C rating

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C is a measure of how much current a battery can deliver, relative to its capacity. The maximum safe current that can be drawn from a battery is the capacity of the battery times its C rating.

Exceeding the C rating of a battery can damage it, and in the case of a LiPo, can cause it to puff or catch fire.

The C rating does not allow you to compare batteries of different voltages; for that we must calculate a maximum power rating. If you have calculated your battery capacity in watt-hours, then the C rating is equal to watts per watt-hour.

[edit] Examples

  • A 3-cell LiPo rated at 2100mAh, 20C, can deliver up to:
2100mAh = 2.1Ah, 20 * 2.1 = 42A (Amperes)
Alternatively, a LiPo cell =3.7V; 3 * 3.7V = 11.1V; 42A * 11.1V = 466W (Watts)
  • A 4-cell LiPo rated at 1500mAh, 30C, can deliver up to:
1500mAh = 1.5Ah, 30*1.5 = 45A
Alternatively, a LiPo cell = 3.7V, 4 * 3.7V = 14.8V; 45A * 14.8V = 666W

[edit] Higher mAh vs higher C rating

Many people have a tough time when comparing batteries. Is it better to get one with a higher mAh or a higher C rating? Unfortunately, the answer is "it depends". The problem is that the two work hand in hand. An easy way to understand the difference between the two is to think of a car.

You can think of mAh as the size of your gas tank. Imagine two identical cars except one has a 16 gallon tank and one has a 20 gallon tank. Obviously, the car with the bigger gas tank can travel longer.

You can think of C as how fast the gas can be delivered to the engine. Imagine how much more powerful your engine would be if you could feed it gas through a garden hose versus trying to feed it gas through a straw. The higher C means you can draw more current for more power.

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