# Internal Resistance. How High is to High



## major (Apr 4, 2008)

Sunking said:


> Recently there was a thread of a person with 3 dead AGM 12 volt batteries and looking for repair/replacement options and the subject of Internal Resistance was brought up. Consensus was to replace all the batteries which was sound advice.
> 
> A higher quality battery with a better warranty and *300% more cycle life* was suggested, but an objection was raised because the Internal Resistance was too high. So that brings up the question how much is too much?
> 
> ...


Hi Sun,

Thanks for taking this subject to a new thread. But I would like you to correct a misrepresentation. I compared the cycle life at the 80% DOD which was given for Brand A to the 80% DOD cycle for Brand B and see 400 vs 550. That is a long way from 300%. And I'd say about as important to the user as the difference in internal resistance. I guess it is up to him to choose a 5.1 kW reduction in peak power vs a 37.5% increase in cycle life.

Other considerations which I do not have the facts would be difference in mass, cost and warranty. Also, Brand A has a long history of EV use (with favorable results) and Brand B is an unknown in the EV application.

Should readers of this thread be interested, this started here: http://www.diyelectriccar.com/forums/showthread.php/agm-battery-issue-3-12-died-82510p3.html 

Regards,

major


----------



## Elithion (Oct 6, 2009)

Sunking said:


> Lithium batteries have much higher Internal Resistance.... than the AGM


Maybe.

Let me put that into real numbers.

(For those who wondered what the real world applications of Short Discharge Time could be, here is a great example.)

First:
For a battery of given voltage and capacity, its resistance is directly proportional to its Short Discharge Time (which is also the Short Discharge Time of the cells or batteries inside it). 


```
resistance [Ω] = short_discharge_time [s] * voltage [V] / capacity [Ah] / 3600
```
So, to compare the resistance of such a battery using various chemistries, all we need to do is compare the Short Discharge Time of its cells or batteries.

Second:
The Short Discharge Time of a small selection of cells or batteries (from this article), in order of better to worse:



High power Li-ion pouch cells: 20~40 s
VRLA: 65~86 s (AGM is one type of VRLA)
Typical Li-ion prismatic cell: 90~140 s
Conclusion:
As far as internal resistance goes, high power Li-ion pouch cells are better than VRLA batteries, standard prismatic Li-ion cells are worse (though, the best prismatic cell is only slightly worse than the worst VRLA battery).


----------



## Sunking (Aug 10, 2009)

major said:


> Hi Sun,
> 
> Thanks for taking this subject to a new thread.


You are welcome, but this is not directed at you. More for my benefit to get a better understanding of EV battery requirements. I have a lot of experience with Telco and Utility high capacity battery plants, and all the various chemistry and manufactures associated with it. In addition I have also gained quite a bit of experience in the last 10 years with off-grid solar systems so my exposure to lithium is minimum, as their is no real application for solar and lithium as of yet.




major said:


> But I would like you to correct a misrepresentation. I compared the cycle life at the 80% DOD which was given for Brand A to the 80% DOD cycle for Brand B and see 400 vs 550.


Major I am referring to 50% DOD as that is the absolute maximum one would ever take VRLA batteries down to. Even 50% is too much in my world as the cost of cycle life is extremely significant. I assume you are using 80% DOD to compare Lithium to Lead acid which IMO is not applicable. Sorry if that mislead you. 



major said:


> Also, Brand A has a long history of EV use (with favorable results) and Brand B is an unknown in the EV application.


That is where my experience differs from yours. I learned about Concorde AGM batteries through the solar world. Concorde has been around for a very long time. Like Rolls/Surrette is one of the highest quality batteries money can buy. They are mostly known in the Air/Space/Millitary industry meeting the extremely high standards and regulations of those industries, and have been around a long time and have been to the moon and back. 

As to specific Energy Density is real easy to compare. All we have to do is look at the specs of each battery to figure that out. FWIW we are talking about BCI Group 31 battery so dimensions are equal as that is a physical size standard.


