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Mar 06, 2012, 06:29 AM
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A simple way to estimate the true C rating of a LiPo battery


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 2200 mAh (2.2 Ah) battery rated at 30C should be able to deliver 2.2 x 30 = 66 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. There are also a couple of links to a discussion thread and a database of people's results.

To go to the web calculator, click on

www.jj604.com/LiPoTool/
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Mar 06, 2012, 06:31 AM
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jj604's Avatar
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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!

I have included a link on the web page to take you directly to the discussion thread about this approach if you want to know more.

Collecting results
There is also a direct link to a database reference (ie: no chit-chat please ) thread where we hope to accumulate values of measured C to assist ordinary modellers with battery choice. If you find the results you are getting are useful and reflect your real life flight experience, please post your results there using Mark’s requested guidelines.

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.
Mar 06, 2012, 12:18 PM
BrainFart RC-Pilot
TreeDiver's Avatar
The value displayed, should it be after charge? Or if the battery is half full, inside the hangar at arround 22degrees tempature.

Maybe I did overlooked it?

Useful guide.
Maybe handy if the calculator has extra tab for main lead IR (cell number needs to be added then as well as value for calculation).

Thanx and cool

TreeDiver (100C rc-pilot)
Mar 06, 2012, 06:09 PM
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jj604's Avatar
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TreeDriver, best to use the discussion page for questions about the method. Mark and others monitor that one regularly.

https://www.rcgroups.com/forums/show...1577989&page=1

IR doesn't change very much with state of charge but I always measure mine the next morning (or at least more than an hour) after they have been fully charged just for consistency.

John
Quote:
Originally Posted by TreeDiver
The value displayed, should it be after charge? Or if the battery is half full, inside the hangar at arround 22degrees tempature.

Maybe I did overlooked it?

Useful guide.
Maybe handy if the calculator has extra tab for main lead IR (cell number needs to be added then as well as value for calculation).

Thanx and cool

TreeDiver (100C rc-pilot)
Sep 18, 2015, 04:11 PM
Registered User

LiPo battery with unknown C rating


Hello,
I recently bought a LiPo battery from ebay http://www.ebay.co.uk/itm/3-7V-2000-...item19e6ddac14 that I used once with my quadcopter until it discharged, and noticed later it has no C rating on it, nor on its ebay site. I haven't charged it since then.
Could you please help me how to calculate the exact C value?
Sep 18, 2015, 06:42 PM
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jj604's Avatar
Thread OP
Written on the pack it says 7.4Wh and it is called a 3.7V 2000mAh Polymer Li Lithium cells LiPo so it is a 1S 2000mAh single cell.

As for C rating the Ocean Sun website does not list that particular cell.

http://www.osbattery.com/eng/product2.asp

However the data sheets for all their other cells show discharge curves up to 2C only and at that rate they are showing significant voltage drop. I am guessing that (unless they produce a special range for modelling) the Ocean Sun cells are all standard low discharge designed for smart phones and GPS applications. It is probably not suitable for a quadcopter.

Here is the closest equivalent in size I could find but it is a bit bigger than yours.

http://www.osbattery.com/eng/pdf/li/DS_6330130P0.pdf

They recommend a maximum discharge of 2.4A. That is a 1C rating.

You could estimate the C rating using the LipOTool in this thread if you have an IR meter capable of measuring single cells but they are rare. IR can also be measured using a Voltmeter and known current load but it is very tricky for single cells. There is no way to calculate C without test results.

The best thing is to see how the quad flies and how warm the pack gets. If it gets more than mildly warm during normal flight it is over stressed. A surface temperature of 60deg C is commonly regarded as maximum allowable for reasonable cell life. Certainly no more than 70deg C.
Last edited by jj604; Sep 18, 2015 at 06:48 PM.
Oct 26, 2015, 07:23 AM
Registered User
Won2Race's Avatar
jj604, is the webpage still available?
Oct 26, 2015, 03:51 PM
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jj604's Avatar
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I just clicked on the link and it seems to be working fine.
Jul 14, 2017, 01:40 PM
Dutch RC Reviewer
Dutch 79's Avatar
Nice, my infinity race spec 1500 mAh 90c comes in at 34C well done designing the stickers infinity

7.6ohm according to my charger
Jul 14, 2017, 05:11 PM
Registered User
I hope you meant 7.6 mohms!
Jul 14, 2017, 07:17 PM
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jj604's Avatar
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Gents, the discussion thread is here:

https://www.rcgroups.com/forums/show...rformance-tool

John


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