|By Jim Broesch
We all know that LiPo batteries have had a huge impact on our hobby. However, even after years of using LiPos the “C rating” is one of the more controversial topics of discussion. First let’s look at the C rate, then we’ll see how it ties into the internal resistance. The C rate is basically the maximum charge rate. For example: a 1S battery (3.7 Volts nominal) with a capacity of 1000 milliamp hour (1000 mAh) and a charge rating of 1C is designed to be charged at 1 Amp (1000 mA) and should reach full charge in 1 hour. Some batteries claim they can be charged at multiples of the C rating. This is somewhat oxymoronic given the definition of the C rate. However, it is fine to charge at a lower rate than the C value. For the example above we could charge at C/2 or 500 mA. In this case it would take twice as long to recharge – thus putting less stress on the battery. I always try to charge at this C/2 rate. One thing to keep in mind is that most LiPo chargers will not charge for longer than two hours. This is a safety precaution to help avoid problems with the batteries getting too hot from accidental overcharging. More commonly, however, we are interested in how much current the battery can discharge. Again, using the example above, we might have a battery that specifies a 20C discharge rating. This (theoretically) tells us we can safely pull 1 Amp * 20 = 20 Amps of current from the battery. Of course, our charge would only last 1/20 of the time, 3 minutes to be exact.
So, to make more sense of all of this we need to dive a little deeper. If we look at the figure below, we see what engineers call the Thevenin equivalent circuit.
|Despite the impressive sounding name the idea is quite simple: we can think of a real-world battery as a theoretically ideal battery in series with a theoretically ideal resistor. Why we care is that many of the key performance characteristics of the battery are defined by the value of this resistor. The resistor is what sets the maximum rate at which current can be put in or taken out of the battery. Since resistors get hot when they conduct electricity the smaller the resistance the better. And this is where LiPos shine. They have a very small internal resistance. So, we can (usually!) pull large currents from our LiPos without them catching fire.
Now, where the confusion with the C rating comes from has two sources. First, the term “C factor” has no real definition. Even the Wikipedia article on LiPos makes no mention of the term. Vendors are free to put more-or-less whatever value they want on the battery. So, theoretically we should be able pull current out of a 50C battery twice as fast we can from a 25C battery. In practice, however, this is where the link between the C rating and the internal resistance becomes important. And the reason is that the internal resistance of the cells in the battery can, and will, change over time. Thus, we may be able to pull more current out of new 25C battery than we can out of an old 50C battery.
The C value does give you a general idea of the charge/discharge characteristics of a battery. It is not useless. On the other hand, for applications where understanding the charge or discharge characteristics are important, the C rating is a very imprecise tool. Much more important is the internal resistance. Some battery chargers are capable of testing the internal resistance of the batteries they are charging.
From a practical point of view this means this explains some of the rituals we observe with our LiPo batteries: Keeping batteries at half voltage or so helps keeps the internal resistance low. An aging battery, independent of what voltage the battery is stored at, will see an increase in the internal resistance. Thus over time the C value will decline. This why many RC pilots will replace their batteries after a year – regardless of their condition otherwise. Understanding the relationship between the internal resistance and the C value provide a significant insight into our fascinating hobby.