# LiFePO4 Charge Curve Question



## bdmiko (Jul 17, 2013)

Does this look like a regular charge curve for a LiFePO4 battery? I have a set of 3 year old batteries that have never been used. Using a Powerlab 6 I am cycling the batteries and get the following charge curve:









Before charging the batteries are at 3.28 volts. The Powerlab is set to charge to 3.6 volts @30 amps. When charging starts the voltage quickly jumps up to 3.5+ volts for a period of time, then slowly drops for about 45 minutes and then slowly goes back up until the battery reaches 3.6 volts.

This isn't what I was expecting based on other charge curves I have seen and I am wondering if this is normal behavior.


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## skooler (Mar 26, 2011)

Strange..... 3.28 volts is virtually 'full' might be worth cycling themnthe other way and see if that shows differently.

What are the batteries?


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## bdmiko (Jul 17, 2013)

On the next battery I'll try discharging first. I haven't tried discharging first and then charging, but the batteries are definitely not full. They are 60Ah batteries and the first charge takes about 45Ah. On average during discharge they are putting out just over 60Ah.

One other unexpected thing I noticed is that the batteries have a memory effect. As soon as the batteries are fully charged to 3.6 volts, if I remove the charger the batteries will drift down to 3.3 volts. The same thing happens after discharging to 2.7 volts. The batteries will drift up to 3.1 volts or higher.


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## skooler (Mar 26, 2011)

What you describe as memory effect is not a memory effect (that's a completely different thing).

What you are experiencing is the battery 'recovering'.

On discharge, the internal resistance causes a certain amount of voltage 'sag' - when the load is removed the voltage recovers.

On charge, have a google for 'open circuit voltage'. for LiFePO4 this is about 3.34v - the cell will drop back to about this figure when the charger is removed and you show a tiny load to the cell.


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## Sunking (Aug 10, 2009)

bdmiko said:


> if I remove the charger the batteries will drift down to 3.3 volts. The same thing happens after discharging to 2.7 volts. The batteries will drift up to 3.1 volts or higher.


That is not memory, it is normal and a result of the batteries Internal Resistance. You cannot determine state of charge on a battery that is under charge or discharge. Only at rest after they have been rested for a few hours. 

A 60 AH cell in new condition is going to have an Ri of roughly 1.2 milli-ohms. If the resting voltage is say 3.4 volts and you apply a 2C discharge rate of 120 amps the battery voltage will drop to 3.25 volts. Remove the quickly and the voltage will jump up and recover to 3.4 volts.

Same on charge if the battery is at 3 volts and you apply say 60 amps the voltage only goes up to 3.1 volts regardless what you have the voltage set too. A fully charge LFP battery at rest is 3.4 volts. You can apply 3.6 volts all day long and the resting voltage will not go up.

FWIW the curve you have shown is not a charge curve, it is a discharge curve. Charge voltage goes up with time, not down.


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## bdmiko (Jul 17, 2013)

Sunking said:


> FWIW the curve you have shown is not a charge curve, it is a discharge curve. Charge voltage goes up with time, not down.


That is the thing I don't understand. This is a charge and discharge curve. The first 1.5 hours is the charge @ 30Amps and the rest of the curve is a discharge at 8 amps.

So during the charge, the voltage actually drops and then goes back up.


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## bdmiko (Jul 17, 2013)

skooler said:


> On charge, have a google for 'open circuit voltage'. for LiFePO4 this is about 3.34v - the cell will drop back to about this figure when the charger is removed and you show a tiny load to the cell.


Thank you. I did not realize this for LiFePO4 batteries.


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## Coulomb (Apr 22, 2009)

bdmiko said:


> Thank you. I did not realize this for LiFePO4 batteries.


Lead acid, if in poor condition, can do the same thing. After 10 minutes or so of charging, the internal resistance goes down (something about the acid getting less dilute, I think). The lower internal resistance means the charger won't raise the voltage as much for the same current, so the terminal voltage goes down for a while.


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## Sunking (Aug 10, 2009)

bdmiko said:


> That is the thing I don't understand. This is a charge and discharge curve. The first 1.5 hours is the charge @ 30Amps and the rest of the curve is a discharge at 8 amps.
> 
> So during the charge, the voltage actually drops and then goes back up.


OK that helps. But begs the question as to why you charged a charged battery to start with? You started at 3.6 volts which is fully charged. The last half from 45 minutes to roughly 1.5 hours looks normal. 

I use a ICharger so not real familiar with PowerLabs, but they are roughly the same and mine would NOT allow me to charge a fully charged battery. Is it possible you had it programmed to cycle charge automatically? 

