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Old Mar 28, 2010, 03:43 PM
flyit, crashit, fixit.
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Lipo C-ratings puzzle: what's most important, C-rating or mAh capacity??

Here's a bit of a Lipo puzzle. What's most important: C-rating or mAh capacity??
All Lipos mentioned here are 3-cell.

My bottom line is that I want to obtain my Amp/Volt/Watt target, without hooking up a truck battery. To achieve this, it seems to me that it will always be better to use a larger capacity (mAh) Lipo, rather than going for the highest <C> rating (and save $$$$). Hope that's not stating the obvious

How this conclusion was reached:

Bench-tested my setup with a brand-new 2200mAh 20C (no-name) 175gram Lipo. This Lipo is a bit too big for the plane unfortunately: it prefers around 130grams, and smaller volume.
The setup draws 20A at 11V. Dropped from 12.5V start voltage. Very acceptable.
Power was measured at 220W, which ties in with P=IV (220W = 20A x 11V).
So that’s my target, 220Watts.

Then I tried a $10, no-name 1500mAh 15C (claimed), 105gram pack. With this I expected that the pack should easily deliver 20A (which is, after all, only 13C). No such luck . .
Result: 15A at 10V (=150W).
Oh: less current, AND a bigger Voltdrop, giving 32% less Watts! Hmmm. Disappointment. Thinks . . Lipo is really only around 10C or less maybe??? (optimistic claims from producer?). In any event this Lipo cannot supply enough Amps, and it can’t even do it fast enough.

Thinks, . . a bit more: . . ok, bought a top quality 1500mAh 20C KOKAM (130grams), which cost 30Euro. That should handle it ok.
Result: 18A at 11V (=198W). Oh!!
Much better: an improvement for sure, BUT, I am still missing 10% (20W) of my expected power somehow. Why? Internal resistance?? Climate change (AGW)?? Not enough beer??

Then yet another test result, using two (no-name) 1000mAh 15C (75grams each) in parallel:
Result: 17A at 11.5V (=195W).
Not bad for two $7 Lipos. Luckily I’ve a few more to hand!

CONCLUSION:
So, one Kokam 1500mAh 20C (130gram), gives about the same power result as two cheapo 1000mAh 15C (total weight 150gram), which are slightly larger in volume & weight, but the pair have 30% more capacity, at about a third or less of the price .
Both these options don’t quite come up to the 2200mAh 20C no-name though!

Selecting a larger <C>value will definitely improve power capabilities, but the monetary cost of the increased performance is exponential. You pay much more for a higher <C>, but quoted/claimed values are still likely to fall short of anticipated results, IMO (so far!).
. . well, that’s my opinion based on results of recent bench-testing.*

2200mAh with 10C could (should), in theory, supply 22A (~20A reality), and would seem to be a better plan than using a high quality 1500mAh with claimed 20C (=30A) or even 30C (=45A), as these will disappoint in the WATTs (and/or cash) department.

i.e. <THERE’S NO SUBSTITUTE FOR CAPACITY !> (Bob’s 33rd law!).

Got all that?? , any comments? Someone must surely know all about this stuff!
Maybe some of you guys with a powermeter have also tested your setups with different Lipo packs and have results to compare?
I’m curious, thanks.

*[OK, I realise that I have only a small amount of data here, and maybe I got a dodgy Kokam (unlikely), or there may be some other factors I have missed. Perhaps I should spreadsheet this data to simplify the explanation (?!?), and apologies if this has already been covered: I only just figured it!]
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Old Mar 28, 2010, 04:15 PM
Southern Pride
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Haralson County GA. USA
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Most recent test (newest LiPolys) near the bottom.

http://www.rcgroups.com/forums/showp...70&postcount=4

A $30 3S rated 25C which can supply much more than 200 watts with ease.

http://www.rcgroups.com/forums/showp...08&postcount=1
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Old Mar 28, 2010, 05:09 PM
flyit, crashit, fixit.
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@everydayflyer

Hi, and thanks for fast response and associated links with info.
I have read a lot of your posts and respect your knowledge in this field.

