LIPO BATTERY GUIDE
I thought we'd kick off our blog section of the site, with a (lengthy) article on LiPo batteries, how to use them, and care for them! Much of the info in this guide comes from 'Rogers Guide' and a huge thanks to him for his permission to reproduce sections of his guide here. Lets get into it!
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 BATTERY||NiMH BATTERY|
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.
VOLTAGE & CELL COUNT
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.
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.
In the early days of LiPo batteries, you might have seen a battery pack described as "2S2P". This meant that there were actually four cells in the battery; two cells wired in series, and two more wired into the first two batteries in parallel (parallel meaning the capacities get added together). This terminology is not used much nowadays; modern technology allows us to have the individual cells hold much more energy than they could only a few years ago. Even so, it can be handy to know the older terms, just in case you run into something with a few years on it.
The voltage of a battery pack is essentially going to determine how fast your vehicle is going to go. Voltage directly influences the RPM of the electric motor (brushless motors are rated by kV, which means 'RPM per Volt'). So if you have a brushless motor with a rating of 3,500kV, that motor will spin 3,500 RPM for every volt you apply to it. On a 2S LiPo battery, that motor will spin around 25,900 RPM. On a 3S, it will spin a whopping 38,850 RPM. So the more voltage you have, the faster you're going to go.
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 fuel 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: 1000mAh = 1 Amp Hour (1Ah)
I said that the capacity of the battery is like the fuel 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. Gel blaster batteries don't really have a standard capacity, because they come in many different sizes, but for R/C cars and trucks, for example, the average is 5000mAh. But there are companies that make batteries with larger capacities. Traxxas, an RC battery manufacturer, 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 game to check their motor temps. Keep that in mind when picking up a battery with a large capacity.
DISCHARGE "C" RATING
Voltage and Capacity have a direct impact on certain aspects of the blasters performance, whether it's rate of fire 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 for Capacity). 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 5 = 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, in the world of RC cars, the Burst Rating would come into play when accelerating the 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. We encourage you to talk to your local hobby shop to have them help determine which battery pack is right for your blaster.
INTERNAL RESISTANCE: The Mystery Number
There is one very important rating we haven't talked about yet: Internal Resistance (or IR). Problem is, you won't find the IR rating anywhere on the battery! That's because the internal resistance of a battery changes over time, and sometimes because of the temperature. However, just because you can't read the rating on the battery doesn't mean it isn't important. In a way, the internal resistance is one of the most important ratings for a battery.
To understand why the IR is important, we have to understand what it is. In simple terms, Internal Resistance is a measure of the difficulty a battery has delivering its energy to your motor and speed control (or whatever else you have a battery hooked up to). The higher the number, the harder it is for the energy to reach its preferred destination. The energy that doesn't "go all the way" is lost as heat. So the internal resistance is kind of a measure of the efficiency of the battery.
1,000 milliohms is equal to 1 Ohm (Ω)
Measuring the IR of your battery requires a special toolset. You either need a charger that will measure it for you or a tool that specifically measures internal resistance. Given that the only tool I have found for this (at least in the hobby world) is almost as expensive as a charger that does this for you, I'd go with a charger for this process.
Some chargers measure each cell's IR separately, and some measure the entire battery pack as a whole. Since internal resistance is a cumulative effect, and the cells are wired in series, if you have a charger that does each cell independently, you need to add up the IR values of each cell, like this:
Suppose we have a 3S (3-cell) LiPo battery, and the measuring the cells independently yields these results.
Cell 1: 3 mΩ Cell 2: 5 mΩ Cell 3: 4 mΩ
To find the total internal resistance for the battery pack, we would add up the values for the three cells.
3Ω + 5Ω + 4Ω = 12 mΩ
For a charger that measures the pack as a whole, all you would see is the 12 mΩ - the rest would be done for you - behind the scenes, as it were. Either way, the goal is to have the IR for the entire pack.
The first reason internal resistance is important has to do with your battery's health. As a LiPo battery is used, a build up of Li2O forms on the inside terminals of the battery (we'll go more in depth on this later in the Discharging section). As that build up occurs, the IR goes up, making the battery less efficient. After many, many uses, the battery will simply wear out and be unable to hold on to any energy you put in during charging - most of it will be lost as heat. If you've ever seen a supposed fully charged battery discharge almost instantly, a high IR is probably to blame.
To understand how Internal Resistance works in our applications, first we have to understand Ohm's Law. It says that the current (Amps) through a conductor between two points is directly proportional to the difference in voltage across those two points.
The modern formula is as follows: Amps = Volts / Resistance.
In the formula, the resistance is measured in Ohms, not milliohms, so we'd have to convert our measurements. If we use our previous 3S LiPo, and plug it into the equation along with a 1A draw, we can find out how much our battery pack's voltage will drop as a result of the load. First, we have to change the equation to solve for volts, which would look like this:
Amps x Resistance = Volts
So plugging in our numbers and solving the equation would look like this:
1A x 0.012 Ω = 0.012V
So our battery would experience a tiny drop in voltage when a 1A load is applied. Considering our 3S LiPo is around 12.6V when fully charged, that's not a big deal, right? Well, let's see what happens hypothetically, when we increase the load to 10A.