What I see is a Discharge for first 45 minutes, followed by a charge for 45 minutes, followed by a 8 an hour discharge. Only possible explanation I can think of is you had a COLD battery when you started like from the fridge cold. *Was it COLD when you started? * If it was cold would explain it as the Ri would have been very high and lowered when it warmed up with charge current flowing. But 3.6 volts screams fully charged. 

If it were me I would rule the test invalid, and start over with a Fully Discharged Cell followed by a C/3 charge (20 amps), allowed to rest an hour or two, then followed by a discharge. Except this time make dang sure you have programmed the charger correctly and do not run it automated to cycle automatically. 

A simple discharge a disregard how many Amp Hours, let it rest.
A Full charge counting Amp Hour in, and let it rest.
A discharge counting Amp Hours.

Additionally measure Ri at Full Charge and Discharge. At Full Charge depending on battery manufacture model should be around 1 to 1.2 milli-ohms, and discharged slightly lower if LFP. Pull the specs and find Ri to compare as it is the best indicator of battery health. Just make sure to do the test at room temps. You have to let the battery rest to cool to room temps. 

When you are done from Amp Hours discharged and Ri is going to tell you a lot. If within specs you are good to go, and will also give you a base line. When Ri starts to rise you know the end is near.


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## bdmiko (Jul 17, 2013)

Sunking said:


> What I see is a Discharge for first 45 minutes, followed by a charge for 45 minutes, followed by a 8 an hour discharge. Only possible explanation I can think of is you had a COLD battery when you started like from the fridge cold. *Was it COLD when you started? *


The batteries at rest were at 3.28 V so I figured I would charge before discharging. The first 90 minutes the battery is definitely charging, even as the voltage drops.

The batteries have been sitting in my basement for the last year, right around 70 degrees so they are not cold.

I took a couple more screen grabs of the battery that is on the charger right now to show Charging Amps, Volts, and Resistance. From the graphs you can see that the battery is charging at 30amps even as the voltage is dropping.

Amps









Volts









Resistance


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## GerhardRP (Nov 17, 2009)

bdmiko said:


> Resistance


Do you know how the system calculates internal resistance?


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## Sunking (Aug 10, 2009)

OK the Ri answers your question. Take notice the Ri decreases from time 0 to .6 hours when the voltage bottoms out. What bothers me is LFP usually does the exact opposite. Ri is lowest at low SOC and rises slightly with SOC.

Only LFP I know that does not do that is Nano LFP like those made by A123.


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## bdmiko (Jul 17, 2013)

I do not know the chemistry of these batteries other than they are LiFePO4. The next cell that I cycle, I will do a discharge first as both Sunking and Skooler suggested.


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## GerhardRP (Nov 17, 2009)

Sunking said:


> OK the Ri answers your question. Take notice the Ri decreases from time 0 to .6 hours when the voltage bottoms out. What bothers me is LFP usually does the exact opposite. Ri is lowest at low SOC and rises slightly with SOC.
> 
> Only LFP I know that does not do that is Nano LFP like those made by A123.


My guess is that this is a quirk due to the fact that the cells had been resting unused for a year before this test. A second cycle probably will be normal.


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## IamIan (Mar 29, 2009)

bdmiko said:


> So during the charge, the voltage actually drops and then goes back up.


Some brands of LiFePO4 have shown to have decreasing Ohms with higher SoC .. and with higher temperatures ... thus as the SoC increased the Ohms can go down which would cause the V=IR effect to go down with it on the terminal voltage .. and as the cell warmed the Ohms also went down .. thus also decreasing the V=IR effect on terminal voltage.

See Attached

- - - - - - - 

All of that having been said... it might also be helpful to keep in mind how the PL6 and PL8 function .. This can might also be an indication of some calibration needing corrected in the software.

The PL6 and PL8 have an internal 'adjustment' in the software to compensate for IR drop across the length of the connected terminal wire to PL6/PL8 ... if that is mis-calibrated it might be showing up as an incorrectly high 'adjustment' under higher Amp rates... and not show up as much (if at all) under lower amp rates.

The PL6 and PL8 resistance measurement is done via the (hz) of the current ripple/fluctuation .. and an internal algorithm to interpret it into the mOhms it displays .. the intentional current ripple is not usually identifiable on the graphs because it happens very fast (up to 125khz)... again if that algorithm that interperts it is miscalibrated it might output weird data.


