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Old May 22, 2013, 11:04 AM
Retired in NC
Rich in ILM's Avatar
USA, NC, Wilmington
Joined Sep 2010
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Proper way to measure internal resistance in a Lipo?

I wonder to inventory all my batteries and add IR to the measurements. Is there a specified Lipo temperature IR?
Also, when is it best to measure? At the end of the charge?
I found a lot on IR theory but not much on measuring techniques.

Thanks!
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Old May 22, 2013, 11:16 AM
Pro Jet Pilot
Cody f86saber's Avatar
Canada, NB
Joined Apr 2006
433 Posts
A fully charged Lithium pack will have a lower internal equivalent resistance value than an almost depleted one. The term 'depleted' is relative to the pack chemistry and production process, and will vary depending on these factors. The internal resistance also has an inverse thermal coefficient, meaning that a warm pack has a lower internal equivalent resistance than a cold pack, compared to most conductors which increase in resistance in proportion to thermal rise. My benchmark settled on 20 degrees Celsius at approximately 50% charge capacity, because this also provides median point that is reliable, and is easily replicated by other battery owners and testers. You can, however, test the IR at any temperature and any charge capacity that you wish after it has been left to stabilize for a few minutes. Just by picking a pack up in your hand, the thermal transfer to the pack will change the IR, so its all relative. Hope this helps!
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Old May 22, 2013, 11:22 AM
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Rich in ILM's Avatar
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Very helpful.

Thanks!
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Old May 22, 2013, 11:33 AM
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Cody f86saber's Avatar
Canada, NB
Joined Apr 2006
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No problem. As long as you can measure different packs at the same temperature and approximately similar charge status, you'll be presented with numbers that are relevant to their performance capability and current condition. Have fun!
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Old May 22, 2013, 12:08 PM
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hoppy's Avatar
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Quote:
Originally Posted by Rich in ILM View Post
I wonder to inventory all my batteries and add IR to the measurements. Is there a specified Lipo temperature IR?
Also, when is it best to measure? At the end of the charge?
I found a lot on IR theory but not much on measuring techniques.

Thanks!
Quick and dirty IR measurement method.
http://www.rcgroups.com/forums/showp...83&postcount=4

Values may not correspond to other meters but if you used to compare your batteries against each other it works well.
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Old Oct 25, 2013, 08:53 AM
Old_Robin
United Kingdom, England, Oxon
Joined Aug 2012
51 Posts
This is a profoundling interesting and informative way of SIMPLY monitoring the resistance of 11.1 V 3S lipos. I just love the simplicity of his rig using cheap and easy available 12V Bulbs...

LiPo INTERNAL RESISTANCE TESTING - a simple 12v load tester (17 min 37 sec)


Best regards __ Old_Robin
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Old Oct 25, 2013, 04:33 PM
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Stark Flyer's Avatar
Canal Fulton, Ohio
Joined Jun 2003
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Old_Robin, That is a really good video explanation.

I have virtually the same setup, difference is, I use 3 12volt car tail lights. As you stated it is very important that you do the test the same way each time. Being close to the same temperature is important.

Don't worry about trying to duplicate any manufactures' internal resistance numbers. They each have their own standard way of testing. One of them being at 1C and many use a variable load, pulsing it at 1,000 cycles per second.

I use the internal resistance number as a reference to tell the condition of the battery. I test my new battery packs and write the number and date on the pack. I then have a reference for later testing (for those that don't want to use a spreadsheet).

Sometimes you can salvage a multi cell pack by testing each cell's internal resistance. If one of them is high while the others are low then you have a chance of eliminating the bad cell and making it into a pack with one less cell (3S to 2S as an example). Or, instead of measuring each cell's internal resistance I just look at each cell's voltage while the load is applied. The individual cell with the lower voltage has the highest internal resistance and it is bad.

Again great video!!!

Dick
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Old Nov 09, 2013, 09:07 PM
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United States, CA, Grass Valley
Joined Sep 2013
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Thank you so much to Old_Robin.

Quote:
Originally Posted by Old_Robin View Post
This is a profoundling interesting and informative way of SIMPLY monitoring the resistance of 11.1 V 3S lipos. I just love the simplicity of his rig using cheap and easy available 12V Bulbs...

http://www.youtube.com/watch?v=usRE3gLFnOc

Best regards __ Old_Robin
I have seven 1 cell eflite lipos for my Nano. I'm having trouble today with very short flights. One, it was colder weather than I have been flying in. Two, I've been horrible about using my timer. Also, I'm practicing inverted flight, so I figured the timer wasn't going to tell me much. You know, ten to fifteen seconds of flight, crash, turn heli back on skids, start over.

All my voltages after charge are fine but I thought that wasn't the issue. Now that I know this Internal Resistance Test, I can judge the health of my little lipos.

From past experience I've got to say that running below the LVC with these little guys is really bad news. So now I'll probably be shelling out some cash for new lipos. Oh well! Such is helicopters.

Why did I run without timer? I guess I learn very slowly.

How would I judge how long to time something like inverted crash and start over flying anyhow?
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