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TFL 1317 Lucky Oct Petron Saint 53 Inch Gasoline convert to Electric RC Boat RC Crawler Extreme TFL 1317 Lucky Oct (Petron Saint).Gasoline convert to Electric.SSS 5694 1000KvFLier 400A ESC12s Lipo Battery.Prop Size 65mmCredit to Dayak Horizon Hobby SGE...

A Guide to Understanding LiPo Batteries.

Foreword
This guide, written over the course of years, has one sole aim: to educate hobbyists new and old about LiPo batteries with a focus on safety. The guide and I have been criticized over the years for "misleading" people on the dangers of LiPo batteries or the practices of charging, using, and storing them. Some of these criticisms were justified, and the guide changed to better reflect reality (see the change log at the bottom for details of those changes). Some of the attacks were not justified and came from those in the industry whose business I was disrupting by recommending against certain practices. In those cases, the guide was not changed. My only interest in keeping those who come to us for guidance safe and educated, with no exceptions. So read on, learn, ask questions, and be safe!

Introduction
Lithium Polymer (AKA “LiPo”) batteries are a 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.

This guide was written after many hours of research. It is as accurate as I can make it without actually being a chemical engineer (though in researching this article, I did talk to a few of them). That said, this guide isn't intended to be taken as definitive. It is a living document, and as common knowledge regarding LiPo batteries changes, so too will this guide.

Let's first talk about the differences between LiPo batteries and their Nickel-Cadmium and Nickel-Metal Hydride counterparts.

LiPo Batteries
Pros
Much lighter weight, and can be made in almost any size or shape.

Much higher capacities, allowing them to hold much more energy.

Much higher discharge rates, meaning they pack more punch.

Cons
Much shorter lifespan; LiPos average only 150–250 cycles.
The sensitive chemistry can lead to a fire if the battery gets punctured.
Need special care for charging, discharging, and storage.

V.
NiMH Batteries
Pros
Longer lifespan than LiPos, usually into the 1,000 cycles range
Much less sensitive, and doesn't usually pose a fire risk
Simpler chargers and routines required for use.

Cons
Much heavier, and limited on size.
Lower average capacity, and less efficient overall.
Lower discharge rates; they lack tremendous punch.

What Do All the Numbers Mean?
The 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.

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.
About Nominal Voltages
I thought (mistakenly) that this was common knowledge, but after a handful of emails on the topic, it was clear I needed to clarify what nominal voltage is.

'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.

Capacity
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. 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 - that is our most popular battery here in the store. 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.

Q: Why do we use voltage, and not capacity, to determine how charged a battery is?

A:The reason we use voltage to determine the charge level of a battery stems from our difficulty in measuring capacity. Voltage is simple to measure — if you've ever used a voltmeter to measure a AA battery, you understand how trivial it is to measure voltage.

Capacity, however, is nearly impossible to measure accurately. We can measure how much energy is going into a battery (at least somewhat accurately), but we can't measure how much is actually in the battery.

Think of it like beakers of water. For voltage, the beaker is transparent, and we can easily see the amount of water in the beaker in the same way we can measure voltage whenever we like. On the other hand, we have the beaker representing capacity, and it's opaque — we can't see through it, and so the only way to know how much is inside is to empty it and measure the water (energy) as it's leaving the beaker (battery).

Because amperage and voltage are intertwined, as we will discuss later in detail, the voltage of a battery does correlate, approximately, to the capacity left in the battery, and while there are times when the voltage can deceive you, in general, it's okay to rely on voltage as our primary measure of how full a battery is.

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 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:

50C = 50 x Capacity (in Amps)
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, 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.

There is a lot of vitriolic comments on the Internet about what C Rating is best. Is it best to get the highest you can? Or should you get a C Rating that's just enough to cover your need? There isn't a simple answer. All I can give you is my take on the issue. When I set up a customer 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 our example customer is purchasing a Slash VXL R/C truck. That motor, according to Traxxas, has a maximum continuous current draw of 65A and a burst draw of 100A. Knowing that, I can safely say that a 2S 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 65A the Velineon motor will draw. Similarly, the Burst Rate of 150A easily covers the 100A the motor could draw.
However, the ratings on the motor aren't the whole picture. The way the truck is geared, the terrain the truck is driving on, the size of the tires, the weight of the truck... 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 truck will never go over.

For most applications, a 20C or 25C battery should be fine. But if you're driving a heavy truck, or you're geared up for racing, or you have a large motor for 3D flying applications, you should probably start around a 40C 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.
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.

Internal Resistance is measured in milliohms (mΩ).
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 R/C 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 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 the Slash VXL from the previous section and plug those numbers in. If you recall, our Velineon motor has a maximum continuous current rating of 65A. Let's assume we manage to hit that mark when driving 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 my R/C vehicle?" Well, let's continue on with our example to show you.

The Velineon motor our Slash VXL uses 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 car or truck or airplane or boat or helicopter 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 dependant 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 vehicle 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 car or airplane 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 car or airplane faster.

This is a case of correlation not equalling causation. It's really the internal resistance making a battery faster, not the C-Rating.

Proper Care & Treatment: Charging
It's important to use a LiPo compatible charger for LiPos. As I said in the Introduction, 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.

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, 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.

NEVER Leave a Battery Charging Unattended!

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