Using my starter base: that is, my system pulling 20A at 11V, being supplied by a more-than-adequate source operating at 10C even though itís rating is 20C.

I bought the Kokam 1500 20C thinking that it would easily be able to supply my required 20A without too much volt-drop, and was very surprised that it didnít reach the 20A target, only delivering 18A at 11V. Almost, but not quite. I can live with it, but it bugs me that it doesnít cut it 100%, only 90% !

Can you please explain why can it not deliver the current? I can only think quality, or KokamĎs optimism.
As itĎs regarded as a high quality Lipo (I thought, anyway), I am disappointed, as I thought it would manage 20A without much trouble.

Obviously as weíre talking planes here, weight and physical size are major considerations, so I was looking for a suitable Lipo which could give me 20A. Thatís full gas, which I wouldnít be using non-stop anyway.

Thanks for your trouble.
Cheers.
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Old Mar 28, 2010, 05:48 PM
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Some really like Kokams but not I. I wasted a lot of money on them years ago when they were very expensive and performed very poorly. My 20C rated Kohkams did good to deliver 50% of their capacity at 10C.

Charles
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Old Mar 28, 2010, 05:59 PM
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Quote:
Originally Posted by Merlin45 View Post
@everydayflyer

Hi, and thanks for fast response and associated links with info.
I have read a lot of your posts and respect your knowledge in this field.

Using my starter base: that is, my system pulling 20A at 11V, being supplied by a more-than-adequate source operating at 10C even though itís rating is 20C.

I bought the Kokam 1500 20C thinking that it would easily be able to supply my required 20A without too much volt-drop, and was very surprised that it didnít reach the 20A target, only delivering 18A at 11V. Almost, but not quite. I can live with it, but it bugs me that it doesnít cut it 100%, only 90% !

Can you please explain why can it not deliver the current? I can only think quality, or KokamĎs optimism.
As itĎs regarded as a high quality Lipo (I thought, anyway), I am disappointed, as I thought it would manage 20A without much trouble.

Obviously as weíre talking planes here, weight and physical size are major considerations, so I was looking for a suitable Lipo which could give me 20A. Thatís full gas, which I wouldnít be using non-stop anyway.

Thanks for your trouble.
Cheers.

I am not sure why you think smaller lipos, with C ratings (1500-15C, 1500-20C, equivalent 2000-15C) will provide the same current at the same output voltage as as a 20C 2200mAHr pack--which as you say is too heavy and big for the application.

It is obvious that the C rating (even though it is an ill-defined quantity) doesn't directly correlate into providing your definition of acceptable power--even the 20C 2200 pack would fail to provide your definition of acceptable power for the C rating I think.

I will agree with you that cost is not always a good indicator of the internal resistance of a pack. However it isn't completely anti-correlated either ("high"cost does not equate with high resistance).
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Old Mar 29, 2010, 05:33 AM
flyit, crashit, fixit.
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Quote:
Originally Posted by everydayflyer View Post
Some really like Kokams but not I. I wasted a lot of money on them years ago when they were very expensive and performed very poorly. My 20C rated Kohkams did good to deliver 50% of their capacity at 10C.

Charles
thanks for your reply. Then, as I suspected, Kokams are simply overrated in that case, and I wonít be buying any more of them either, as they are still not exactly cheap!
Could you recommend a 1500 x-C that does deliver please? I thought about the Turigy 20C at HK, which costs $10.95.
Bob
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Old Mar 29, 2010, 05:46 AM
flyit, crashit, fixit.
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Originally Posted by Alan Hahn View Post
I am not sure why you think smaller lipos, with C ratings (1500-15C, 1500-20C, equivalent 2000-15C) will provide the same current at the same output voltage as as a 20C 2200mAHr pack--which as you say is too heavy and big for the application.

It is obvious that the C rating (even though it is an ill-defined quantity) doesn't directly correlate into providing your definition of acceptable power--even the 20C 2200 pack would fail to provide your definition of acceptable power for the C rating I think.

I will agree with you that cost is not always a good indicator of the internal resistance of a pack. However it isn't completely anti-correlated either ("high"cost does not equate with high resistance).
thanks for taking the trouble to respond. I do agree with what you say (direct correlation/ill-defined etc).

