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Old Apr 09, 2012, 10:46 AM
Ignint McNugget
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Maybe I missed it, but what is the eqn used to calculate "true C rating"?

Also, a slightly quicker route to the degrees symbol is ALT+248.
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Old Apr 09, 2012, 10:55 AM
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Additional theory and empirical data basis for the calculator is contained in the data thread. Specifically, post #3 has pertinent information:

http://www.rcgroups.com/forums/showp...85&postcount=3

Mark
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Old Apr 09, 2012, 12:24 PM
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Quote:
To evaluate the performance of a pack, I take it to the field and fly.
If I am purchasing a pack that alleges to be something it is not I have been ripped off. I now have a way of finding this out.

I don't fly hard and don't particularly over stress my lipos but I am finding some have started to puff. I now find that using these tools that the packs are not capable of meeting my requirements but they should have based on what the manufacturer states.

Similarly you may not be taking a pack near its rated limits and so for you they are perfect but it could be summerised like purchasing a car that should have a 2 litre engine only to find it in fact only has a 1.8. It seems to drive okay for you but you have still been ripped off.
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Old Apr 09, 2012, 01:14 PM
like a rock!
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>> I am finding some have started to puff ... not capable of meeting my requirements
Like it or not, this insight is a result of flight tests

Before you make the big leap from "not capable of meeting my requirements" to "been ripped off", you should have a very careful look at your setup and the history of those packs.

True, there are some rotten packs out there also by big names (which brand has all those single-cell-failures?)
But a lot of what's written on the web about problems with their "2 liter car that has been found by scientific methods to have only a 1.8 liter engine" comes from people who think the car vendor put it into 2nd gear, he knows best and you aren't supposed to change that...
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Old Apr 09, 2012, 02:08 PM
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Originally Posted by gravityKills View Post
>> I am finding some have started to puff ... not capable of meeting my requirements
Like it or not, this insight is a result of flight tests

Before you make the big leap from "not capable of meeting my requirements" to "been ripped off", you should have a very careful look at your setup and the history of those packs.

True, there are some rotten packs out there also by big names (which brand has all those single-cell-failures?)
But a lot of what's written on the web about problems with their "2 liter car that has been found by scientific methods to have only a 1.8 liter engine" comes from people who think the car vendor put it into 2nd gear, he knows best and you aren't supposed to change that...
those single cell failures wouldnt happen to be hyperion would it, I have had a bunch take a crap on me
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Old Apr 09, 2012, 02:35 PM
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Originally Posted by Andy from Sandy View Post
I don't fly hard and don't particularly over stress my lipos but I am finding some have started to puff. I now find that using these tools that the packs are not capable of meeting my requirements but they should have based on what the manufacturer states.
Whilst I find that IR is indeed a good indicator of inflight performance, I haven't managed to correlate puffing with IR or how hard the cells have been worked. In particular I have two batteries which are slightly puffed but the IR remains low, they fly well and come out of the model cool. Two others (different brand, same size and capacity) have higher IR, heat up in use but remain unpuffed.

I'm sure it's outside the scope of this tool to predict a 'puffing tendency', but I'm convinced that puffing is not just down to how the battery is used and feel it must be telling us something about the quality of the cells - or should we just not worry about it?
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Old Apr 09, 2012, 02:58 PM
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>> be hyperion would it
not in my experience, but I only own two packs per heli, one charges, one flies.
Those Hyperion packs I've killed died in balance

What can help is to strap the LiPos in tight, to hold the layers together and prevent puffing under load. It's a vicious cycle, as a puffed cell runs less efficiently, causes more heat, which causes more puffing (and common sense tells me, the harder I make them work, the better I should be prepared for a fire).
Typical causes under the control of the user are overcurrent (overloaded motor), and too high operating temperature.
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Old Apr 09, 2012, 03:00 PM
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I purchased a lipo with a stated C rating that should of been capable of supplying the current required for my application. After little use I find the pack has started to puff.

I then find rcgroups and see that people who have tested packs have seen a correlation between what a pack or cell is capable of and its IR or as Wayne calls it ESR. I have rightly or wrongly assumed based on their work that if my pack is to perform as stated it then should also have a certain IR. Like you though I have similar packs performing in a similar way. One thing I note though and that is just because a pack doesn't meet its marked c rating doesn't mean it will not fly your model as you want to fly it.

