.
The C Rating is simply a measure of how fast the battery can be discharged safely and without harming the battery. One of the things that makes it complicated is that it’s not a stand-alone number; it requires you to also know the capacity of the battery to ultimately figure out the safe amp draw (the “C” in C Rating actually stands forCapacity). Once you know the capacity, it’s pretty much a plug-and-play math problem. Using the above battery, here’s the way you find out the maximum safe continuous amp draw:
Calculating the C-Rating of our example battery: 50 x 5A = 250A
The resulting number is the maximum sustained load you can safely put on the battery. Going higher than that will result in, at best, the degradation of the battery at a faster than normal pace. At worst, it could burst into flames. So our example battery can handle a maximum continuous load of 250A.
Most batteries today have two C Ratings: a Continuous Rating (which we’ve been discussing), and a Burst Rating. The Burst rating works the same way, except it is only applicable in 10-second bursts, not continuously. For example, the Burst Rating would come into play when accelerating a vehicle, but not when at a steady speed on a straight-away. The Burst Rating is almost always higher than the Continuous Rating. Batteries are usually compared using the Continuous Rating, not the Burst Rating.
Is it best to get the highest you can? Or should you get a C Rating that’s just enough to cover your need?. When I suggest a friend with a LiPo battery, I first find out what the maximum current his or her application will draw. Let’s look at how that works.
Let’s assume that my friend is purchasing a 450 quadcopter. In which the motor consumes 5amps at 50% throttle and at full burst throttle it consumes 15amps. Knowing that, I can safely say that a 3S 5000mAh 20C LiPo will be sufficient, and will in fact have more power than we need. Remember, it has a maximum safe continuous discharge rating of 100A, more than enough to handle the 60A the 4 motors of the quadcopter motor will draw. Similarly, the Burst Rate of 150A of lipo easily covers the 100A the motor could draw.
However, the ratings on the motor aren’t the whole picture. The way the quadcopter is designed, the air flow, the size of the propellers, the weight of the frame… all of these things have an impact on the final draw on the battery. It’s very possible that the final draw on the battery is higher than the maximum motor draw. So having that little bit of overhead is crucial, because you can’t easily figure out a hard number that the copter will never go over.
For most applications, a 20C or 25C battery should be fine. But if you’re flying in heavy wind, or you’re geared up for racing, or you have a large motor for 3D flying applications, you should probably start around a 45C battery pack. But since there is no easy way to figure this out, I encourage you to talk to your local hobby shop to have them help determine which battery pack is right for your application or you can contact me to know more about it.
Proper Care & Treatment: Charging
It’s important to use a LiPo compatible charger for LiPos. LiPo batteries require specialized care. They charge using a system called CC/CV charging. It stands for Constant Current / Constant Voltage. Basically, the charger will keep the current, or charge rate, constant until the battery reaches its peak voltage (4.2v per cell in a battery pack). Then it will maintain that voltage, while reducing the current. On the other hand, NiMH and NiCd batteries charge best using a pulse charging method. Charging a LiPo battery in this way can have damaging effects, so it’s important to have a LiPo-compatible charger.
The second reason that you need a LiPo-compatible charger is balancing. Balancing is a term we use to describe the act of equalizing the voltage of each cell in a battery pack. We balance LiPo batteries to ensure each cell discharges the same amount. This helps with the performance of the battery. It is also crucial for safety reasons – but I’ll get to that in the section on discharging.
While there are stand-alone balancers on the market, I recommend purchasing a charger with built-in balancing capabilities, using a balance board like the one pictured to the right. This simplifies the process of balancing, and requires one less thing to be purchased. And with the price of chargers with built-in balancers coming down to very reasonable levels, I can’t think of a reason you would not want to simplify your charging set up. We’ll talk more about chargers in the next section.
Most LiPo batteries come with a connector called a JST-XH connector on the balance tap. One of the big problems with this connector is it’s lack of surface area; namely, one’s inability to get a good grip on the connector. This makes it hard to unplug from a balance board, and a user usually just ends up pulling on the wires. This can break the connector, and potentially short out the battery. A unique product, called Balance Protector Clips (or AB Clips) is a great way to solve this problem. They clip around the balance connector, and give a user more space to grab on to the it. They are usually inexpensive, and a great way to prevent balance connector fatigue. To the left, you can see a balance connector with and without the Balance Protector Clips.