Iím not saying that smaller will provide the same current as larger.
What I am saying is, that it seems that higher capacity with lower-C, is more likely to satisfy than a smaller capacity, even when compensated for by a higher-C rating. So better to buy a cheapo with more mAh and lower <C>.

My (no-name) 2200 20C pack supplies 20A (=9C) to the system easily, and I have taken that to be my target. A much larger battery would not give a higher Ampere reading, because the system is happy with 20A, but lower capacity, with higher-C should crack it, so I thought!

BUT, I had thought that a high-quality (?) Kokam 1500 20C pack SHOULD be able to supply 30A;- well letís say 25C to be on the safe side! However, I only require it to supply 20A, which to my thinking would be be absolutely no problem, IF the C-rating is realistic. It only managed 18A (=90%) though.

So, would a Kokam 1500 25C or 30C actually achieve the target? I doubt it, but donít know for sure. Not prepared to spend big money to find out, as the 20C doesnít live up to itís claim.

The fact that it can only supply 18A is what gets me. WHY, why, why??
Where's my thinking gone wrong?

From what Charles says, Kokam are just not all theyíre cracked up to be, which seems to answer the question. However, it also raises another:

Is this only a question of the claimed-C/or Cell-quality? and; is it unrealistic to assume that a 1500 20C (real) can supply 20A, or are there other, better Lipos? (or are they all unrealistic?).

Thanks for your thoughts on this.
Bob
---
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Old Mar 29, 2010, 08:40 AM
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Originally Posted by Merlin45 View Post
Here's a bit of a Lipo puzzle. What's most important: C-rating or mAh capacity??
All Lipos mentioned here are 3-cell.

My bottom line is that I want to obtain my Amp/Volt/Watt target, without hooking up a truck battery. ...
In theory -- and that's assuming that manufacturers are honest/accurate about the mAh and C ratings they give their packs -- 1,000mAh rated 20C and 2,000mAh rated 10C are going to be able to comfortably deliver the same amps, volts, watts. But, as has already been said here, some manufacturers seem to have a history of being over-optimistic.

But what's also important for most people is time/quantity, and in the above example the 2,000mAh pack will keep your model running for twice the duration of the 1,000mAh pack. So, personally, if given a choice, and if other factors such as dimensions and cost didn't rule it out, I would always go for more mAh over higher C rating
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Old Mar 29, 2010, 08:57 AM
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Can you repeat your tests because they don't seem to make much sense ? I assume you're using exactly the same motor and prop all the way through and each battery is freshly charged ? In which case what you have is

Test 1 20A @ 11V (on the 2200mAh 20C). O.K. that's a reference point
Test 2 15A @ 10V looks o.k but that's a much weaker battery (max 22.5A rather than max 44A in Test 1).

Now we get to
Test 3 18A @ 11V. With the same load (motor/prop) it shouldn't be possible to get less current at exactly the same voltage as Test 1. If it's running at 11V the current should be 20A again. If the current is really lower the voltage should be lower too.

Test 4 17A @ 11.5V. Even stranger you're now showing a HIGHER voltage but a LOWER current. That's not possible either if the load is still the same as in the other tests.

Something odd going on but perhaps it's not just the battery performance .

Steve
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Old Mar 29, 2010, 03:01 PM
flyit, crashit, fixit.
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Originally Posted by abenn View Post
In theory -- and that's assuming that manufacturers are honest/accurate about the mAh and C ratings they give their packs -- 1,000mAh rated 20C and 2,000mAh rated 10C are going to be able to comfortably deliver the same amps, volts, watts. But, as has already been said here, some manufacturers seem to have a history of being over-optimistic.

