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C Rate: Unraveling the Dynamics of Solar Battery Discharge

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Solar batteries are an essential part of any renewable energy system – they store solar energy for when sunlight is scarce. To maximise solar batteries’ performance, one must have a firm grasp of the battery C rate. This article defines the C rate and breaks it down, discussing the C20 rating, battery discharge rates, battery c rate charts and the impact on different battery types.

What is Battery C Rating?

The battery C Rate is the value at which a battery is charged and discharged. The battery’s expansion here is the measurement of the battery’s current. The general method of rating and labelling the capacity of a battery is at the 1C Rate.

For example,

  • A fully charged battery with a capacity of 120 amperes should deliver a current of 120 amperes per hour at a C rate of 1.
  • If a 120 A battery discharges at a C rating of 0.5, it delivers 5A over two hours.
  • Under a C rate of 2, it delivers 20A over 30 minutes.

All values above are known as the battery C Rating. This is critical to understand because most batteries’ available stored energy is dictated by the change in charge and discharge current due to the majority of batteries.

Why are C Ratings Different Between Different Batteries?

Depending on the chemistry and design, a battery has varying C ratings. The battery’s C rating has two significant influencing factors: Thermal heating and mass transfer.

Thermal heating:

Thermal heating arises from the battery’s internal resistance, creating an excessive amount of heat that has to be dissipated into the atmosphere. During charging at very high currents, the heat generated inside the battery cannot be removed swiftly enough from the environment, thus increasing its temperature at a faster rate.

Mass transfer:

Despite the nanoscale of Lithium ions, there is mass transfer at the fastest time, or diffusion-limiting current, during rapid charging. While electrodes are made of clean nanosheets that enable nanoparticles with a big surface area, hundreds or thousands of layers limit the amount of Li+ ions that diffuse into the separator. This mass transfer limitation is due to the transference number of Li+, which is less than 1.

Efforts to exceed the limiting current cause solvent decomposition, heating, and the battery to disintegrate. Since distinct materials have different rates, the average Lithium nickel manganese cobalt oxide (NCM) battery has a C rating of 1C, and the maximum C rate is 10C for 18,650 batteries. Similarly, the C rating of a LiFePO4 lithium battery is 1C, and the maximum C rate is for 3C LiFePO4 prismatic.

Battery C Rating Chart

The chart below shows the different battery C ratings and their corresponding discharge times. When we calculate them, the battery C rating should use the same calculation as the same energy.

C-rate Time
5C 12 minutes
2C 30 minutes
1C 1 hour
0.5C or C/2 2 hours
0.2C or C/5 5 hours
0.1C or C/10 10 hours
0.05C or C/20 20 hours

How to Calculate the C Rating For the Battery?

A battery has its C Rating, which is defined by the time of charge and discharge. A C Rate can be increased or decreased; thus, it will automatically affect the time in which it takes to charge and discharge the battery.

The C Rate charge or discharge time is changed according to the rating. This means that for,

  • Rating 1: 1C = 60 minutes
  • Rating 2: 0.5C= 120 minutes
  • Rating 3: 2C= 30 minutes

The formula for calculating the C rating:

I = Cr * Er, hence,

Cr = I / Er

[C-rate (C) = charge or discharge current in amperes (A) / rated capacity of the battery(Ah)]

In which, Er = Rated energy (Ah); Cr = C Rate; I = Current of charge or discharge (Amps)

To calculate the charge and discharge time, the formula is,

t = Er / I

In which, t = Time; Cr = C Rate

t = 1 / Cr (to view in hours); t = 60 minutes / Cr (to view in minutes)

Here is an example,

For a 0.5C rate and a 2300mAh battery:

  1. Step 1: Convert mAh to Amps, 2300mAh / 1000 = 2.3A
  2. Step 2: Apply the formula, 2.3A x 0.5 C = 1.15 A available
  3. Step 3: Calculate charging and discharging,
    1. In minutes, 60 (min) / 0.5C = 120 minutes
    2. In hours, 1h / 0.5C = 2 hours

How to Find the C Rating of a Battery?