----------



## Sunking (Aug 10, 2009)

Correct me if I am wrong here but it sounds like you are describing Peukert Law. 



Elithion said:


> Maybe.
> 
> Let me put that into real numbers.
> 
> ...


----------



## Elithion (Oct 6, 2009)

Hello Dereck ,



Sunking said:


> Correct me if I am wrong here but it sounds like you are describing Peukert Law.


No, not at all. For all practical purposes, Peukert doesn't apply to Li-ion: they're too "perfect", meaning that their charge efficiency is 100 %, unlike L.A.


----------



## major (Apr 4, 2008)

Sunking said:


> Major I am referring to 50% DOD as that is the absolute maximum one would ever take VRLA batteries down to. Even 50% is too much in my world as the cost of cycle life is extremely significant. I assume you are using 80% DOD to compare Lithium to Lead acid which IMO is not applicable. Sorry if that mislead you.


The only cycle life given for Brand A was for 80% DOD. Brand B spec which you supplied has a graph which shows the 80% DOD. So I compare the 80% figure. I suspect the percentage difference to be similar at 50% DOD, about 35% better for Brand B, NOT 300%. If you have evidence to support this 300% better cycle life at equivalent operation, please provide it. If not, please do not claim 300%.

Brand B spec claims 1000 cycles at 50%. Brand A was 400 cycles at 80%. Brand A would certainly be higher than 400 at 50%. Show me the math you use to get 300% better cycle life for Brand B at 50% DOD.

Sources are cited in the referenced thread.

The user referred to a 70% DOD in his application, so it is not unreasonable to use 80% for comparison. I agree it is important to consider life cycle cost. And the level of discharge is an important factor. There are also other factors to consider. But let's get the facts straight, please.


----------



## major (Apr 4, 2008)

Sunking said:


> FWIW we are talking about BCI Group 31 battery so dimensions are equal as that is a physical size standard.


Yes, size is the same per Group 31. Brand A is 77.8 pounds. What is the weight of Brand B?


----------



## Siwastaja (Aug 1, 2012)

For lead acid, internal resistance is only part of the losses, you have also to take the peukert factor (coulomb or Ah loss) in account. It has a completely different mechanism from internal resistance and yet both affect lead acid at the same time. 

Peukert effect is given in the battery datasheet in the form of how many ampere-hours you can get out of battery at different discharge rates (current or time). Recommended DOD% comes on the top of this!

For lithium, Peukert is always 1.00, so you always get full ampere-hours out. Just add the DOD% you want to maintain.

Anyway, no sensible person would choose lead acid today anyway, so this does not matter anymore. Given the fact that Peukert (and how it differs from internal resistance) always seemed too difficult to understand for surprisingly many people, this is a very good thing; lithium is extremely easy to understand.

Calculate your losses, efficiency % and voltage sag during full power by using R = UI and P = UI and you'll know if the internal resistance is too high. But bear in mind that practically no manufacturer report their cells internal DC resistance in their datasheets. You have to derive it from the discharge curves.


----------



## Sunking (Aug 10, 2009)

major said:


> Yes, size is the same per Group 31. Brand A is 77.8 pounds. What is the weight of Brand B?


Sun Extender PVX-1290T Spec
Internal Resistance = 3.19 milli-Ohms

Cannot find much in the way of spec sheet for the Odyssey 31-PC2150 except for this. About all you can gather is the Odyssey has less capacity and about 2 pounds heavier thus lower Specific Energy Density.


----------



## Sunking (Aug 10, 2009)

Elithion said:


> No, not at all. For all practical purposes, Peukert doesn't apply to Li-ion: they're too "perfect", meaning that their charge efficiency is 100 %, unlike L.A.