But what's also important for most people is time/quantity, and in the above example the 2,000mAh pack will keep your model running for twice the duration of the 1,000mAh pack. So, personally, if given a choice, and if other factors such as dimensions and cost didn't rule it out, I would always go for more mAh over higher C rating
agreed: <manufacturers OVER optimistic claims> being the key to the puzzle here .
Also, for a given capacity, those with lower C-rating are relatively smaller & lighter, I think.
What I mean here is just that: get the biggest capacity you can for space/weight limitations, giving less importance to C-rating. Saves you cash into the bargain.
Thanks for your input!
Cheers, Bob
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Old Mar 29, 2010, 03:13 PM
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Originally Posted by slipstick View Post
Can you repeat your tests because they don't seem to make much sense ? I assume you're using exactly the same motor and prop all the way through and each battery is freshly charged ? In which case what you have is

Test 1 20A @ 11V (on the 2200mAh 20C). O.K. that's a reference point
Test 2 15A @ 10V looks o.k but that's a much weaker battery (max 22.5A rather than max 44A in Test 1).

Now we get to
Test 3 18A @ 11V. With the same load (motor/prop) it shouldn't be possible to get less current at exactly the same voltage as Test 1. If it's running at 11V the current should be 20A again. If the current is really lower the voltage should be lower too.

Test 4 17A @ 11.5V. Even stranger you're now showing a HIGHER voltage but a LOWER current. That's not possible either if the load is still the same as in the other tests.

Something odd going on but perhaps it's not just the battery performance .

Steve
Interesting points, and has got me thinking, thanks. Iím figuring this out as I go along, so ideas help a lot.
First I can confirm that all tests are done under same circumstances, which are, I admit, not ideal (more below).

Are you sure about that relationship (V & I)? I mean different Lipos, different characteristics.
Test-3 had higher <C>, and therefore Volts wouldnít suffer (drop) as much as Test-2 Lipo, though supplying the same current. Correct me if Iím wrong.

I too am convinced that something is not quite right here: I think itís the <C> claims, but . . . (hereís the below bit, mentioned above!) . .

. . itís very hard doing these tests alone: I canít watch the powermeter, notice and write down values, operate the throttle, measure RPM, hold the plane, answer the phone and drink my beer all at once . You know what I mean! I did this at home, with no available assistants at the time.

Tests were short (>15 seconds), all with same load, and all lipos are fully charged to start. This was no endurance test.

As I was having problems with the RPM-measuring-gadget (not enough hands & feet), elected to leave that out, but wouldíve been good data to have. Though I did notice Volts/Amps/Watts in real time, when it came to writing-down time, I had to go for the values retained within the meter, (rounded to nearest 0.5-number): Amin, Vmax, Pmax.

I realise that this is not going to provide the most accurate results, but tíwas the best I could manage.
It wasnít my intention to go into this especially deeply, but got curious when the Kokam test disappointed, and then moreso after test-4.

So, the values were the extremes recorded and not averages, but the method used is consistent.
I will repeat the tests, but with a helper, and hopefully RPM too! (when I get time that is!).

Thanks for your suggestions so far.
Cheers, Bob
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Old Mar 29, 2010, 03:19 PM
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Realistic <C> ratings???

Would still like to know:
Is it unreasonable to expect a 1500 20C to deliver 20A whilst retaining a reasonable voltage?

And does anyone have data showing that a <C> rating is (almost) reliable?

All suggestions appreciated!
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Old Mar 29, 2010, 03:28 PM
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In my first response ,post#2 I provide a link to many test and the second link shows a 25C rate 2200 at 50 amps which is 50/2.2=22.7C . The volts and capcity delivered are on the graphs.

Charles
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Old Mar 29, 2010, 03:49 PM
flyit, crashit, fixit.
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Yes Charles, thanks again.

I did spend time on those links you posted, but quite honestly, I couldnít see the wood for the trees.
So many links to go through, and I ainít getting any younger .

Iím just hoping that you, or someone, will have recent data and opinions specifically pertaining to my preferred pack size. IĎm just lazy I guess, but I call it optimistic.
Cheers
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Old Mar 29, 2010, 04:07 PM
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Quote:
Is it unreasonable to expect a 1500 20C to deliver 20A whilst retaining a reasonable voltage?
20 amps / 1.5Ah =13.33 C which is 66.66% of the claimed C rate so yes it should do it. The thing is there is a lot of difference in LiPoly quality and their ability do deliver anywhere near the claimed C rate.
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