The smallest batteries are often rated at the 1C rating or the one-hour Rate. If your battery says 3000mAh at the one-hour Rate, then the 1C rating is 3000mAh. The C rate of your battery is generally on its label and also on the battery data sheet.

Different battery chemistries sometimes show different C rates.

For instance, lead-acid batteries are usually rated at a very low discharge rate, e.g. 0.05C, also known as the 20-hour Rate. Your battery’s chemistry and design will determine its maximum C rate. Lithium batteries, for example, will often withstand discharging C Rates much higher than other chemistries like alkaline.

NOTE: If you can’t find the battery’s C rating on the label or data sheet, please consider calling the battery manufacturer.

Application Requiring High C Rates

The number of applications and devices requiring a high C Rate discharge battery is rapidly growing. This includes everything from industrial to consumer applications: RC models and drones, robotics, and vehicle jump starters.

The common thing is that all of them have to handle a large amount of energy in a very short period of time. For instance, most jump-start systems need a 35C Rate discharge, and in the RC sector, people use high-rate discharge batteries that exceed the 50C Rate.

Some offerings available on the market even claim to have a C Rate higher than that. However, these announcements are based on the maximum pulse discharge rates, which means that the battery is forced to fully discharge over a few seconds. In reality, most applications require a 2C-10C Rate.

What are the Effects of C Rating on Lithium-ion Batteries?

Lastly, the C rating is highly dependent on the type of battery used; the battery has different applications. Since some electronics need a lot of power to supply them to many batteries with high c-ratings. An example of a high c rating application is the motorcycle starter. It only takes a few seconds, but it has to power the motors quickly.

Furthermore, some applications require a discharge time with a low C rating. An example is the solar light, which you need to power for the whole night or several nights. C-rate is a unit that explains the current value of the discharge time of lithium-ion batteries under different conditions. Now, you can settle for the best C rating for your next battery selection.

Dissecting Discharge Rates

In many cases, identifying the discharge rate of the battery is necessary to consider not only to enhance the battery’s capacity but also to ensure optimal performance and durability.

  • A fast discharge, or a higher C rate, refers to the process of drawing power from the battery fast. Sometimes, it is required for applications with rapidly changing power demands, such as power tools or emergency systems. On the other hand, high C rates may reduce the service life of the battery.
  • A medium discharge ensures an appropriate balance between the battery’s life and the energy drawn.
  • A slow discharge is characterised by a C20 and below Rate. In such a case, the power output of the battery must be stable and consistent for an extended period. Such applications include residential solar power systems.

Impact on Charging and Discharging

Fully charged and discharged times C rate provides an easy way to calculate how long a battery can take and discharge fully or reversely. For instance, a C10-rated battery can take 10 hours to discharge fully, while its C rate is rated for a 30-minute discharge. This is a fast and intense drainage of energy and usually occurs at a rate higher than 2C. It is common in applications that may need power quickly.

Optimising Battery Performance

Select a battery with the same capacity and discharge time as the energy requirement of the project. Choosing the best battery in this context may lead to underperforming or over-performing the battery and shorten its life. Calculate your operation’s energy demand.

Besides, one should constantly track the charge and discharge rates of the batteries, as it is a vital means of protecting the health of your solar battery. It helps prevent overloading, which accelerates battery decay.

The C rate plays a crucial role in the interactions with solar batteries due to the fact that it affects their discharge and charge limitations. Regardless of whether you have a residential solar system or you are using some batteries for your portable tools, learning about the C rate will help you save energy sustainably and make your batteries last longer. Understanding the detailed descriptions of battery discharge rates and how they relate to the type of batteries helps you make appropriate and informed choices regarding sustainable energy.

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Naveen Kumar
Naveen Kumar

October 13, 2023


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