While Li-ion charge efficiency is good at 95 to 98%, but not really much better than AGM at 90 to 95%


----------



## major (Apr 4, 2008)

Sunking said:


> Sun Extender PVX-1290T Spec
> Internal Resistance = 3.19 milli-Ohms
> 
> Cannot find much in the way of spec sheet for the Odyssey 31-PC2150 except for this. About all you can gather is the Odyssey has less capacity and about 2 pounds heavier thus lower Specific Energy Density.


Yes, so these two batteries are pretty much the same, spec wise. A few % here or there, one way or the other.

Now, address your 300% better cycle life claim. And I never said the resistance was too high or raised an objection to it. I simply stated it was "higher". In fact, both these batteries have relatively low resistance and at that level for the user of a 500A controller, doesn't make a lot of difference, and I never said or implied otherwise.



major said:


> The Odyssey is better by spec having 5000A vs 4000A short circuit current meaning 20% lower resistance and 20% higher efficiency with less internal generated heat. The SunXtender claims higher cycle life of 550 vs 400 at 80% DOD.





Sunking said:


> A higher quality battery with a better warranty and *300% more cycle life* was suggested, but an *objection* was raised because the Internal Resistance was *too high*.


Show me 300% or edit your statement about it, please. Then get on with your thread


----------



## Elithion (Oct 6, 2009)

Sunking said:


> While Li-ion charge efficiency is good at 95 to 98%, but not really much better than AGM at 90 to 95%


You may be thinking of the _energy _efficiency of charging (measured in Wh / Wh.). Which is quite understandable.

But actually I was referring to the _charge _efficiency of charging Li-ion cells (the "Coulombic efficiency" or "Faraday efficiency" )(measured in Ah / Ah); the charge efficiency of Li-ion cells is indeed 100 %. Hence, for Li-ion, the Peukert is ~1.00.


----------



## Siwastaja (Aug 1, 2012)

Sunking said:


> While Li-ion charge efficiency is good at 95 to 98%, but not really much better than AGM at 90 to 95%


Do you know what charge efficiency means? If not, please educate yourself. If yes, please show some proof for this kind of groundbreaking claim. Quite a few people here have tested lithium cells and verified that same Ah goes in, same Ah goes out. This means 100% charge efficiency.

Efficiency is NOT charge efficiency. Total energy efficiency (or "efficiency" for short) means voltage sag and Ah loss combined. The former is Internal Resistance, the latter is Peukert effect.

And BTW, both charge efficiency and total energy efficiency vary depending on current.


----------



## Sunking (Aug 10, 2009)

Siwastaja said:


> Do you know what charge efficiency means? If not, please educate yourself.


I think so as I am a PE with 30 + years of experience working with battery plants. Charge-current efficiency is the ratio of the current used for electrochemical conversion of the active material to the total current supplied to the cell on recharge. There are many sources you can check but I prefere to use what I call the Battery Bible aka Handbook of Batteries published by McGraw Hill and written by David Linden and Thomas B Reddy. AGM batteries when constant voltage method is used at 2.2 vpc is nearly 99% efficient. 

Chapter 24 which covers VRLA in full. Look at the graphs on pages 24.37 and 24.38


----------



## Sunking (Aug 10, 2009)

Siwastaja said:


> And BTW, both charge efficiency and total energy efficiency vary depending on current.


You are preaching to the Choir.  Besides Charge Efficiency and Turn Around Efficiency is irrelevant to the discussion as it is about AGM. Not AGM vs LFP


----------



## major (Apr 4, 2008)

Sunking,

Where did you get the 300% figure?

edit: I see you did if fact edit your post #1 from 300% to "significant". Thankyou.


----------



## EVfun (Mar 14, 2010)

Elithion said:


> You may be thinking of the _energy _efficiency of charging (measured in Wh / Wh.). Which is quite understandable.
> 
> But actually I was referring to the _charge _efficiency of charging Li-ion cells (the "Coulombic efficiency" or "Faraday efficiency" )(measured in Ah / Ah); the charge efficiency of Li-ion cells is indeed 100 %. Hence, for Li-ion, the Peukert is ~1.00.