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## bdmiko (Jul 17, 2013)

IamIan said:


> and as the cell warmed the Ohms also went down .. thus also decreasing the V=IR effect on terminal voltage.


During charging the cells are not warm to the touch, however the charge is C/2 which is the recommended limited for charging.



IamIan said:


> All of that having been said... it might also be helpful to keep in mind how the PL6 and PL8 function .. This can might also be an indication of some calibration needing corrected in the software.
> 
> The PL6 and PL8 have an internal 'adjustment' in the software to compensate for IR drop across the length of the connected terminal wire to PL6/PL8 ... if that is mis-calibrated it might be showing up as an incorrectly high 'adjustment' under higher Amp rates... and not show up as much (if at all) under lower amp rates.
> 
> The PL6 and PL8 resistance measurement is done via the (hz) of the current ripple/fluctuation .. and an internal algorithm to interpret it into the mOhms it displays .. the intentional current ripple is not usually identifiable on the graphs because it happens very fast (up to 125khz)... again if that algorithm that interperts it is miscalibrated it might output weird data.


Is the calculation done to compute the voltage at the battery or the output amps of the PL6? 

In my configuration the balance port is used during charging to monitor the voltage. Just to double check that the reported voltage was correct, today while charging a battery, I checked the voltage with a voltmeter and it matched the voltage that the PL 6 reported.


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## IamIan (Mar 29, 2009)

bdmiko said:


> During charging the cells are not warm to the touch, however the charge is C/2 which is the recommended limited for charging.


Were they cold to the touch ? ... The largest -dOhm/dTemp is shown at lower temperatures... higher 'warm' temperatures see smaller -dOhm/dTemp.



bdmiko said:


> Is the calculation done to compute the voltage at the battery or the output amps of the PL6?


The calibration for the unit and the internal algorithms it uses are co-dependent ... it can effect the PL indicated/measured Voltage (battery or supply), Amps (Battery or supply) and thus Ah , Ohms (battery).

Any significant mis-calibration should be a rare event .. and it is something that they usually fix/adjust in the software... but it might require an administrative / developer log in (depending on specifics)



bdmiko said:


> In my configuration the balance port is used during charging to monitor the voltage. Just to double check that the reported voltage was correct, today while charging a battery, I checked the voltage with a voltmeter and it matched the voltage that the PL 6 reported.


The double check would make it pretty unlikely for both devices to be off in the same direction to the same amount ... You could do a similar double check of the amps and operating khz as well.

But .. I suspect , that if you continue to see this effect .. it is most likely due to an initially decreasing internal Ohms (same thing Sunking pointed out above)... which some types of LiFePO4 do .. like lower temps .. this for the types that do this they see more -dOhm/dSoC at the lower SoC points .. and a smaller -dOhms/dSoC at higher SoC points.

It is a bit unusual to see the magnitude of the effect you saw .. might be interesting to see if it persists or not in future cycles (like GerhardRP posted above).


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## Sunking (Aug 10, 2009)

bdmiko said:


> Is the calculation done to compute the voltage at the battery or the output amps of the PL6?


C/2 is the maximum recommended rate the manufacture is comfortable with to prevent over heating issues. That does not mean you should charge at C/2.

The actual voltage on the battery terminal is not important. In fact you battery terminal voltage is irrelevant and will NOT EQUAL the charger voltage until current stops flowing or reduced enough.

In a nutshell most all lithium chargers, even your P8 is a CV charger with adjustable current limit. When you connect battery at say 3 volts and current limit is set to say 10 amps, the chargers voltage is going to fold back to match the battery IR charge curve. The charger goes into current limit which essentially is a CC mode even though the charger is CV.

As the battery charges its voltage begins to rise along with the chargers voltage until the charger reaches its voltage set point of say 3.5 volts. At that point the current is going to start to Taper off as the battery voltage begins to equalize with the chargers voltage. You terminate the charge when current tapers off to say 3 to 5% of C.

So as you can see the voltage is only relevant at the chargers terminals, not the battery. It is current that determines the termination of charge. 

No where is one tidbit you need to know about Hobby Chargers programming. It assumes (default) you are charging at 1C, and termination is based on 3 to 5% of C. So if you set the current to 30 amps it assumes you have a 30 AH cell and will terminate charge when current tapers down to 1 amp. However you can change that to terminate at 1.8 to 3 amps on a 60 AH cell.


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## BVH (Jul 4, 2014)

The FMA PL* chargers have a battery capacity input field under the "fuel" tab. This info, when input by the user, (default is "0") is used as a factor in calculating termination according to the FMA techs. 89+ Amp hours is the max user input.


