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Old Nov 04, 2005, 04:43 AM
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Lipo cell internal resistance

Well, I am happy to say that this is the one area in RC Groups that really makes my brain smoke! Could this be why I took college physics all those years ago???

Anyway, is there some way to calculate the internal resistance for packs from individual cells? There doesn't seem to be any data from TP, PQ etc. on the specs. of their packs, or cells for that matter.

I was thinking about the internal resistance for use with the motor calc. programs like MotoCalc and Ecalc.

Norm
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Old Nov 04, 2005, 05:38 AM
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For practical use you can just measure the voltage at a couple of different current loads and calculate the effective resistance from that e.g. if you have a pack which gives 11.5V at 5A and goes down to 11V at 10A the resistance is

11.5 - 11 / 10 - 5 = 0.5/5 = 0.1 ohms

Since that will be a 3S lipo the resistance/cell will be 0.033

All the calc programs need is a value per cell to work out the reduction in voltage under a particular load.

Steve
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Old Nov 04, 2005, 06:25 AM
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More than you ever wanted to know about battery testing.
http://www.mpoweruk.com/testing.htm
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Old Nov 05, 2005, 12:31 AM
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http://www.rcgroups.com/forums/showp...4&postcount=39

Some published specs for Kokams.

I did some Ir testing a while back and consistantly came up with numbers higher than I expected. The Ir also varies depending on the load your applying. So testing at very lite loads gives different readings from testing at heavy loads. State of charge is a factor also as well as temps.

Its a moving target.

Larry
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Old Nov 05, 2005, 12:33 AM
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Oh yea - do a Google search on "measure battery internal resistance" you will get dozens of pages.

Larry
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Old Nov 05, 2005, 03:51 AM
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For Motocalc I simply take cell voltage at the mid-point of the discharge curve, subtract it from the nominal cell voltage, and divide by the discharge current to get internal resistance.

In reality resistance is somewhat non-linear, reducing as current increases. However, if the measurement is taken at a high current draw (ie. close to the cell's maximum continuous rating) error is minimised for most applications. Of more concern is that internal resistance is highly temperature dependent. Since Motocalc does not simulate this, the only way to account for it is to have separate entries for different temperatures.

I get measurements from manufacturers' graphs (if available) or independant tests. One site that has a lot of useful discharge graphs is Elektromodellflug.
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Old Nov 06, 2005, 09:03 AM
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do a search with the phrase battery impedance

I a different Ezone thread, there was a construction article on a dynamic battery impedance tester, originally from EDN magazine. I just finished mine last week. I have yet to start cataloging me cells. I modified the circuit so I could test both a A and 10 Amp pulses Seems to give reasonable results
EP
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Old Nov 06, 2005, 12:55 PM
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That brought up a few hundred hits, how about a direct link?
TIA
Jim
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Old Nov 07, 2005, 10:58 AM
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here's the link

http://www.rcgroups.com/forums/showt...ery+impedance+

I think that there is a simple error in the schematic.
I had to wire the op amp as a unit gain to make it work.
EP
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Old Nov 13, 2005, 09:01 AM
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huh, everyone's wrists broke ?

update:
I had a large pile of poorly performing LiIon packs.
I disassembled about 5 packs. Due to the PTC's low current limit in the LiIon cells, I have to use them at about 2C, so the packs are big. 3S4P of US18650 sized cells. I found that the good cells are around 0.13ohms, where as the trashed cells are anywhere up to about 0.3ohms. When cold the PTC's could be around 0.02~0.04 ohms or so, when I pulse the current at 1 amp, I should not see the impedance rise too much due to the PTC. However when I use the 10 amp pulsing, I can see the measured impedance rise within a couple seconds. The duty cycle of the pulsing is 50%. Now I have to rebuild the packs using just the good cells.
EP

PS technical comment. Don't worry about getting a low impedance fet. The op-amp control in the schematic doesn't take advantage of it, all the power drained out of the cell under test is just turned to heat. I used a surplus CPU heatsink for the FET's (2 of them) I also heatsunk the schottkey reverse voltage protection diode.
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Old May 27, 2014, 12:50 PM
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I found this thread whilst searching for advice on measuring battery IR. I also found this site (http://batteryuniversity.com/) and have found it to be an invaluable resource for just about anything you wish to know about batteries so I am posting it for others who happen upon this thread :0)
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Old May 27, 2014, 12:59 PM
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OHM's Law.. I E R .. build a known load get a watt meter and do the calculations yourself from one cell to how ever many you need just build it to handle whatever voltage you put in it..all known IR meters on the market to test IR in lipos say They do a estimate value not a true value..
what I described is a true value way to find out.. and dividing the total ohms by cell count dont tell you anything One cell could have 90% of the ohms you are reading MY WAY will test and your math can figure out True OHMs.. No guess work or estimated values..Yes the Truth hurts so .LOL
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Old May 28, 2014, 09:01 AM
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"Alls I know" is that cell IR values go up with pack use/abuse. When a pack is near it's end of full useful life, values will be up and quite uneven per cell.
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Old Jun 15, 2014, 01:13 PM
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Quote:
Originally Posted by E-Challenged View Post
"Alls I know" is that cell IR values go up with pack use/abuse. When a pack is near it's end of full useful life, values will be up and quite uneven per cell.
Is there a tester available or chargers that can accurately test this?

bob
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Old Jun 15, 2014, 01:22 PM
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Quote:
Originally Posted by steelgtr View Post
Is there a tester available or chargers that can accurately test this?

bob
Most mid-high end chargers have an IR measurement feature. iChargers and Powerlabs do for sure. There is also the Wayne Giles dedicated ir meter if you really want a dedicated device.
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