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 current than the above figure (i.e. over 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.
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, it’s not everything.
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 inferior quality, which means IR will rise quickly, and voltage sag will become more and more pronounced, negatively impacting the lifespan of the battery.
- Feb 2015 – Article Created
- Jan 2018 – Updated