When buying LiPo batteries, you should see cell count (S), capacity (mAh) along with C rating (C). The first two are self-explanatory, but what is C rating?
LiPo Battery C Rating Explained
C Rating is an indicator of continuous discharge rate of a LiPo. It’s designed to allow users to easily calculate the maximum constant discharge current you can draw from the LiPo pack safely without harming the battery pack.
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 20A in the example)? You could, but it’s not recommended as batteries it’s bad for the battery’s health, and the battery might not be able to deliver the desired power anyway.
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 causing the battery to get warm when discharging at high current. It might even “go puffed” (become swollen) in some extreme cases.
Since V=I*R, to maintain the same desire current, the voltage required would be proportional to resistance. That means if the IR is high in your battery, you will have bad voltage sags. When voltage drops, your motor will lose RPM and the aircraft will feel “lack of punch”.
Note that IR can change over time, even a change of temperature can affect it. And 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 could be an useful tool, unfortunately many RC LiPo manufacturers nowadays overstate C-rating values for marketing purposes. Therefore, while it’s an indication of how good a LiPo is, it’s not everything. Try to find out about batteries’ true and overall performance by looking up third party tests and reviews whenever possible.