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Old Mar 17, 2012, 12:01 AM
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Wrong Zac, the higher the capacity (i.e. a 6500 mAh pack vs your 4400mAh) pack the lower the IR should be. A small lipo like a 800mAh pack will be much higher then a 5000mAh pack. Cell count has nothing to do with IR. Also, like mentioned, temperature, brand, charge all come into account.
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Old Mar 17, 2012, 12:20 AM
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Wrong Zac, the higher the capacity (i.e. a 6500 mAh pack vs your 4400mAh) pack the lower the IR should be. A small lipo like a 800mAh pack will be much higher then a 5000mAh pack. Cell count has nothing to do with IR. Also, like mentioned, temperature, brand, charge all come into account.
So on average?? What is typical IR for 4400 4S pack?? Is there any reference that companies have? Is there a chart? Trying to find info...Do I use the IR formula??
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Old Mar 17, 2012, 03:18 AM
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http://www.jj604.com/LiPoTool/

IR Calculator
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Old Mar 17, 2012, 03:48 AM
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Use what vlad has posted to get an estimate of where you should be... roughly 2mOhms per cell for a 26C rating on your 4400mah pack, 1 mOhm per cell equates into a 37C rated battery. Or divide by total mOhms if your charger doesn't do individual cells and take the average.
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Old Mar 17, 2012, 03:55 AM
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Zac,

If you want a typical value, go to the calculator and you will find that a cell IR of 2.7milliohms at 22degC for a 4400mah pack will result in an FOM of 1. FOM of 1 indicates a good Lipo, so the answer to your question is that a good average 4S4400 pack would be (4 x 2.7) + resistance of leads and connector, say 4-5 milliohms. This gives a total of about 15 milliohms. Likely range is 12 - 25 milliohms, but beware that temperature is a big factor.

Wayne
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Old Mar 23, 2012, 05:15 AM
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Is the simple LiPo performance tool available for use offline please? I can see this would be very useful at the field when used in conjunction with Wayne's meter for the many of us that are big on electric. Thank you.
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Old Mar 23, 2012, 05:37 AM
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Originally Posted by Andy from Sandy View Post
Is the simple LiPo performance tool available for use offline please? I can see this would be very useful at the field when used in conjunction with Wayne's meter for the many of us that are big on electric. Thank you.
Andy,

I'm not very good on software etc, but my understanding is that Mark's calculator, which is an Exel file can be downloaded onto your smartphone so that you can use it anywhere. So if you have an ESR meter and a 'phone with you, you can instantly calculate a safe max current to take from your lipo at that particular temperature
This also means that in cold conditions, the calculated safe max current will be lower which is a warning to avoid 'winter failure' problems. Careful throttle control early on in the flight is the obvious answer.
Vlad gave the link above from John Julian's calculator which I think works on almost any system (http://www.jj604.com/LiPoTool/)

Wayne
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Old Mar 23, 2012, 05:47 AM
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Thank you Wayne.

I have now got the calculator running standalone on my PC. I carry a small netbook to the field so I can install it on that.
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Old Mar 23, 2012, 09:03 PM
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Andy, the calculator is just a simple Excel spreadsheet. You can use it on anything that will run Excel.

For anyone wanting the stand-alone version, The speadsheet is available for download as a zip file on Mark Forsyth's thread here.

http://www.rcgroups.com/forums/showp...69&postcount=1

My version is just a web conversion of the speadsheet. It has the advantage it will run on any browser on a range of hardware without Excel, but of course you need an internet connection.

http://www.jj604.com/LiPoTool/

It works just fine at our field on my iPhone using the mobile phone connection. Data traffic is very small.

John

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Thank you Wayne.

I have now got the calculator running standalone on my PC. I carry a small netbook to the field so I can install it on that.
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Old Mar 24, 2012, 07:50 PM
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Thanks John. I have one of Wayne's meters on order and in conjunction with your spreasheet this is going to be invaluable for testing LiPos.

If I have a motor / ESC combo that requires 50 Amps for example but the IR of the pack wouldn't allow that much current to flow what is the effect on the LiPo?
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Old Mar 24, 2012, 08:19 PM
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Originally Posted by Andy from Sandy View Post
If I have a motor / ESC combo that requires 50 Amps for example but the IR of the pack wouldn't allow that much current to flow what is the effect on the LiPo?
Current draw is determined by motor, prop, and battery voltage. If the motor/prop combination is trying to draw current in excess of the battery's ability to safely source that amount of current, the net result will be overheating and early demise of the battery.

To use the spreadsheet, plug in the highest measured cell value and it will calculate the maximum safe continuous current for your pack. Next, measure current draw on your power system at full throttle with the same pack to ensure that it is no greater than that recommended by the formula. As long as you're in the ballpark you'll be ok. If actual current draw is far in excess of that recommended by the formula, it's wisest to get a better battery or use a smaller prop.

Mark
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Old Mar 24, 2012, 08:20 PM
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Originally Posted by Andy from Sandy View Post
Thanks John. I have one of Wayne's meters on order and in conjunction with your spreasheet this is going to be invaluable for testing LiPos.

If I have a motor / ESC combo that requires 50 Amps for example but the IR of the pack wouldn't allow that much current to flow what is the effect on the LiPo?
Any idea where I can purchase one of these meters??
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Old Mar 24, 2012, 08:22 PM
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US distributor for Wayne's awesome little meter is David at ProgressiveRC: http://www.progressiverc.com/esr-meter.html

You can also PM Wayne if David happens to be out of inventory.

Mark
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Old Mar 25, 2012, 02:04 AM
ancora imparo
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QR code for LiPoTool

If you have a smart phone that reads QR codes, then you don't need to even enter the URL of the LiPoTool.

Just read it from here - or print it out and stick it on your battery box.

John
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Old Mar 25, 2012, 12:24 PM
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Quote:
the net result will be overheating and early demise of the battery
When the pack was new in my example it used to be able to supply the 50Amps but as time has gone by the IR has gone up. Does it act to limit the current flow as if the IR were a resistor in series with the ESC?
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