C rating is one of the most important factors to consider when buying LiPo batteries. In this tutorial we will explain what C rating is and how it’s calculated.
Make sure to check out our tutorial about mini quad LiPo batteries.
LiPo Battery C Rating Explained
C Rating is an indicator of the continuous discharge rate of a LiPo. It allows users to easily calculate the maximum constant current you can draw from the LiPo pack safely without harming the battery.
Max Current Draw = Capacity x C-Rating
For example, if you have a 3S 1000mah 20C LiPo pack, your safe max 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 too quickly is bad for the health of the battery. It will raise the internal resistance faster than discharging within the battery’s limit.
Discharge Rate and Charge Rate
C rating is also used to refer to how quickly you charge a LiPo battery (or how much current to pump into the LiPo when charging).
You might have heard of common sayings like “charge your LiPo at 1C”. This means charging the battery at its capacity number, for example for an 4S 1500mAh pack, 1C means 1500mA which is 1.5A. 2C would be 1.5A x 2 = 3A.
Interesting if you charge at 1C, it will take exactly one hour to fully charge a battery, 2C would be 30mins, and 6C would be 10mins etc…
Higher quality batteries can handle higher charge rates at little to no degradation but lower grade cells have a higher probability of overheat at higher charge rates. For safety, it’s recommended to charge at 1C whenever possible. Or stick to the recommendations from manufacturer to prolong battery life and high performance.
To explain these issues, we need to understand what internal resistance (IR) is. Every LiPo cell has internal resistance, and this is what limits the max current draw of a LiPo battery.
Ohm’s Law: Voltage (V) = Current (I) x Resistance (R)
Energy that is lost to resistance will turn into heat, and this is what causes the battery to get warm when discharging at high current. The pack can even become swollen (or “puff”) in extreme cases.
Since the formula V=I*R is constant, to maintain the desired current, the voltage required is proportional to resistance. That means if you have a high IR in your battery, you will suffer from bad voltage sag when using the throttle aggressively. When the voltage of your battery drops, your motor will lose RPM and the aircraft will lack “punch”.
Note that IR will increase as the battery gets older, but it can be affected by other factors too, like a change in temperature (why battery performs worse in the cold winter?)
Also note that some battery packs have low IR, probably because of the high C-rating, but it’s not always true that high C-Rating batteries necessarily have low IR.
Can You Trust C Rating?
C rating can be a useful tool to help you select a LiPo pack, unfortunately many RC LiPo manufacturers nowadays overstate C-rating values for marketing purposes. Therefore, while C rating is an indication of how good a LiPo is, you can’t always trust it.
The best thing to do is to find out about a battery’s true overall performance by looking at third party testing and reviews.
Remember that cheap batteries are usually of inferior quality, which means IR would rise quickly. Voltage sag will become more and more pronounced overtime, negatively impacting the lifespan of the battery.
- Feb 2015 – Article Created
- Jan 2018 – Updated