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## bdmiko (Jul 17, 2013)

As suggested by several posters I ran a discharge first then a charge. Ran the battery down to 2.7 volts then did a full charge. The charge curve is a more normal charge curve now. The charge and partial discharge cycle graph is below:










It took about 20Ah to drain the battery to 2.7 volts so the batteries were either not stored fully charged or they lost some charge during the last 3 years.


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## bdmiko (Jul 17, 2013)

IamIan said:


> Were they cold to the touch ? ... The largest -dOhm/dTemp is shown at lower temperatures... higher 'warm' temperatures see smaller -dOhm/dTemp.


Out of curiosity I put 2 batteries side-by-side, one that was being charged and one that was not being used. Using an IR thermometer I measured the temperature of the batteries. During charging the battery being charged was about 3 degrees F warmer than the unused battery.



Sunking said:


> C/2 is the maximum recommended rate the manufacture is comfortable with to prevent over heating issues. That does not mean you should charge at C/2.
> 
> The actual voltage on the battery terminal is not important. In fact you battery terminal voltage is irrelevant and will NOT EQUAL the charger voltage until current stops flowing or reduced enough.
> 
> ...


Thank you for the information on the chargers. Your description on how they work makes a lot of sense. I did an initial setup of the PL6 by tweaking an existing preset but I think it may be worthwhile going through the settings again.


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## pm_dawn (Sep 14, 2009)

Sunking said:


> The actual voltage on the battery terminal is not important. In fact you battery terminal voltage is irrelevant and will NOT EQUAL the charger voltage until current stops flowing or reduced enough.
> 
> So as you can see the voltage is only relevant at the chargers terminals, not the battery. It is current that determines the termination of charge.



The voltage of the battery terminal is hugely important if you want your IR-readings to be accurate. Otherwise the R of the Charging cables will be in that mix. 

Regards
/Per


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## Sunking (Aug 10, 2009)

pm_dawn said:


> The voltage of the battery terminal is hugely important if you want your IR-readings to be accurate.


How is voltage important when in CC mode. If you have the voltage set to 3.6 volts and the charger is in constant current, at no point is there 3.6 volts anywhere to be seen. All you need to know is what the current is doing. When it begins to taper off, then you will see 3.6 volts and can terminate the charge.


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## pm_dawn (Sep 14, 2009)

@Sunking:

Was it really that hard to understand ?

If you want the IR readings from the RC-Charger to make any sense you pretty much need to have sense leads to the battery terminals, to make sure you measure the correct voltage over the battery terminals when doing the IR testing. Otherwise resistance of the charging cables will be presented also in the IR reading.

It might be that the Power Lab chargers does not show any IR unless sense leads are connected to the balance port, but the iCharger does have the ability to do IR calc both with and without the sense leads.

I use good connectors and also some heavy cables (16mm2) when charging with my iCharger 4010 duo. It still shows a bit of difference between the IR that includes the cables and the value that shows only the cell.


My objection to the way you described the need for sense leads was to highlight the fact that there are measurements done that makes them needed.

also to be picky: you can actually keep a higher charge current longer if you have the sense leads, that way you remove the charging cable voltage drop from the CV voltage, allowing the charger to have a higher than CV voltage until the battery shows the set CV voltage.....


But to the topic of the thread, that is for sure a strange charge curve.
I can see if I can get some curves from my old Thundersky cells that has been sleeping fot a while...

Regards
/Per


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## Sunking (Aug 10, 2009)

pm_dawn said:


> @Sunking:
> 
> Was it really that hard to understand ?


No I know what you are driving at. 



pm_dawn said:


> It might be that the Power Lab chargers does not show any IR unless sense leads are connected to the balance port, but the iCharger does have the ability to do IR calc both with and without the sense leads.
> 
> I use good connectors and also some heavy cables (16mm2) when charging with my iCharger 4010 duo. It still shows a bit of difference between the IR that includes the cables and the value that shows only the cell.


That is my point the error is so small it can be ignored.


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## pm_dawn (Sep 14, 2009)

The error is not that small that it can be ignored when it comes to the IR.

But if you don't care, thats fine for me.
I care !

Regards
/Per


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## Sunking (Aug 10, 2009)

pm_dawn said:


> The error is not that small that it can be ignored when it comes to the IR.
> 
> But if you don't care, thats fine for me.
> I care !
> ...


Not enough error to even be seen on a graph unless you make it the size of a wall. 5 loop feet of 16 mm2 cable =.0015 Ohms. And on a Icharger or Power Lab charger 5 loop feet is too long. More like 2 to 3 feet. 

