Lithium Polymer batteries (hereinafter referred to as "LiPo" batteries) are a newer type of battery that is now used in many consumer electronic devices. They have been gaining popularity in the radio control industry for the past few years and are now the most popular choice for those seeking long run times and high power.
LiPo batteries offer a wide range of advantages, but each user must decide if the advantages outweigh the disadvantages. More and more people are finding that they do. As long as you follow the rules and treat the batteries properly, there is nothing to fear from LiPo batteries.
First, let's talk about the differences between LiPo batteries and their counterparts, Nickel-Cadmium and Nickel-Metal Hydride.
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One way we identify any battery is through a rating system. This helps us compare the characteristics of a cell and determine which battery is appropriate for our needs. There are three main ratings you need to know about a LiPo battery.
So what does all this mean? Let's break it down and explain each one.
A LiPo cell has a nominal voltage of 3.7V. For the above 7.4V battery, it means there are two cells in series (meaning the voltages add up). You might hear people talking about a "2S" battery pack - this means 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 voltage of a battery pack. This is why the battery industry decides to discuss and compare batteries. However, it is not the cell's full charge voltage. LiPo batteries are fully charged when they reach 4.2V/cell, and the minimum safe discharge, as we'll discuss later, is 3.0v/cell. 3.7v is in between and is the cell's nominal load.
In the early days of LiPo batteries, you may have seen a battery pack described as "2S2P." This meant that there were actually four cells in the battery; two cells were connected in series, and two of those were connected in parallel (combining the capacities). This terminology is not commonly used today; modern technology allows individual cells to hold much more energy than they could only a few years ago. Even so, it can be useful to know the old terms if you encounter something from a few years ago.
A battery pack's voltage will primarily determine how fast your vehicle will go. Voltage directly affects the RPM of an electric motor (brushless motors are rated in kV, which means RPM per volt). So, if you have a brushless motor rated at 3,500kV, that motor will spin at 3,500 RPM for every volt you apply. In a 2S LiPo battery, that motor will spin at around 25,900 RPM. In a 3S, it will spin at 38,850 RPM. So the more voltage you have, the faster you will go.
A battery's capacity is essentially 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 tells you how much drain can be applied to the battery over one hour. Since we discuss a motor system's drain in amps (A), the conversion is:
1000 mAh = 1 Amp Hour (1Ah)
I said the battery's capacity is like a fuel tank - this also means it determines how long you can run on a charge. The higher the number, the longer the runtime. Airplanes and helicopters don't really have a standard capacity. They come in many different sizes, but for R/C cars and trucks, the average is around 5000mAh - this is the most popular battery in the store. However, there are companies making batteries with larger capacities. Traxxas even has a 12000mAh one! That's very big, but big capacities also have their drawbacks. As capacity increases, so does the physical size and weight of the battery. Another consideration is heat buildup in the battery. More capacity means more current flowing through the pack, which means more heat is generated. And too much heat is bad for the battery.
The "C" rating of a battery is a measure of how quickly a battery can be discharged safely and without harm. It's a multiplier for the battery's capacity, so a 5000mAh (or 5Ah) battery with a 10C rating can provide a current of 50A. The higher the "C" rating, the more power can be drawn from the battery without damaging it. A higher C rating generally means a higher-quality battery, but it also often comes with a higher price tag. It's important to match the battery's C rating to the demands of your application. If your vehicle draws more current than the battery can safely provide, it can lead to overheating and damage to the battery.
There are other factors to consider when choosing a LiPo battery, such as physical size, shape, and weight. LiPo batteries come in various shapes and sizes to fit different applications. For example, batteries designed for quadcopters are often flat and rectangular to fit in the tight spaces of the aircraft's frame, while batteries for RC cars and trucks may be cylindrical to fit in the battery compartments of the vehicles. Additionally, the weight of the battery can affect the performance of the vehicle, especially in applications where weight is critical, such as racing.
Another consideration is the connector type used on the battery. There are several different types of connectors used in the RC hobby, each with its own advantages and disadvantages. Some of the most common connector types include Deans, XT60, and Traxxas. It's essential to choose a battery with a connector that is compatible with your vehicle and charger.
In conclusion, LiPo batteries offer many advantages over traditional NiMH batteries, including higher energy density, higher discharge rates, and lighter weight. However, they also require special care and handling to ensure safe operation. By understanding the key characteristics of LiPo batteries and following best practices for charging, discharging, and storage, you can enjoy the performance benefits of LiPo batteries while minimizing the risks.
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