Uhm, Peukert's calculations have nothing to do with charging a lead acid battery. Try for yourself, no matter what rate you discharge a lead acid battery the amp hours needed to recharge it is what was removed plus about 5% to 10%. Lead acid batteries have noticeably less than 100% Coulombic efficiency, but it isn't affected by the discharge rate.


----------



## major (Apr 4, 2008)

Sunking said:


> If my calculations are correct a 144 volt, 100 AH LFP pack would be on the order of .2 ohms .....
> What say you?


Internal Resistance. How high is too high? Your example would be too high for most. The peak power from a 144V pack with 0.2 Ω internal resistance is 26 kW. So that would be about 26 hp to the wheels. This would be insufficient power for a passenger EVcar to drive at highway speeds reasonably well.

Battery internal resistance relates to the specific power or kW/kg. Most often batteries are a compromise with regards to power and energy, and also cost, life, etc. So it depends on the application. For the EV drag racer, internal resistance is evil and any is too much. For grocery getters running AGMs, 2 to 5 mΩ/12V doesn't make much difference.


----------



## Sunking (Aug 10, 2009)

major said:


> For the EV drag racer, internal resistance is evil and any is too much. For grocery getters running AGMs, 2 to 5 mΩ/12V doesn't make much difference.


That was the answer I was looking for I think. Would seem there would have to be an AH associated with a voltage. In this example we are talking about 12 volt AGM @ 100 AH. I could be wrong here but I think a lithium is higher than that for a 12 volt 100 AH pack. Well actually I guess it would be 14.4 volts


----------



## major (Apr 4, 2008)

My explanation pales in comparison to Davide's work. For a solid understanding study his article from post #3.


----------



## Sunking (Aug 10, 2009)

Elithion said:


> The Short Discharge Time of a small selection of cells or batteries (from this article), in order of better to worse:
> 
> 
> 
> ...


David thank for the link. I missed it the first time around until Major pointed it out for me. Humor me for a minute. I understand what the article is saying and do not have any real problems with it except for one thing.

I work in the telecom sector and we use a lot of conductance meters (aka MHO METER) made specifically for VRLA batteries as it is really the only way to judge the health of a VRLA battery. Flooded we natural use Hydrometers and thermometers. 

Back to the conductance meter because the article referenced to it. As the article points out it uses 1 Khz signal to measure the internal resistance of a cell or string of cells. I understand that is not DC resistance. It is the DC resistance plus reactance. So with that said I think a MHO meter can be used as a Go No-Go test. For example if I wanted to select a cell with say less than 5 milli-ohms and my MHO meter reads 6 milli-ohms I can rest assured the actual internal resistance is 5 milli-ohms of less. Do you concur?

FWIW in real life practice the MHO Meter readings are used as a baselline and the actual readings are of no importance. New batteries are fully charged and allowed to stabilize for a few days, and meter readings are recorded for a base line to be measured against in future readings. If the reading start going astray from base line it indicates a bad cell or something going on. In addition we use a DLRO meter to measure connector resistance. A DRLO is a 4-point ohm meter using a 10 amp current to measure very low resistances down to .1 micro-ohms accuracy.


----------



## Elithion (Oct 6, 2009)

Sunking said:


> It is the DC resistance plus reactance.


No.

The DC resistance of a cell has nothing to do with its impedance at 1 kHz.



Sunking said:


> For example if I wanted to select a cell with say less than 5 milli-ohms and my MHO meter reads 6 milli-ohms I can rest assured the actual internal resistance is 5 milli-ohms of less.


No.

Think of it this way. If you had a 1 Ohm resistor in parallel with a 1 F capacitor:


the DC resistance is 1 Ohm, as it the impedance
The 1 kHz impedance is 159 uOhm, and the reactance is 160 uOhm
Similarly, it is entirely possible that the AC impedance at 1 kHz is significantly less than the resistance at DC. Or the other way around. There is no telling.


