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Old 28-04-2017, 08:55   #5746
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by alctel View Post
Thanks for all the feedback guys.

So currently there is no charger/controller on the market that just has a straight switch on/off when it hits a certain voltage?

Note I'm not talking about the battery protection system with relays here, I'm just talking about the chargers so I never even hit the BPS voltages
Hi Alctel,

Check my post about the Genasun MPPT's we have here that drop to a float of 13.4(26.8). That is effectively off, at least until the batteries are loaded enough to pull them down a few % off of full charge. In testing of a recent installation of 2 x 150Ah x 12v (300Ah) on a small race boat, after a 15min absorption @ 14.2v, we wait around for 30min with lights on, wasn't enough load...and eventually had to turn on a 110V vacuum through an inverter (80+A) for a good minute or two to get the voltage down to 13.4.

However, wouldn't use these on any Li system that doesn't have a full BMS with balancing and HVC protection. Not that we ever work on Li systems without those features anyhow....
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Old 28-04-2017, 12:39   #5747
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Hi Alctel,

Check my post about the Genasun MPPT's we have here that drop to a float of 13.4(26.8). That is effectively off, at least until the batteries are loaded enough to pull them down a few % off of full charge. In testing of a recent installation of 2 x 150Ah x 12v (300Ah) on a small race boat, after a 15min absorption @ 14.2v, we wait around for 30min with lights on, wasn't enough load...and eventually had to turn on a 110V vacuum through an inverter (80+A) for a good minute or two to get the voltage down to 13.4.

However, wouldn't use these on any Li system that doesn't have a full BMS with balancing and HVC protection. Not that we ever work on Li systems without those features anyhow....
I have an controller for my existing panels that is customisation I will do something similar with

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Originally Posted by toddedger View Post
I had my GV-10 's custom made at 13.8. At a fraction C rate my bank doesn't have any trouble sucking in the power 13.8. When I requested that lower voltage the Genesun engineer called me up and said with that voltage my batteries would never fully charge. I told him yah that's the idea. I use a switch after each charger to be able disconnect manually if I want.
I think I am going to do this too, at 13.8 as well. My usage cycles should mean it's not going to be at 13.8 for very long, esp at night so should avoid damaging the cells?

I wish there was a marine charger that just shut itself off at a certain voltage

The controller for my side panels has to be right by my SSB tuner so I like the fact that the GV-10s are really quiet.
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Old 28-04-2017, 14:19   #5748
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

The difference between 13.8V or 14.0V is irrelevant when charging "fresh" cells if the battery doesn't get held at that voltage, but when they do lots of fractional cycles, the little bit of extra voltage helps with maintaining the system in good health.


The amount of time needed to go from 13.8V to 14.0V is very short, and it would be even shorter climbing to 14.2V afterwards so it is hard to claim that the extra voltage is going to cause "damage"...
It is just that cell balance starts getting a bit cranky past 14.0V.


Whether at 13.8V or 14.0V, unless the charging current is extremely low, they still benefit from being absorbed a little. The termination current for zero absorption is about C/30.


The problem with solar controllers is not properly overcome by "floating" at the battery resting voltage or lower (which indeed terminates charging), because the cycle restarts fresh again the next morning (or after some set amount of time) and the battery is unable to discharge.
The old LA strategy is still there: in doubt always try to charge to full.
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Old 28-04-2017, 14:36   #5749
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

If you're not hard-cycling, discharging right after your defined "full", then better not to fill them up in the first place.

When not being actively used, better to keep them down at 40-70% SoC, completely disconnected from charge sources.

Keeping them at 13.8V "full" for a few hours doesn't "damage" them, but doing it regularly, or higher voltages, but definitely takes off some unknown # of cycles off their lifetime.
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Old 28-04-2017, 14:47   #5750
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Properly programmed controllers will not go to a new full charge cycle every day, unless the bank voltage sags below the "reset" voltage level of the controller. Which of course varies from controller to controller....
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Old 28-04-2017, 20:09   #5751
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

You can try to program everything you want, the functionality needs to be there in the first place. Try setting the reset voltage on a GV-10.