Most LiPo batteries need to be charged rather slowly, compared to NiMH or NiCd batteries. While we would routinely charge a 3000mAh NiMH battery at four or five amps, a LiPo battery of the same capacity should be charged at no more than three amps. Just as the C Rating of a battery determines what the safe continuous discharge of the battery is, there is a C Rating for charging as well. For the vast majority of LiPos, the Charge Rate is 1C. The equation works the same way as the previous discharge rating, where 1000mAh = 1A. So, for a 3000mAh battery, we would want to charge at 3A, for a 5000mAh LiPo, we should set the charger at 5A, and for a 4500mAh pack, 4.5A is the correct charge rate.
However, more and more LiPo batteries are coming out these days that advertise faster charging capabilities, like the example battery we had above. On the battery, the label says it has a “3C Charge Rate”. Given that the battery’s capacity is 5000mAh, or 5 Amps, that means the battery can be safely charged at a maximum of 15 Amps! While it’s best to default at a 1C charge rate, always defer to the battery’s labeling itself to determine the maximum safe charge rate.
Due to the potential for fire when using LiPo batteries, regardless of the likelihood, certain precautions should be taken. Always have a fire extinguisher nearby; it won’t put out a LiPo fire (as I will further explain below, LiPo fires are chemical reactions and are very hard to put out). But a fire extinguisher will contain the fire and stop it from spreading. I prefer a CO2 (Carbon Dioxide) extinguisher – it helps to remove oxygen from the burn site, and will also cool down the battery and surrounding items. Another safety precaution is to charge the LiPo in a fire-resistant container. Most people opt toward the LiPo Bags on the market today, like the one pictured to the left. They are a bit pricy, but are more portable than other solutions. Finally, never charge your LiPo batteries unattended! If something does happen, you needs to be around to react quickly. While you don’t have to always be in the same room, you shouldn’t leave the house, or go mow the lawn, or anything else that will prevent you from taking action should the battery catch fire.
Picking Out the Right Charger
A multi-chemistry charger, which means it can charge NiMH, NiCd, and Lead Acid batteries as well as LiPo batteries. It can even charge the newest LiFe batteries that some use for receiver packs in airplanes and cars. It has a built-in balancer that handles up to 6S LiPo batteries, and can charge up to six amps.
Watts = Voltage x Amperage
See, wattage, voltage, and amperage are intertwined. You can convert voltage to amperage, and vice-versa. This is important in determining what kind of charger you need. Let me show you how.
Let’s say that I have a 6S 5000mAh LiPo battery, and I want to charge it at 1C, which would be 5A. If I have a B6ac+ v2 AC/DC Charger, I can set up the charger to charge at 5A for a 6S battery. But when I go to charge the battery, the most it ever charges at is around 3.5A. What gives? If we use the formula above, we can plug in our voltage (22.2V) and our Amperage (5A) and we get this:
22.2v x 5A = 111W
So the formula is saying that if we want to charge our 6S 5000mAh LiPo pack at 5 Amps, we would need a charger that is capable of delivering at least 111 Watts of power. Our B6AC+v2 can only deliver 50 Watts approx.
So you can see why a higher wattage charger might be important if you want to charge larger batteries quickly. For these kinds of chargers.
As always, it’s best to talk to your local hobby shop and have them set you up with a charger that will fit your needs. Local support is always a handy thing!
Parallel vs. Series Charging
Absolutely no for a 6s lipo! Parallel charging can be very dangerous. Even experts from well-known battery manufacturers “consider parallel pack charging to be highly dangerous and should not be attempted even by experienced users”. The problem with parallel charging (or even using your batteries in parallel) is that, when hooking up batteries in parallel, you are doubling the capacity of the batteries while, and this is important, maintaining the voltage of one of the individual batteries. What this means is that your charger, which normally monitors the battery while charging to prevent overcharging, cannot see all of the individual batteries’ voltages – it can only see one.
Another problem with parallel charging is the inequality of the batteries. If the two batteries (and the cells contained therein) were from the exact same production lot, had the exact same chemical composition and age and charge history and everything else – in other words, if they were completely identical – parallel charging would be okay. But a consumer (that’s you) will never be able to replicate those conditions, or even come close. The more those parameters differ, and considering the questionable balance charging techniques that many battery chargers use, the higher the chance of over charging and thermal runaway (more on that in the next section).