----------



## Sunking (Aug 10, 2009)

Elithion said:


> The DC resistance of a cell has nothing to do with its impedance at 1 kHz.


Dave I do not believe that is correct as an AGM equivalent circuit is a series RC. All I am saying is the 1 Khz impedance will be greater than the internal resistance.


----------



## Elithion (Oct 6, 2009)

Sunking said:


> an AGM equivalent circuit is a series RC.


If it were, there would be 0 current at DC: a series capacitor cannot carry any DC current.

(If you cold please note that my name is "Davide".)


----------



## Sunking (Aug 10, 2009)

Elithion said:


> If it were, there would be 0 current at DC: a series capacitor cannot carry any DC current.
> 
> (If you cold please note that my name is "Davide".)


OK Davide please accept my apology. 

I strongly disagree because the Capacitor is the _Gas Tank_ of any battery. That is where the energy is stored and released. A picture is worth a thousand words.












*Rm* is the resistance of the metallic path through the cell including the terminals, electrodes and inter-connections.
*Ra* is the resistance of the electrochemical path including the electrolyte and the separator.
*Cb* is the capacitance of the parallel plates which form the electrodes of the cell. DC power source. At 1 Khz the impedance is insignificant, thus Ra and RM make up the majority of the total internal resistance. _This is where I derive the internal resistance is less than the 1 Khz impedance._

*Ri* is the non-linear contact resistance between the plate or electrode and the electrolyte, and is the source of self discharge.
The above is the basics of a Conductance Meter aka Mho-Meter operating at 1 Khz. With that said IMO the internal resistance will be something less than the impedance measured at 1 Khz. It has to be something less.

I agree with the article in that you can put a load on a battery to measure internal resistance and it will be more accurate. All I am saying is a Conductance Meter is a decent evaluation tool for a Go or No-Go test. Especially if you are testing a known battery to see if its performance has declined and needs replaced. Greater than 90% of all battery failures are from high resistance which cannot be repaired.


----------



## Elithion (Oct 6, 2009)

Source


----------



## Sunking (Aug 10, 2009)

Davide same difference. 1 Khz impdedance will still be higher than the actual DC resistance.


----------



## frodus (Apr 12, 2008)

Nope not quite.

Lets ignore Ra and Rm a minute and only look at Ri and Cb in your example.

Impedance of a Cap is dependant on Frequency and the it's Capacitance. It's impedance decreases with increasing frequency, causing the parallel equivalent to decrease as well.

Xc = 1/ _ω_C
Xc is the Impedance of the capacitor
_ω_ is frequency in radians
C is the capacitance
With a FIXED capacitance, as _ω_ increases, The impedance goes down.

Then we have 1/Ztot = 1/Ri + 1/(Xc)

Here's a nice simulator you can play with.
http://keisan.casio.com/has10/SpecExec.cgi

Enter 5 mOhm for R
Enter in 5 mF for C

Then try 0 and 1000 Hz for frequency, also try 10,000 Hz and 20,000 Hz.

With 0 Hz, The impedance is: 0.005 Ohms
With 1000 Hz, The impedance is: 0.0049394335096009 Ohms
With 10,000 Hz, The impedance is: 0.0026851463607316 Ohms
With 20,000 Hz, The impedance is: 0.0015165723552668 Ohms

So, DC impedance is higher than AC impedance.


----------



## Sunking (Aug 10, 2009)

frodus said:


> Nope not quite.
> 
> Lets ignore Ra and Rm a minute and only look at Ri and Cb in your example.
> 
> ...


Sorry to be blunt but, *Duh!* That is what I have been saying in every post. There is no such thing as DC impedance. 