10A x 0.012 Ω = 0.120V
Now we see that when we increased the load 10X, we also increased the voltage drop 10X. But neither of these examples are very "real world". Let's use an example from R/C cars for this demonstration - A Velineon motor has a maximum continuous current rating of 65A. Let's assume we manage to hit that mark and use that.
65A x 0.012 Ω = 0.780V
Wow, more than 3/4 of a volt! That's around 6.2% of the total voltage of our battery pack. Pretty respectable, but it's still a reasonable drop in voltage.
"So, yeah, the voltage drops. But so what? What does that actually mean? How does it effect me?" Well, let's continue on with our example to show you.
The Velineon motor has a Kv rating of 3500. That means it spins 3,500 RPM per volt. On a fully charged 3S LiPo we'll see this (assuming no voltage drop):
12.6V x 3500RPM = 44,100 RPM
Now, assuming we can hit that 65A draw on our unloaded motor (which we can't in real life, but for the purposes of demonstration we can), here's the RPM on the same motor with our voltage drop from before:
11.82V x 3500RPM = 41,370 RPM
Difference of 2,730 RPM
See the drop in performance? That's the effect Ohm's Law has on our hobby. A lower internal resistance means your motor goes faster and has more power.
This begs the question: how low should it be? Unfortunately, there's no easy answer for this. It's all dependent on your use case and battery. What is great for one battery may be terrible for another. Based on my online research, combined with my own experience and findings, I would say, as a general rule, a per cell rating of between 0-6 mΩ is as good as it gets. Between 7 and 12 mΩ is reasonable. 12 to 20 mΩ is where you start to see the signs of aging on a battery, and beyond 20mΩ per cell, you'll want to start thinking about retiring the battery pack. But this is only a guide - there is no hard rule set here. And if your charger doesn't give you the per cell measurements, you'll have to divide your total count by the number of cells in your battery to get an approximate per cell rating.
INTERNAL RESISTANCE AND "C" RATING
There are many people out there that believe a higher C-Rating will make their blaster perform better. We know from our previous discussion on C-Ratings that you need to account for the power draw your motor has when picking out the right C-Rating for your battery, but does more equal better? Many people say yes.
But there isn't anything intrinsic to the C-Rating that substantiates their claims. It's simply not true that a higher C-Rating makes your blaster faster.
However, there is a correlation between the C-Rating of a battery and the internal resistance of that battery. In general, batteries with a higher C-Rating also have a low internal resistance. This isn't always the case, as there are always variances in manufacturing, but the general idea seems to hold true, and a lower IR will make a blaster motor go faster.
This is a case of correlation not equaling causation. It's really the internal resistance making a battery faster, not the C-Rating.
PROPER CARE AND TREATMENT - CHARGING
It's important to use a LiPo compatible charger for LiPos. As I said in the earlier, 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 balancer's on the market, I recommend purchasing a charger with built-in balancing capabilities. This simplifies the process of balancing, and requires one less thing to be purchased. And with the price of chargers with built-in balancers at 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 shortly.
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.
The safest charge rate for most LiPo batteries is 1C, or 1 x capacity of battery in Amps.
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. They can be 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 THE RIGHT CHARGER
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, purely as an example, that I have a 6S 5000mAh LiPo battery, and I want to charge it at 1C, which would be 5A. If I have a 80W AC/DC Charger, I can set up the charger to charge at 5A for a 6S battery. But if 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 something like a 6S 5000mAh LiPo pack at 5 Amps, we would need a charger that is capable of delivering at least 111 Watts of power. Our charger in the example can only deliver 80 Watts.
So you can see why a higher wattage charger might be important if you want to charge larger batteries quickly. 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!
PROPER CARE AND TREATMENT: Discharging (Using the Battery)
LiPo batteries offer plenty of power and run time for us radio control enthusiasts, but that power and run time 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 lithium, an alkali metal, which reacts with water and combusts. When heated, Lithium also combusts when reacting with oxygen. The process of using the battery, in the sometimes extreme ways that we do in the R/C or blaster world, causes there to be excess atoms of Oxygen and excess atoms of Lithium on either end (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 lithium corrosion, or lithium “rust”. The Li2O causes the internal resistance of the battery to increase. The practical result of higher internal resistance is that the battery will heat up more during use.
Higher Internal Resistance = Higher Operating Temperature
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, and eventually the LiPo pack begins to swell. This is a good time to stop using the battery - its trying to tell you that it has come to the end of its life. Further use can 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.
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, in a perfect world, takes around 300-400 charge/discharge cycles to reach a tipping point. A typical lifetime of a LiPo battery is closer to 150-250 cycles, because 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 blaster (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 blaster would continue to operate, allowing you to further degrade the battery. That's why balancing is so important. It's worth noting that most older blasters do not have a Low Voltage Cutoff, however many of the newer versions do have this as a feature. In general, do not continue to run your blaster once you detect any loss of power.
