C-Rate and C-Rating

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Wanna know a secret?  Get confused about C-Rates of cells and all that?  Here’s the Magic Decoder Ring.

“C-Rate” is a simple calculation of the amount of current going in or out of a battery.  “C-rating”, on the other hand, is a tested value of what C-Rate a battery can charge or discharge without damage.

They are not interchangeable terms.

That’s it.  That’s my big secret.  Am I the only one?  Did everybody else out there understand this?

Here’s the Battery University page on C-Rate“Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.”  You take a battery, discharge it at some specific C-Rate and decide if it can take it without blowing up.  If you discharge it at 10C and it’s fine, then discharge it at 15C and it starts getting hot, then it’s probably a good bet that it’s a 10C rating.  (Yes, that’s hugely oversimplified as far as the actual process goes, but don’t quibble details with me.)

And here, from the RadioControlZone, is a great explanation of the whole thing:

What is C in relation to batteries?

C ratings are simply a way of talking about charge and discharge rates for batteries.

1C, = 1 time the rated mah capacity of the battery. So if you charge your 650 mah pack at 1C, you charge it a 650 miliamps, or .650 amps.

1C on a 1100 pack would be 1.1 amps.

2 C on your 1100 pack would be 2.2 amps

Motor batteries are often rated in Discharge C and charge C.

So a 1100 mah pack (1.1 amp hour) might be rated for 10C discharge, so you can pull 11 amps ( flow ) without damaging the battery.

Then it might be rated at 2C charge rate (flow), so you charge it at 2.2 amps (2200 mah)

How did I do? Things clearing up?

If you have a 500 mah pack – any kind – and it is rated at 16C that means it can deliver 8 amps.

If you have a 1000 mah pack – any kind – and it is rated at 8C that means it can deliver 8 amps.

If you have a 1000 mah pack – any kind – and it is rated at 12C that means it can deliver 12 amps

If you have a 1500 mah pack – any kind – and it is rate at 8C that means it can deliver 12 amps

If you have a 1500 mah pack – any kind – and it is rated at 20 C that means it can deliver 30 amps.

If you have a 3000 mah pack – any kind – and it is rated at 10 C that means it can deliver 30 amps.

So, if you need 12 amps you can use a pack with a higher C rating or a pack with a higher mah rating to get to needed amp delivery level.

The formula is Current /capacity=rating, or to find the safe current discharge rate, rating x capacity = current.  Armed with this, let’s look at a 20ah 5C cell, like the Farasis:

A = 20ah x 5C = 100A

Pretty simple.  Now, remembering that when you parallel cells, you add up capacity while not adding voltage, if you parallel four of these to get an 80ah pack, you’re going to get 400A out of it (safely).

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4 responses to “C-Rate and C-Rating

  1. I understand completely, (slightly beforehand, and well, now). My confusion comes to a pack (mine is 3000mAh rated at 40C discharge cont, 50C burst), factory supplied with XT-60 connectors, that is capable of more than 60A draw from it (120A), continuously. I’m only using it for a 75A (max, surge). No continuous draw. I doubt the wires supplied could withstand 120A continuously, let alone the XT-60 connector (rated for 60A). Thanks for any help with this.

  2. Pingback: “Battery Horsepower”? | The Electric Chronicles·

  3. Pingback: Basic Battery Power Revisited | The Electric Chronicles: Power in Flux·

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