Let me repeat. The Resistance (battery internal resistance) in a SERIES CIRCUIT; will always be lower than Impedance at a given frequency. Or put another way the; Impedance will be higher than Resistance at a given Frequency. Assuming the Frequency is greater than 0


----------



## frodus (Apr 12, 2008)

Sunking said:


> Sorry to be blunt but, *Duh!* That is what I have been saying in every post. There is no such thing as DC impedance.
> 
> Let me repeat. The Resistance (battery internal resistance) in a SERIES CIRCUIT; will always be lower than Impedance at a given frequency. Or put another way the; Impedance will be higher than Resistance at a given Frequency. Assuming the Frequency is greater than 0


No, you haven't been saying this in every post. You keep saying DC resistance will be lower than AC impedance. This is completely wrong. It may not be.

Reguarding terminology.... An impedance is an impedance. Just because you calculate the impedance of something in steady state (i.e. DC signal with no AC component) doesn't mean the result isn't an impedance. Impedance takes into account Resistance, Capacitance and Inductance. Without Frequency, Capacitance and Inductance drop out and you're left with Resistance.... but it is STILL an impedance technically.

Reguarding your example of an ideal battery, Your image shows a resistance (Ra and Rm) in series with a parallel (Ri and Cb). What I'm trying to tell you, is that the total impedance of Ri and Cb in parallel for a single cell goes down with Frequency, not UP.


----------



## PStechPaul (May 1, 2012)

I understand what you are saying, but I think the models are wrong (or at least misleading). The battery capacity does not act like a capacitor because its SOC can increase without much change in voltage, depending on chemistry. So the capacitance element might be better modeled with a two-way DC-DC converter and a capacitor, where the voltage at the input is held relatively constant while the output charges a capacitor at a current proportional to the charging current applied. In the reverse direction, it is a wide range DC-DC converter with a regulated output voltage that remains constant as the charge (and voltage) on the capacitor drops.

It may be possible to apply an AC signal to a battery (using AC coupling), which will nullify the voltage of the battery and essentially apply alternating charge and discharge current to the battery. Thus it will show an impedance based on the vector sum of the internal resistance and the effective series capacitance.

But I think it is much better to apply a DC load and measure the voltage change.


----------



## Siwastaja (Aug 1, 2012)

Sunking;

Think it simply as a parallel battery and a capacitor. The capacitor can supply more current for short peaks. When measured at 1 kHz, this "short peak" is about 1 millisecond. Small parasitic parallel capacitance within the cell can supply this peak, hence, _lowering_ the internal resistance.

It seems you are thinking about series capacitance, but that naturally cannot be there. It wouldn't pass DC. But it is quite natural that a battery -- which is mostly an electrochemical device and _cannot_ be modeled using a capacitor symbol -- has also _some_ parasitic capacitance due to construction of plates or layers of sheet. This capacitance increases peak current capability (i.e., decreases impedance), but it is not enough for most practical purposes; in an EV, the parallel capacitance should be high enough to supply higher current for several _seconds_ (e.g., acceleration), not milliseconds. 

Hence, measuring at 1 kHz makes sense only for marketing people who want to get good numbers.


----------



## Elithion (Oct 6, 2009)

Siwastaja said:


> measuring at 1 kHz makes sense only for marketing people who want to get good numbers.


It's not that sinister, actually.


1 kHz impedance is easy to measure with available equipment
An out of bounds 1 kHz impedance is a good indicator of a faulty cell
1 kHz impedance is relatively consistent, unlike DC resistance
There happens to be a dip in cell impedance at about 1 kHz
Cell manufacturers think in terms of Nyquist plots, in which DC doesn't appear directly
Some (most?) cell manufacturers truly do not understand the concept of DC resistance
Unfortunately, that resulted in a lot of confusion and miscommunication between the cell manufacturers and the users. A few of us are trying hard to rectify that.


----------



## Sunking (Aug 10, 2009)

frodus said:


> Reguarding your example of an ideal battery, Your image shows a resistance (Ra and Rm) in series with a parallel (Ri and Cb). What I'm trying to tell you, is that the total impedance of Ri and Cb in parallel for a single cell goes down with Frequency, not UP.


