The Subject of the Weekend: Understanding the C-Rate

Discharge rate, or C-Rate, is what will determine how much current the battery can deliver to the controller and motor.  It’s confusing.  I’m going to make it the Subject of the Weekend, and start with what I have right now.

Let’s first turn to Battery University for a good overview of the concept, here.

As I understand it, you have two things going on.  First, you have the internal design of the battery itself.  There’s only so much that a battery can deliver at it’s maximum.  Second, like motors, you have the maximum that a battery will deliver without melting down or blowing up.  That’s more a function of a specific aspect of the construction – the internal resistance.  You have a lot of current load, you have a lot of resistance, you’re going to make a lot of heat.

Consequently, discussions about determining the C-rate of a particular battery talk about cycle life.  You want to find out how long the things last at any given discharge rate.  Here’s a typical graph from Battery U:

Now, look at this thread on EndlessSphere about testing Headways, and this post specifically.  There he says: “For sure these cell can handle 50A continuous! …at 30A continuous, they reach 39 degree C frtom the ambient of 24. I expect they should reach around 50-55 degree C at 50A. In a scealed bex maybe they could reach around 60 degree C .”

The question is, to understand what this means in terms of a C-rating, how long can you run these at a 50 amp load and how does it affect the overall life of the cells?

More to come…



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