But what about batteries that are built in parallel? Doesn’t the fact that batteries are sometime constructed in parallel mean that parallel charging is safe?
Like I said before, if you can match all those parameters to be identical, parallel charging is okay. At the factory where they make the packs, manufacturers are constantly monitoring and testing the materials that make up the cells. Each cell is labelled with a UPC code that contains the entire chemical makeup and history for that cell. When a manufacturer assembles a battery, a computer scans and sorts the cells into compatible matches. Only then is it safe to parallel charge these cells. However, when you charge a LiPo battery at home, even if it is already constructed in parallel, you don’t need any fancy parallel charging boards – the work is already done for you, and you simply need to charge it as you would a normal battery.
Please, please, don’t use parallel charging cords. There is no safe way to use these. Parallel charging simply tempts fate. You will be able to find thousands of successful attempts to parallel charge batteries around the web. These are simply people that have not yet managed to burn their home down. Every battery expert I have talked to agrees on this.
Series charging is the safer of the two methods (meaning it’s no less dangerous than straight-up normal charging) IF you set up your charger the right way. If you want to charge six single-cell LiPo batteries — that are all the same capacity — you can wire them up in series, set up your charger as if it were a six-cell LiPo, and balance charge your LiPos. The act of balance charging them is essentially making the charger individually charge each cell, making sure they are all kept at safe levels. If you only have a single-port charger, series charging is the only safe way to charge multiple batteries at the same time, but wiring them up properly is pretty complex andnot something a beginner should attempt.
And the absolute best way to charge multiple batteries at the same time is to have a multi-port charger.
Proper Care & Treatment: Discharging (Using the Battery)
LiPo batteries offer plenty of power and runtime for us radio control enthusiasts. But that power and runtime comes at a price. LiPo batteries are capable of catching fire if not used properly – they are much more delicate than the older NiMH/NiCd batteries. The problem comes from the chemistry of the battery itself.
Lithium-Polymer batteries contain, quite obviously, lithium. Lithium is an alkali metal, meaning it reacts with water and combusts. Lithium also combusts when reacting with oxygen, but only when heated. The process of using the battery, in the sometimes extreme ways that we do in the R/C world, causes there to be excess atoms of Oxygen and excess atoms of Lithium on either end (be it the cathode or anode) of the battery. This can and does cause Lithium Oxide (Li2O) to build up on the anode or cathode. Lithium Oxide is basically corrosion, albeit of the lithium kind; not iron oxide, which is otherwise known as “rust”. The Li2O causes the internal resistance of the battery to increase. Internal resistance is best described as the measure of opposition that a circuit presents to the passage of current. The practical result of higher internal resistance is that the battery will heat up more during use.
As we touched on earlier, some modern chargers can read the internal resistance of the battery in milliohms (mΩ). If you have one of these chargers, you can get a sense of how your LiPos are performing, and how their internal resistance increases as they age. Simply keep track of the internal resistance reading each time you charge your battery, and chart the increase over time. You will see how just the process of using the LiPo battery begins to wear it out.
Heat causes the excess oxygen to build up more and more. Eventually the LiPo pack begins to swell (due to the oxygen gas build up). This is a good time to stop using the battery – its trying to tell you that it has come (prematurely or not) to the end of its life. Further use can, and probably will, be dangerous. After the pack has swollen, continued use can cause even more heat to be generated. At this point, a process called Thermal Runaway occurs.
Thermal Runaway is a self-sustaining reaction that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Basically, when this reaction starts, it creates heat. This heat leads to a product that increases resistance (more Li2O), which causes more heat, and the process continues until the battery bursts open from the pressure. At this point, the combination of heat, oxygen, and the humidity in the air all react with the lithium, resulting in a very hot and dangerous fire.
However, even if you stop using the battery when it swells, you still have to render it safe (a process I’ll get into later on in the LiPo Disposal section). If you puncture a LiPo that has swollen and still has a charge, it can still catch fire. This is because the unstable bonds that exist in a charged battery are in search of a more stable state of existence. That’s how a battery works; you destroy a stable chemical bond to create an unstable chemical bond. Unstable bonds are more apt to release their energy in the pursuit of a more stable bond.