PROPER CARE & TREATMENT: Storage
In the old days, we used to run things 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 never 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.
Discharge the LiPo battery as far down as you safely can. You can do this a number of ways. Most computerized LiPo chargers have a discharge feature in them. If you don't have a charger with a discharge feature, you can run down the battery in your vehicle - keep in mind that you risk a fire doing this, so take care to have the necessary safety equipment around. Alternatively, you can build your own discharge rig with a taillight bulb and some wire. Simply solder a male connector of your choosing to the tabs on a taillight bulb, and plug the battery in. Make sure to have the battery in a fireproof container while doing this.
Place the LiPo in a salt water bath. Mix table salt into some warm (not hot) water. Keep adding salt until it will no longer dissolve in the water. Ensure that the wires are all entirely submerged. The salt water is very conductive, and it will essentially short out the battery, further discharging it. Leave the battery in the salt water bath for at least 24 hours.
Check the voltage of the LiPo. If the voltage of the battery is 0.0V, great! Move onto the next step. Otherwise, put it back in the salt water bath for another 24 hours. Continue doing this until the battery reaches 0.0V.
Dispose of the battery in the trash. That's right - unlike NiMH and NiCd batteries, LiPos are not hazardous to the environment. They can be thrown in the garbage with no problem.
Alternatively, if you don't feel like going through this process yourself, you can bring the battery in to us and we will dispose of it for you. If you're not in our area, check with your local hobby shop to see if they offer a similar service.
EQUIPPING YOUR LiPO: CONNECTORS
|Deans Connectors are really the king of connectors. They've been around seemingly forever, and have been the top choice for the discerning R/C enthusiast for quite some time now. They are somewhat difficult to solder, especially for novice users. Deans connectors slide together smoothly, and are very well designed. Like almost every modern connector, they are polarity protected.|
|Only through the sheer force of Tamiya's market share did these connectors take on their name. Originally called a 'Molex' connector, these connectors were the de facto standard of the hobby industry for years. Popularized by Tamiya in their bazillion R/C cars, these connectors came on every vehicle until very, very recently. This is a terrible connector with lots of resistance. You are more likely to melt these connectors than anything else. If you have a LiPo that has a Tamiya connector on it, cut it off and solder on one of the other connectors.|
|JST SMP-02 Connectors are the standard connections you will find inside most gel blasters, and LiPo batteries purchased from vendors in Australia.
Normally the female connector is attached to the battery pack, with the male connector attached to the charger of power device. These connectors provide average performance and we recommend switching to deans or another type of connector if you're upgrading your blaster.
XT-60s have gained a little bit of ground in the last few years. The XT-60 connector is getting some adoption due to their prevalence on the LiPo batteries coming directly out of China. We do like these plugs; it's easy to solder to. It's relatively small and compact as well. You could do worse. If you're ordering batteries direct from overseas, these are the most common connectors today. Other connectors have come out in recent years, but their adoption rate is minimal. Of the above connectors, the only ones you seriously want to avoid are the Tamiya. Other than that, go with whatever connector makes sense for what you're doing.
QUICK NOTE ON SOLDERING:
Soldering is as much an art as it is a tool, and there is a right way to solder when you're talking about battery packs.
Never Cut Your Positive and Negative Wires at the Same Time: This is a great way to damage your battery pack and risk a fire. Cutting both wires at the same time will short out the battery pack, which will generate a lot of heat. Think of it this way — when a welder completes a circuit and welds two pieces of metal together, that's called Arc Welding — and the same principal is at play in Arc Welding as it would be when you touch positive and negative on your battery together. Cut, solder, and heat-shrink (if necessary) one wire at a time. It might take a little longer, but it's far and away the safest way to solder a battery.
Get Your Polarity Right: One sure-fire way to destroy a speed control is to solder your connector on backwards. Reversing the polarity is never a good idea. Be mindful of the markings on the connector — most brands include a simple "+" for positive and "-" for negative to indicate which contact is which. Red is positive and black is negative. If your battery or speed control (or whatever you happen to be soldering) doesn't use the red/black paradigm, usually the lightest color wire is positive and the darkest is negative. If both wires are black, look for one of them to have white dashes on it — that's the positive wire.
Those are some basic safety tips on soldering. If you're not happy with your ability to solder, keep at it! Like I said above, soldering is as much an art as it is a tool. Keep working at it and eventually you'll be amazed at how nice your solder joints look!
So there you have it - now you know most of what you need to know about LiPo batteries. I make no claims that this article teaches you everything there is to know about LiPos, but hopefully it helps give you some insight into how they work. It is certainly an exciting time for the hobby, and things are changing on a frequent basis. Just remember to have fun, and if you don't know something, ask questions! The only dumb question is the one you don't ask!
SEE YOU ON THE FIELD!
- Grant / "THURISAZ"