I saw this lipo C estimator tool posted on another forum by member jjr06 and thought it would be helpful for all of us..
One of the most misunderstood and confusing numbers around is the “C rating” manufacturers put on their LiPos.
Most folks understand that a battery can be safely used at a maximum current which is the Capacity multiplied by the C rating.
For example a 5000 mAh (5.0 Ah) battery rated at 30C should be able to deliver 5 x 30 = 150 Amps.
But that all depends on how the manufacturer derived the C rating and how honest they (or the battery label design department) are. It also depends what you mean by “safely used”.
Most label values of C are optimistically high and consistency between manufacturers is lacking. There is no agreed standard for measuring C and many manufacturers use a single value across a range of batteries when in fact it may vary with size. C rating has very largely become a marketing ploy rather than a reliable measure.
Mark Forsyth and others have developed a simple way to make a reasonable “real world” estimate of a conservative continuous C rating that will ensure that our LiPos are long lived and deliver expected voltage under load. It is not theoretical, but a practical “rule of thumb” based on a great deal of measurement and observation on LiPos in action over a number of years.
It uses the Internal Resistance (IR) measurement made by some of the high end chargers. If you have an iCharger that measures IR or an FMA CellPro 10S, CellPro 10XP, PL6, or PL8. – or have one of Wayne Giles purpose built LiPo ESR/IR meters – calculating C takes only a moment.
I have converted Mark’s spreadsheet to a web page to make it more accessible. There is no need to download or understand Excel and it can be accessed from anywhere (including an iPad or Smartphone). You can even test a pack at the field if you suspect a problem, or check for a reduced recommended current in cold weather.
Just enter the maximum measured cell IR in mOhms and the battery capacity in mAh and it will calculate a recommended maximum average current and “true” C rating for the pack.
To go to the web calculator, click on
http://jj604.com/LiPoTool/
One of the most misunderstood and confusing numbers around is the “C rating” manufacturers put on their LiPos.
Most folks understand that a battery can be safely used at a maximum current which is the Capacity multiplied by the C rating.
For example a 5000 mAh (5.0 Ah) battery rated at 30C should be able to deliver 5 x 30 = 150 Amps.
But that all depends on how the manufacturer derived the C rating and how honest they (or the battery label design department) are. It also depends what you mean by “safely used”.
Most label values of C are optimistically high and consistency between manufacturers is lacking. There is no agreed standard for measuring C and many manufacturers use a single value across a range of batteries when in fact it may vary with size. C rating has very largely become a marketing ploy rather than a reliable measure.
Mark Forsyth and others have developed a simple way to make a reasonable “real world” estimate of a conservative continuous C rating that will ensure that our LiPos are long lived and deliver expected voltage under load. It is not theoretical, but a practical “rule of thumb” based on a great deal of measurement and observation on LiPos in action over a number of years.
It uses the Internal Resistance (IR) measurement made by some of the high end chargers. If you have an iCharger that measures IR or an FMA CellPro 10S, CellPro 10XP, PL6, or PL8. – or have one of Wayne Giles purpose built LiPo ESR/IR meters – calculating C takes only a moment.
I have converted Mark’s spreadsheet to a web page to make it more accessible. There is no need to download or understand Excel and it can be accessed from anywhere (including an iPad or Smartphone). You can even test a pack at the field if you suspect a problem, or check for a reduced recommended current in cold weather.
Just enter the maximum measured cell IR in mOhms and the battery capacity in mAh and it will calculate a recommended maximum average current and “true” C rating for the pack.
To go to the web calculator, click on
http://jj604.com/LiPoTool/
| Some other stuff that may be helpful.. The one thing to watch is the temperature, as IR values are both temperature and method dependent. Follow the guidelines in red. The ESR meter, iChargers and FMAs all give slightly different answers but are close enough for practical purposes, and the discussion as to which one is “correct” is academic for this purpose. The Hyperion chargers use a method that gives different values and are not recommended for this particular approach. A caution IR measurement is a complex and controversial subject and gives rise to a lot of opinion! Collecting results If you find the results you are getting are useful and reflect your real life flight experience, please post your results here Just to reiterate; this is a practical “rule of thumb” tool – a bit like the well known Watts/Pound rules. It attempts to estimate a conservative safe value of C for a pack from individual cell readings. It is entirely possible to discharge a pack at higher rates successfully. Degradation of cycle life and voltage drop during discharge change continuously with current draw, there is no abrupt end point beyond which you should not go! However, if you want to discharge packs at high C rates, you really need to spend time understanding the voltage discharge curves that experienced testers post on the forums. Hope to hear back from those who can provide some real world data which will help us all determine which lipo manufacturers are telling the biggest porkies and able to make more informed lipo purchases. Sticky worthy? |
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