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Basic Lipo-Battery Knowledge
While some RTR cars are still delivered with NiMh (nickel metal hydride) batteries, and they can still be a good choice for beginners or specific uses, by and large, Lipo (lithium polymer) batteries have taken over the RC car market. Many RTR’s are either already delivered with them included, or the battery choice is left up to the customer, and in all RC car racing today Lipo batteries are used.
LiPo batteries come in a huge variety of type and size. But what do all the markings on a Lipo battery mean, and how do you know which is the right choice for you? Let’s take a look at some basic Lipo battery knowledge.
A Lipo battery is constructed from separate cells, all connected to form the specific battery. One Lipo cell has a nominal voltage of 3.7V. When connecting these in series, the voltage increases, meaning you get 7.4V for a 2 cell battery, 11.1V for a 3 cell battery, 14.8V for a 4 cell battery etc. Capacity on the other hand can be increased by connecting more cells in parallel.
On batteries, or in product descriptions for batteries, you see numbers or codes like 2S2P. These numbers are a way to describe the battery configuration in a short way, and in the example they mean that the battery has 2 cells (2S) connected in series, and 2 cells sets connected in parallel (2P).
Higher voltage means more power, higher capacity give longer runtimes.
Here’s a table with the voltage for the Lipo types used for RC cars:
- 1S battery: 1 x 3.7V = 3.7V
- 2S battery: 2 x 3.7V = 7.4V
- 3S battery: 3 x 3.7V = 11.1V
- 4S battery: 4 x 3.7V = 14.8V
- 5S battery: 5 x 3.7V = 18.5V
- 6S battery: 5 x 3.7V = 22.2V
- 8S battery: 5 x 3.7V = 29.6V
In addition to "normal" Lipo batteries, there are also special LiHV, or High Voltage Lithium Polymer batteries available. These are very similar to normal Lipo batteries, but they allow you to charge the battery up to 4.35V per cell, while regular lipos must be charged up to 4.2V maximum.
Another important aspect of any Lipo battery is the C rating, which you will always again find on the battery label or product description. The C rating stands for discharge rate, and describes how fast a battery can be discharged safely. If a battery is rated at 50C and has a capacity of 5000mAh, it means that safely handle a discharge rate of 50 x 5000mAh = 250.000mAh = 250Ah. Meaning that this number is the maximum sustained load you can safely put on the battery. As you can see from the example, both the capacity and C rating are important for the actual discharge rate of a battery.
Many batteries today have two C ratings - a continuous rating, and a burst rating. The burst rating describes what the battery can handle in short bursts, and not continuously. Batteries are usually compared using the continuous rating and not the burst rating. The C rating can be useful to help you select a Lipo battery, but unfortunately many manufacturers overstate C values for marketing purposes, which means you can’t always trust it 100%.
Another very important aspect of a Lipo battery is IR, meaning Internal Resistance. As opposed to the other features we have mentioned, you will not find the IR rating stated anywhere on the battery. The internal resistance of a battery pack changes over time, and depending on conditions like temperature, so it would be impossible to give a correct number. That does not mean it isn't important though, as in some ways, the internal resistance is among the most important ratings for any battery.
To make it simple, we can describe IR as a measure of the difficulty a battery has delivering its energy to your motor and speed control. So the higher the IR number, the harder it is for the energy to reach them. In short, the internal resistance is a measure of how efficient the battery is.
Choosing a Lipo battery
- Size and connectors
This is the first thing to take into account, as you need to choose a battery with a physical size that actually fits in the car. You also need a battery with the right connectors for your car/charger. Of course connectors can be exchanged, but the easy solution is to buy a pack with the right connectors.
- Voltage
Battery voltage, or cell count, is another decision that you will need to make. Many cars and their electronics are made for a specific voltage, like a max 2S (7.4V) battery, but for some cars it’s up to you to choose among several alternatives. Higher voltage batteries makes more power, but are heavier and often more expensive. The most important thing is of course not to go over the voltage that your electronics are able to support. And there is no idea to go for a super expensive battery that makes your car so powerful that it is un-driveable.
- Capacity
What capacity you need is directly related to the size and weight of a car, as well as the motor used. With a light and small car, you will get enough runtime even with a low capacity battery pack. The more energy the car and electronics draw, the more capacity you will need to get decent runtime. A higher capacity battery will usually also cost more, so the choice comes down to a compromise between price and runtime.
- Discharge Rate
Using a battery with a discharge rate (C) that is too low can result in your battery being damaged. However, today most batteries sold have a high enough discharge rate for most uses. Remember that the maximum current output of a battery depends on the capacity and C rating. Therefore, the smaller the capacity of a battery, the higher the C rating needs to be.
- Hardcase
While most batteries for RC cars have a hard case, there are also soft case Lipo batteries available. For RC car use hardcase batteries is what we always recommend.
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