You are right, I don't care about the error as it is meaningless. All I care about is the finishing current if I want to go to 100% SOC which I never would do. I just do not let the trees block my vision so I can see the Forrest behind the trees.


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## IamIan (Mar 29, 2009)

pm_dawn said:


> Otherwise resistance of the charging cables will be presented also in the IR reading.


The PL6 and PL8 can do that with the 'balance' leads being the 'sense' leads while the main wire carries the bulk of the current.

Both PL6 and PL8 also have software ability to account for the IR of the wire itself even if you are not using the 'balance' leads as described above .. the IR of the wire can be accounted for in the P6 or PL8 software... if you know what the wire IR is... which you could test and determine separately.

Also .. If you are only trying to make comparative (not absolute) measurements ... the IR of the wire won't really matter ... It just blends in ... and is the same offset for all cells you test with that same wire ... you will still be able to sort the cells tested in order of most to least cell IR ... even if you don't know how much the wire IR adds to all the tests... it's the same wire in all the tests.


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## pm_dawn (Sep 14, 2009)

@sunking:

I don't know what kind of batteries you have been playing with.
But in my case when dealing with large LiFePo4 cell of over 100ah, even 1,5mOhms of cable resistance is very significant, since the Cell itself can be in that range and also lower.

And if you look at the NanoTech-lipo packs they are even lower than that, even the small 5000mAh cells.


@Iamian
I know that i can sort and compare the cells even without doing four-wire.
And sometimes I do because I'm in a hurry and just need a rough idea of the capacity.


Kind Regards
/Per


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## Sunking (Aug 10, 2009)

pm_dawn said:


> @sunking:
> 
> I don't know what kind of batteries you have been playing with.
> But in my case when dealing with large LiFePo4 cell of over 100ah, even 1,5mOhms of cable resistance is very significant, since the Cell itself can be in that range and also lower.
> ...



Well I play with RC LiPo and have a 16S 100 AH LFP battery in my LSV.

I agree with you if you were charging a single LFP large format cell at 100 amps. A single CALB 100 AH cell Ri is roughly 1 milli-ohm. At 100 amp charge current would drop .15 volt error. It makes no real difference because once the battery voltage reaches the CV value of the charger, the current tapers, and you terminate when there is only 5 amps flowing. At that point the voltage error is so small it would take lab equipment to measure with any accuracy, and cannot be seen on a chart. 

However I only charge my 16S pack at 30 amps and with 16 cells is series the cable resistances is magnitude of 16 times less than the battery pack and I could care less about a .045 volt error when in CC mode. It goes away when I reach CV mode of 56.8 volts. That is a whopping .07% error. No test procedure or equipment is that accurate. All I care about is when the current tapers off to 5 amps @ 56.8 volts at the charger terminals. That I can measure with some degree of accuracy.

For an EV running say 45S or 144 volts at 100 AH and charging at 100 amps. I hope you use something larger than 16 mm2 cable but even if you did use 16 mm2 cable at 5 feet, the wire resistance in now on a magnitude of 40 times lower than the pack. The error is (.15 volts / [3.6 volts x 45 cells]) x 100 = .*09% error*. Your test equipment cannot measure that. You cannot see it on a graph no matter how large you blow it up. 

But hey all the power to you for trying. Even if you could, does not tell you anything.  What am I missing here?


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## pm_dawn (Sep 14, 2009)

I'm not in this to argue with you Sunking.
I think we both can agree on some things.

I totally see your point with the CV point and voltage drop not being significant when doing series strings and lower Amps. And I agree!

And you seem to agree with me on that when doing single large cells the R of the charging cables can be a significant part of the circuit. Especially when testing IR.

This thread in my view was about testing single cells and plotting chargecurves.
And then trying to compare that to a "normal" LFP curve.

To get the best possible data into a comparison with other curves I would use a 4-wire setup.

There was talk about IR very early in the thread, from that I suggested to use 4-wire setup to get the best results for IR measurements.

I know that I used a qoute from your post that was not about the IR measurement, and that was totally wrong by me. It kind of twisted the context a bit.

I will use 4-wire setup if I like as good accuracy as I can get for IR and capacity measurements. Otherwise I just go 2-wire.

Regards
/Per


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## Sunking (Aug 10, 2009)

pm_dawn said:


> I'm not in this to argue with you Sunking.
> I think we both can agree on some things.


Yes we agree on some things and I see your point for a single cell measurement. I just do not see any benefit when what I want to really know is the Amp Hours goes inza and comes outza.


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