Understanding LiPo Batteries
Lithium Polymer batteries, Also referred to as LiPo batteries, are a relatively newer type of battery now used in many consumer electronic devices today.
They have been growing in popularity in the radio control industry over the last few years, and are now the most popular choice for anyone looking for longer run times and higher power output.
LiPo batteries offer a large range of benefits. Each user must decide if the benefits outweigh the drawbacks of running LiPo Batteries.
For the majority of people, they do. In my personal opinion, there is nothing to fear from LiPo batteries, As long as you follow the rules and treat the batteries with respect.
Pros And Cons Of LiPo Batteries
- Much lighter weight, and can be made in almost any size or shape.
- Much higher capacities, allowing them to hold much more power.
- Much higher discharge rates, meaning they pack more punch.
- 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.
- The rating system is the way we define all batteries. This allows us to compare the properties of a battery and help us determine which battery pack is best suited for each of our needs.
There are three main ratings that you need to be aware of on a LiPo battery.
The discharge rate (c rating), the capacity (mah) and the cell count (voltage).
What does all this mean? Let’s break down each one.
The nominal voltage of each LiPo cell is 3.7V. 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 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 later, is 3.0v/cell. 3.7v is pretty much in the middle, and that is the nominal voltage of the cell.
Voltage is determined by Cell Count.
Take a 2S 7.4V LiPo battery, This means that there are two cells in series, Which means the voltage gets added together.
This is why sometimes you will hear people mention a “2S” battery pack – That means there are 2 cells in Series. A two-cell (2S) pack is 7.4V, And a three-cell (3S) pack is 11.1V, and so on. For each that is added, we are adding an additional 3.7v, That’s easy enough to follow, Right?
Capacity Or mah
The capacity of a LiPo battery is basically a measure of how much power the battery can hold. An easy way I found to understand capacity is to, 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)
Thinking of the capacity of the battery as like your fuel tank – What this means is, The capacity determines how long you can run before you have to recharge. The higher the number, the longer the runtime.
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 choice today.
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 and/or speed control if it isn’t given proper time to cool down, For most people, They don’t stop during a run to check their motor temps.
Just something to Keep in mind when picking up a battery with a large capacity.
LiPo Discharge Rating (“C” Rating)
The “C” in C Rating stands for Capacity. The Voltage and Capacity of the battery has a direct impact on certain aspects of the vehicle.
Whether it’s the speed or runtime. This makes them easy to understand. The Discharge Rating is a bit harder to understand, and this has lead to it being the most mistaken and misunderstood aspect of LiPo batteries.
The C Rating is simply a measure of how fast the battery can be safely discharged without harming the battery. One thing 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.
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 case, 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, 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 and not the Burst Rating.
There is a lot of 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 is not 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 their application will draw. Let’s look at how that works.
However, It is not just the ratings on the motor. 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 what the final draw on the battery will be.
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. If you have a heavy truck and bashing hard off-road you will want a little more like a 50C-75C battery.
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 are wired in series, and the other two are wired into the first two batteries in parallel. Parallel means the capacities get added together.
This terminology is not used much these days, The 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. There are some still out there!
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.
The brushless motors are rated by kV. The kV rating means ‘RPM per Volt’. For example, if you have a brushless motor with a rating of 3,800kV, that motor will spin 3,800 RPM for every volt you apply to it. On a 2S LiPo battery, that motor will spin around 28,120 RPM. On a 3S, it will spin a whopping 42,180 RPM.
Traxxas Velineon motor used in the 1/10 scale trucks has a kV rating of 3500. Meaning the motor spins 3500 rpm per volt. If you were to use a fully charged 3S LiPo pack it would have an rpm of 44,100.
So, As you can see the more voltage you have, the faster you’re going to go.
Is Higher C-Rating Better and Faster?
There are a lot of people that claim a higher rating will make your RC vehicle perform better and faster. There is not anything instinktive to support their claims.
“It” is simply not true that a higher rating will make your vehicle faster. There is, however, A correlation between the internal resistance and rating of a battery.
Generally speaking, A battery with a higher rating typically also have a lower internal resistance. This is not always the same with all batteries. A lower internal resistance with a result in a faster RC vehicle. To clarify the Internal resistance will increase the speed and performance, The C-Rating, however, will not.
Any thoughts or questions are welcome. Leave