Selecting the correct LiPo battery pack for your RC vehicle is not as easy as just selecting the cell count. One of the most important parts of the correct battery pack selection is also considering the C rating. What is the C rating? Well let’s take a look!
What is the C Rating?
All LiPo battery packs have a C rating associated with them. Actually, they have not just one C rating, but a few C ratings that we will discuss shortly. A C rating is a value given to the LiPo battery pack that refers to the maximum discharge rate of the LiPo. Examples of C ratings could be anything between 30C to 65C. Numbers outside of this range are possible as well. LiPo’s typically have the C rating marked right on the front of the LiPo pack. Can you spot the rating on these packs?
How to calculate Max Discharge using the C Rating?
The maximum discharge rate of the LiPo is highly dependent on the capacity of the battery pack. In general the larger the battery pack in terms of capacity measured in mAh, the higher theoretical maximum continuous discharge rate you should expect. With this being said, the capacity of the battery pack is part of the calculation. We first take the capacity of the battery pack in mAh and convert this to Ah. Next we use the maximum continuous discharge rating in order to perform the calculation. Take these two values and multiply them together.
If we look at the image above consisting of a 4000mAh 4s pack, it has a C rating of 45C. Firstly, 4000mAh is converted to 4Ah. We then take 4Ah and multiply it by 45C. The resulting value is 180 Amps.
4Ah x 45C = 180Amps
Our Turnigy Graphene pack is capable of providing 180A continuously according to the rating provided by the manufacture.
Check out this page to help calculate battery pack specifications.
Other C Ratings of a LiPo Battery Pack
There are other C ratings that can be found on LiPo battery packs that aren’t typically spoken about. The second C rating that is also quite important, is the rating provided for charging a LiPo battery. This C rating is specifically for the rate at which you can charge the battery pack at.
Charge Rate C rating
There are many chargers on the market these days that are able to charge at a high charge rate, significantly decreasing the amount of charge time. In order to be certain you can charge your pack quickly, you must verify the charge rate using the charge rate C rating. When LiPo’s first came out, typical charge rates were known as 1C. Typical C ratings for charging in today’s day is anywhere from 2C to 15C where 15C charge rates are crazy!
The Turnigy Graphene pack in the above example has a charge rate of 10C. To determine the maximum charge rate, we first convert the pack capacity to Ah from mAh. 4000mAh is equal to 4Ah. Next we multiply this value by the charging C rating. 10C x 4Ah = 40 Amps maximum charge rate. Keep in mind that this is an extremely high charge rate, in fact I typically do not charge any faster than 3C. An absolute personal maximum is 5C.
Peak Discharge C rating
The last and final C rating that you may come across when looking at LiPo battery packs, is the C rating for maximum peak discharge rate. The peak rating may or may not appear on the battery advertised. Typical peak ratings could be anywhere between 50 to 100% more than the rating for maximum continuous discharge.
What C rating is required for my Application?
The correct answer here really boils down to budgets, battery pack weights/sizing and required discharge rate. Having the battery pack with the highest C rating is always best for the power system and battery pack health.
It is best to have a discharge rate overhead of 30%. If you work out a maximum power system discharge of 100 amps. Your battery pack should should deliver at least 30% more or 130 total amps. Never match system draw to maximum continuous discharge rates of the battery pack.
For example, I would select this 860mAh at 35C for a load that will discharge at a maximum continuous current of 23 Amps.
35C x 0.86Ah = 30 Amps
30 Amps / ( 1 + 30%) = 30 A / 1.3
Having a battery pack that can deliver 1000 Amps for a motor that will only use 10 Amps may seem like overkill, and it probably is. However, there is no reason for concern that the battery pack could over power the motor. Keep in mind that a load only pulls the amount of current that it requires.
In conclusion, the best C rating for your pack is a value that will allow 30% overhead in discharge rate, fits your budget, and is the correct size and weight for your application.