Frodus if you meant the impedance goes down as frequency goes up I agree with you. That is the bases of my point when I say the DC resistance is lower than the AC impedance

For a moment assign a capacitance of C. Something rediculiously small for say a 12 volt 100 AH battery. Pulling a number out of the air say 10 Farrads, with a test frequency of 1000 Hz. Ri is extremely high, say 1 Meg-Ohm. Ra + Rm = 5 milli-Ohm's. At 1000 Hz Z of capacitor = 32 micro-Ohms. So 5 milli-Ohms + 32 micro-Ohms = 5.032 milli-Ohms. The amount of impedance the capacitor added to the test is insignificant and we can conclude the internal resistance is something less than 5.032 milli-Ohms. If I were looking for a 12 volt 100 AH battery with 6 or less milli-Ohms this would pass Go test.

It is not like I am making this stuff up. This is the basis of how any commercial Battery Conductance Meter operates. The circuit I used is what is in the operators manual to describe the operation of the meter.


----------



## major (Apr 4, 2008)

Elithion said:


> The DC resistance of a cell has nothing to do with its impedance at 1 kHz.


I think Davide knows what he is talkin' bout 

I always test for battery or cell internal resistance using a DC load test typically at around 1C and 3C, usually at about 80 to 90% SOC. I don't think I have ever observed a measurement less than the specified 1000 Hz impedance figure over hundreds of samples.


----------



## frodus (Apr 12, 2008)

Sunking said:


> Frodus if you meant the* impedance goes down as frequency goes up* I agree with you. That is the bases of my point when I say the DC resistance is lower than the AC impedance


You contradicted yourself there bud.

My example showed a lower impedance at 1000hz (and at 10,000 and 20,000hz) than a purely DC measurement (i.e. 0hz), not the other way around.

The fact is, the impedance of a cell could be any number of things, higher, lower, the same.... depending on the frequency at which you measure the battery. 

Look at some of the nyquist plots Davide posted..... then read the papers they're associated. They're from PHD's with many times your knowledge on the subject.


It's not like we're pulling this stuff out of our ass.


----------



## Sunking (Aug 10, 2009)

major said:


> I think Davide knows what he is talkin' bout


Never meant to imply he doesn't. I am trying to figure out what I am missing.


----------



## frodus (Apr 12, 2008)

Maybe go check out some of those nyquist plots and read some of the articles that are related to them.... it was above my head too, but it's good reading material.

I think what you're missing is:

You expect things to work one way, but they work another.... we're trying to help you understand how they work in reality.


----------



## MemphisPapa (Feb 16, 2012)

Sunking said:


> That was the answer I was looking for I think. Would seem there would have to be an AH associated with a voltage. In this example we are talking about 12 volt AGM @ 100 AH. I could be wrong here but I think a lithium is higher than that for a 12 volt 100 AH pack. Well actually I guess it would be 14.4 volts


I too was curious about normal internal resistance. I bought some BestGo 160 Ah cells and they have a spec of <1 mOhm (at some temperature like 23C). Of the 32 cells I bought, four of them measure over 1 mOhm with one of them getting to 3 mOhm. They discharge and charge noticeably faster and sag badly under load. I asked BestGo to replace these four cells under their 3year waranty. The average cell internal resistance for the pack is 0.6 mOhm. I got all this info from my Orion BMS which allows you to set some thresholds based on a percentage of the average. So if I set 300% as the max resistance, if the average is 0.5 mOhm then a cell that measured >1.5 mOhm would create a trouble fault. I'm not sure if BestGo will send replacements, but my cells are new with only about 20 charge cycles to a DoD to about 60%. I sent them the Orion BMS data and am waiting for an answer.

UPDATE-I got word that BestGo will be replacing my four cells with high internal resistance with new cells. It will probably take a couple months to get the new cells.


----------