I have been able to log that although at the end of a flight a pack has not been over-discharged it also at some point was not capable of holding its voltage under load which I believe is related to its IR. Either that or it was a lie by the manufacturer that in fact I got a 15C pack and not a 25C pack.
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Old Apr 10, 2012, 11:19 AM
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So I am pretty interested on the IR state of my batteries and testing it with my tricopter performance. My icharge gave me about 10mohm. Since the spread sheet script is protected I looked around different posts for a linear approx between True C and max AMPs. I realize that this is a non-linear relationship. I then got the Specs of my eflite motors, park 450s, and the weight of my tricopter to figure its performance ie WATTs/LBS and what each arm was seeing. I them tested it on my battery I pulled a total of 375 watts with a max amp of 36.75. I calculated a c rating of 16.75. By the way this is a 2200 nano tech 3s 25-50c. I know temp plays big in the calculations. I back calculated my IR to be about 8.0 mohm. I placed that into my other equation and figured that I have about 25watts due to total IR^2 losses which sound about right for eff of 92%.

Although this has a FOM of 0.774 it still delivers enough performance to fly my tricopter very nicely. 147.33 Watts/LBs is near the range which could be used for unlimited 3d performace IAW eflite. Maybe C-rating is really quite lower then on the packs or my cals are justed totally hosed, but the program on this thread sees to be working better then a thumb rule. if you account for losses it may very well be spot on. In playing with the numbers an FOM of 0.4196 would still get me about 100 watts/lbs which will still fly a tricopter ok or can be graduated to flying parkflyers with less power demands.
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Old Apr 10, 2012, 05:15 PM
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I am looking for a charger for my batteries (the link below is the batteries I have)

http://www.hobbyking.com/hobbyking/s...dProduct=11911

I want it to be able to charge 2-3 batteries at a time both from a wall plug and a car outlet.

Any ideas???
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Old Apr 10, 2012, 05:19 PM
ancora imparo
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Striker you are in the wrong thread.
Try posting in this one.
http://www.rcgroups.com/forums/showt...356436&page=10
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Old Apr 11, 2012, 01:16 PM
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Originally Posted by MIDNCOCO View Post
Although this has a FOM of 0.774 it still delivers enough performance to fly my tricopter very nicely.
This is why I think the FOM as it stands is not very useful. It arbitrarily sets a good battery at 22C. It's not adjusted for what you actually need. Also, if I buy a 65C battery and the "real" C comes out to 25C, the FOM will rate it as a great battery just the same as a 25C rated battery with a "real" C of 25C. That doesn't make sense... at least in my brain.
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Old Apr 11, 2012, 01:31 PM
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Originally Posted by ToBeFrank View Post
This is why I think the FOM as it stands is not very useful. It arbitrarily sets a good battery at 22C. It's not adjusted for what you actually need. Also, if I buy a 65C battery and the "real" C comes out to 25C, the FOM will rate it as a great battery just the same as a 25C rated battery with a "real" C of 25C. That doesn't make sense... at least in my brain.
You bring up a good point, perhaps the reasoning behind 22C is so it gives a baseline for comparison across all batteries, if we know one battery has a FOM of 1, and another of 2, then doesn't that accomplish what we are looking for? The better battery? If FOM of 1 = 22C discharge, then FOM of 2 should be 44C, and so on... realistically a 65C pack should be around 3 on the FOM score but we know that companies label packs and use far different ranges for how they come up with C ratings... By using a FOM your looking at what the battery is really capable of.
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Old Apr 11, 2012, 01:36 PM
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Originally Posted by ToBeFrank View Post
This is why I think the FOM as it stands is not very useful. It's not adjusted for what you actually need.
FOM is simply a numerical depiction of capacity normalized internal resistance. It is not intended to be adjusted for what you need. This is for you to determine. I have some lipolys with FOM of ~.4 that work great in my trainer type planes. These same packs will be destroyed in my high performance planes.

The lesson here is that the end-user must understand the demands of his particular application and choose packs that will meet this application. IMO, FOM is an excellent tool in helping to make this determination.


Quote:
Originally Posted by ToBeFrank View Post
Also, if I buy a 65C battery and the "real" C comes out to 25C, the FOM will rate it as a great battery just the same as a 25C rated battery with a "real" C of 25C. That doesn't make sense... at least in my brain.
If two packs measure the same, they should be rated the same. What doesn't make sense is why one vendor would rate a battery at 65C and another vendor would rate a battery with identical measured performance at 25C. That's the whole idea behind the objective lipoly performance tool - to remove all of the garbage and hyperbole of the arbitrary 'C' rating system and level the playing field so the consumer can make informed decisions on purchasing lipolys and on how he can extract maximum long term performance from his investment.

Mark
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Old Apr 11, 2012, 02:52 PM
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I agree Mark. The whole point of the calculator and FOM is to tell the user the truth about the Lipo. The C rating written on the side of the pack is irrelevant as it has generally proven to be fiction.
Surely the user wants to know what the Lipo is really capable of - the calculator and FOM does that and the label does not.

ToBeFrank,
I wouldn't call a 25C Lipo a 'great' battery, but the genuine 25C pack is equal to the 65C marked pack in your example and surely that is what we want to know and makes sense.
This is the 'raison d'Ítre' for the calculator , and for the FOM.

Wayne
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