When a LiPo is punctured, the lithium reacts with the humidity in the atmosphere and heats up the battery. This heat excites the unstable bonds, which break, releasing energy in the form of heat. The Thermal Runaway starts, and you again get a very hot and dangerous fire.
The entire process of building up that lithium oxide usually takes around 300-400 charge/discharge cycles to reach a tipping point. That’s a typical lifetime of a LiPo battery. But when we heat the batteries up during a run, or discharge them lower than 3.0 volts per cell, or physically damage them in any way, or allow water to enter the batteries (and I mean inside the foil wrapping), it reduces the life of the battery, and hastens the build up of Li2O.
In light of this, most manufacturers have taken to putting a Low Voltage Cutoff (LVC) on their speed controls. The LVC detects the voltage of the battery, and divides that voltage by the cell count of the battery. So it would see a fully charged 2S LiPo as 8.4V, or 4.2V per cell.
This is where the advantage of balancing comes in. Because the speed control does not read off the balance tap, it cannot know the exact voltages of each cell within the battery. The speed control can only assume that the cells of the battery are all equal. This is important because, as I mentioned above, discharging a LiPo cell lower than 3.0V causes a usually permanent degradation of the cell’s ability to absorb and retain a charge.
The LVC works to cut-off the motor of the vehicle (or in some cases, pulse the motor) to alert you to a nearly-depleted battery pack. It uses the total voltage of the battery as its reference. Most LVCs cut-off around 3.2V per cell. For our two-cell example battery, that would be 6.4V. But if our battery isn’t balanced, it’s possible for the total voltage to be above the cutoff threshold, yet still have a cell below the 3.0V danger zone. One cell could be 3.9V, while the other could be a 2.8V. That’s a total of 6.7V, which means the cut-off would not engage. The vehicle would continue to operate, allowing you to further degrade the battery. That’s why balancing is so important.
So when running your LiPo, make sure you have the Low Voltage Cutoff enabled, set up correctly, and for the sake of all that is Holy, don’t continue to run it after the LVC has kicked in! It may be a slight nuisance, but it’s worth enduring so that your LiPo batteries remain in good health.
It’s worth noting that most helicopter speed controls and some airplane speed controls do not have a Low Voltage Cutoff, as disabling the motor in mid-air wouldn’t be a good idea. For these kinds of applications, it’s best to set a conservative timer (some aircraft radio systems have a timer function built in) and land when the timer goes off. Whether your R/C vehicle has a LVC or not, it’s not a good idea to fly until the battery dies!
Proper Care & Treatment: Storage
In the old days, we used to run our cars or airplanes until the batteries died, then just set the batteries on the shelf at home, waiting for the next time we could use them. We just stored them dead. But you should not do that with LiPo batteries. Nor should LiPo batteries be stored at full charge, either. For the longest life of the batteries, LiPos should be stored at room temperature at 3.8V per cell. Most modern computerized chargers have a LiPo Storage function that will either charge the batteries up to that voltage, or discharge them down to that voltage, whichever is necessary.
I recommend to our customers that they put their LiPo batteries in storage mode after every run. This isn’t necessary per se, but it does build up good habits. If you do it every time, you don’t have to worry about whether or not you remembered to put it in storage. I have had many customers come to me with batteries that died because they charged it up, intending to use it, but life got in the way and they never remembered to put it back to storage voltage. Lithium-Polymer batteries can be damaged by sitting fully charged for as little as a week. This doesn’t mean they will get damaged every time you leave them for over a week. It just means they can, and I’ve seen it happen. So don’t forget to put your LiPos at storage voltage when you’re done using them.
They should also be stored in a fireproof container of some sort. As I mentioned above, most people tend toward leaving their LiPos in a LiPo bag, as they are portable and protect your workshop from catching fire should the LiPo combust. I have also seen people use empty ammo boxes, fireproof safes, and ceramic flower pots. Whatever you have (or can buy) that will prevent any fire from spreading will be worth it in the unlikely event that anything untoward should happen.
I feel the need to reiterate: the most common problem people have with LiPo batteries is a direct result of improper storage. When a LiPo battery sits for a long period of time (and not at proper storage voltage), it tends to discharge itself. If it drops below 3.0V per cell, the vast majority of LiPo chargers will not charge it. Sometimes, batteries with this problem can be rehabilitated, but just as often, they are a lost cause.
Reference: Rogers hobbycenter