A LiPo Battery packs C rating is a very important specification on a LiPo. It represents the ability for a LiPo battery pack to provide continuous current. Although, the C rating alone does not tell you how much current you can get out of a particular LiPo. There is a calculation for this and that calculation is quite simple to perform.
Consider a LiPo battery pack has a specified maximum continuous discharge C rating of 50C and a capacity of 3000mAh. Do not confuse the C ratings of the LiPo, there can be a continuous C rating and a Maximum Peak C Rating. We only want to use a continuous C rating. Firstly, 3000mAh must get converted to Ah. 3Ah. Next take the C rating and multiple it by the capacity in amp hours. 3Ah x 50C = 150A of maximum discharge current.
The question then becomes, how do you know that the C rating specified on a battery pack is accurate and not just some buy now bait from a marketing team? Or how do you know if a battery pack is weakening with age and or wear? Let’s look at 3 ways that we can do so. After all the C rating is just an arbitrary number that manufactures identify as a safe thermal limit.
Calculate the Actual C Rating of your Battery pack
The most simplistic approach to quickly estimate the actual C rating of a battery pack is to calculate it! To make this process as simple as possible, the calculator has already been complete and is located here: The Real LiPo C Rating Calculator
In order to calculate the C rating of your battery pack, you will need to know the C rating of your pack, the capacity of your LiPo and the average internal cell resistance. The average internal cell resistance is measured using a LiPo battery charger. The other items are simply read on the label of the pack.
It is recommended to follow the procedure in order to reduce error. Information that goes in to the calculator largely effects the overall accuracy of the calculation.
LiPo Battery Specification Comparison
This last method is both simple but also not so simple. The goal for this method is to compare internal resistances of battery packs with the same C rating. Once you know the internal resistance of your pack you can compare to other packs. This is the easy part. Where it gets more difficult is finding specifications of other LiPo battery packs for you to compare against. If you have multiple packs or have used these packs previously, you may already know where the internal resistance should be at. Use this as the baseline comparison. But if you don’t?
Consider having a 5000mAh battery pack that has an average internal resistance of 3.5 milliohms. If you happen to compare your data up against other known data (yours or others) and determine that the internal resistance is 2.5 milliohms, you can draw conclusions. Having a higher internal resistance does suggest that either the C rating of the pack may be weaker than you thought, or that the battery has aged.
Use a Load to Stress Test the LiPo – Manufactures Test
This test is not one that I can recommend, however it would be what I’d expect LiPo manufactures to be completing. Or some variation of the test.
If you have access to a large power dissipation bank that can apply a load to your battery pack, this is one way you can verify discharge current. There are such units that exist on the market today specifically for our RC battery packs.
The idea is that you would take your LiPo pack and wire it to the load bank. The load bank would then be configured to load the battery pack at the specific amount of current that the test requires. What you’d want to do is load your battery pack to the specified C rating. As the battery is being discharged, temperature should be recorded. The duration of the test would last until the pack is discharged to 80% of it’s rated capacity. During this time the LiPo would have to not exceed the manufactures maximum recommended temperature. If the maximum temperature is exceeded then the LiPo could not deliver the amount of power required for the C rating specified.
Concerns with the LiPo Load Test
Not only can this test be dangerous but it also places a lot of stress on the LiPo. For these reasons alone, is why I start off by saying it’s not my recommended test to confirm C rating. a) I don’t want to cause any harm to myself when conducting such a test and b) I don’t want to harm the LiPo’s.
Load Test Alternate
Another version of this test can be completed placing less stress on the battery and also decreased safety risks. This would be testing at a lower C rating then what the battery pack is rated at. However, the difference is that the expected manufactures maximum temperature would also decrease. The expected temperature would have to be verified with the new load to be applied.
I hope these points in this article provide you with an idea as to how LiPo batteries should be tested by the manufacture. It doesn’t matter what any of these tests say about your battery pack. keep in mind that if the battery stays within temperature specifications and operates your RC’s, that is what is most important.
Keeping your LiPo within its limitations can prolong your LiPo Batteries lifespan.