C rating is one of the most important factors to consider when buying LiPo batteries for your FPV drone. In this tutorial I will explain what C rating is, the significance of C-rating and how it’s calculated.
Make sure to check out my tutorial about LiPo batteries for FPV Drone, it has everything you need to know about LiPo.
What’s LiPo Battery C Rating
C Rating is an indicator of the continuous and burst discharge rate of a LiPo battery. It allows users to easily calculate the maximum current you can draw from the LiPo safely without damaging the battery.
Max Current Draw = Capacity x C-Rating
For example, if you have a 3S 1000mah 20C LiPo battery, the safe maximum current draw would be 1000ma x 20C = 20A.
You might wonder, can I draw more amperage than the above figure (i.e. higher the 20A given in the example)? You could, but it’s not recommended.
Discharging a battery at a rate higher than its specification – the battery will overheat and that will increase the internal resistance of the cells faster, which subsequently shortens battery lifespan or even damage it.
What Affects C-Rating
The following factors affects the C-rating of a LiPo battery:
- internal resistance of the cells – chemicals, battery construction, age etc…
- thickness of the wires
The biggest factor is usually internal resistance (IR), it limits the max current output of a LiPo battery.
If your battery has a low C-rating, it has high internal resistance, voltage of the battery will drop as you increase throttle – a common issue as known as “voltage sag”. And with voltage decrease, the motor will lose RPM and ultimately the drone would feel less powerful. (This is an over-simplified explanation of battery would affect the performance of a drone)
Energy that is lost to resistance will turn into heat, and this is why the battery gets warm when it’s discharging.
Higher C Rating Battery Is Bigger and Heavier?
Batteries of the same capacity, the one with higher discharge rate is normally bigger.
Both of these batteries are 4S 650mAh, but the one on the left only weighs 60g, while the one on the right weighs over 86g and much bigger.
And yes, the bigger battery has much better performance and able to output much higher current without getting overheat. But I wouldn’t say one necessarily better than the other. For a less powerful drone, it might not need all that potential power. A lower C-rating battery could be more beneficial because it’s lighter, cheaper, more efficient and actually gives you longer flight time. It all depends on the application.
Can You Trust C Rating?
No you can’t.
C rating can be a useful tool to help you select a LiPo battery, unfortunately many RC LiPo manufacturers nowadays use inflated C-rating numbers for marketing. Therefore, while C rating is an indication of how good a LiPo is, you should only take it as a grain of salt..
Check out my testing and reviews if you are not sure what batteries to buy.
Charging Your Batteries at 1C
C rating is also used to refer to how quickly you should charge a LiPo battery (or how much current to pump into the LiPo when charging).
You might have heard of a common saying “charge your LiPo at 1C”. This means charging the battery at a current equals to its capacity*1C.
For example to charge an 4S 1500mAh at 1C, simply multiply 1500mA by 1, which is 1.5A. At 2C, that would be 1.5A x 2 = 3A.
It’s worth knowing that if you charge a battery at 1C, it will take exactly one hour to fully charge it (regardless the capacity), at 2C that would be 30mins, and at 6C, 10mins.
Higher quality batteries can handle higher charge rates at little to no degradation over time, but lower grade cells will overheat if you charge them with too much current. For safety, it’s recommended to charge at 1C whenever possible. Or stick to the recommendations from manufacturer to prolong battery life and performance.
- Feb 2015 – Article Created
- Jan 2018 – Updated
- Aug 2020 – added info about C-rating and weight/size