john61ct, continuously cycling within a smallish range sounds good, but it just ends up causing grief with internal resistance over time. Now and then, you need to charge properly to avoid this, so either you need a strategy mixing the two, or simply cycle slowly and deeply and recharge properly each time without stressing anything.
Floating them at 13.6+ causes overcharging and then lithium plating and it doesn't wait a few hours to start. It is just that it is a slow process. The termination condition is current down to ~C/30, which is quite a bit of residual current by the way. Keep going and the current goes down to virtually zero very quickly and you start happily losing lithium to electrode plating.
If you are not using them, then the lower the SOC the better and 70% would be way high.
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Old 28-04-2017, 21:10   #5752
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by OceanSeaSpray View Post
You can try to program everything you want, the functionality needs to be there in the first place. Try setting the reset voltage on a GV-10.

john61ct, continuously cycling within a smallish range sounds good, but it just ends up causing grief with internal resistance over time. Now and then, you need to charge properly to avoid this, so either you need a strategy mixing the two, or simply cycle slowly and deeply and recharge properly each time without stressing anything.
Floating them at 13.6+ causes overcharging and then lithium plating and it doesn't wait a few hours to start. It is just that it is a slow process. The termination condition is current down to ~C/30, which is quite a bit of residual current by the way. Keep going and the current goes down to virtually zero very quickly and you start happily losing lithium to electrode plating.
If you are not using them, then the lower the SOC the better and 70% would be way high.
Interesting, what do you believe the cell voltage would be at 70% SOC? It isn't clearly definable looking at the manufacturers charts, even what voltage they consider to be fully charged, all the discharge charts are voltage under load, not rested voltage and the operating range of between 4.0V and 2.8v have a lot of factors that must be taken into consideration as neither of these voltages are rested voltage.

As for the other stuff, it would be interesting to see the charts/graphs that back this up, I certainly haven't seen it in any of my tests on systems with over 6 yr 24/7 cycling, both thousands of mini cycles and hundreds of deep cycles during that period. I have a lot of systems that I can draw data from, not just one or two so I would have thought I'd see these sort of symptoms occurring because the charging control method I use would put the cells in the danger zone you are putting forward.

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Old 28-04-2017, 21:37   #5753
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I was just pointing out that
Quote:
Originally Posted by OceanSeaSpray View Post
in doubt always try to charge to full.
only applies when they are cycling, not always.

And OK, when not in use 30-50% but not actually "lower the better" right?
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Old 29-04-2017, 03:40   #5754
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I was just pointing out that
"in doubt always try to charge to full."

only applies when they are cycling, not always.

And OK, when not in use 30-50% but not actually "lower the better" right?
"In doubt always try to charge to full" exclusively referred to the strategy used by LA chargers, which is why they are so problematic to embed in LFP systems. They really do that and it also provides maximum reserve capacity at any time, which is great. When you have to manage LFPs, you have a completely new dilemma on your hands between the available reserve capacity and extending battery life.

For storage, the lower the better, really. I remember a story about cells that were stored for 10 years at 20% SOC in a university before getting pulled out for a look, and they were fine.

The graph attached here is from "Calendar Aging of Lithium-Ion Batteries", Keil et al, Journal of The Electrochemical Society, 163 (9) A1872-A1880 (2016). Quite an interesting paper as they stored them across the full SoC range.

Keep in mind that electrolyte chemistry varies between manufacturers and cells models, so it doesn't mean that every cell is going to lose capacity at that rate. They were testing small cells here, but still LFP.
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Old 29-04-2017, 04:16   #5755
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Interesting, what do you believe the cell voltage would be at 70% SOC? It isn't clearly definable looking at the manufacturers charts, even what voltage they consider to be fully charged, all the discharge charts are voltage under load, not rested voltage and the operating range of between 4.0V and 2.8v have a lot of factors that must be taken into consideration as neither of these voltages are rested voltage.

As for the other stuff, it would be interesting to see the charts/graphs that back this up, I certainly haven't seen it in any of my tests on systems with over 6 yr 24/7 cycling, both thousands of mini cycles and hundreds of deep cycles during that period. I have a lot of systems that I can draw data from, not just one or two so I would have thought I'd see these sort of symptoms occurring because the charging control method I use would put the cells in the danger zone you are putting forward.

T1 Terry
The flat voltage curves published by the manufacturers are charging or discharging at fairly high C-rates. They are useless, all the features disappear. You need a rested OCV curve and it remains relevant at the very low C-rates we tend to have. Everything goes out the door when there is too much current involved of course.
The one below was collected in a university and they waited for hours each time before measuring. I can definitely see the middle and upper flat regions in operation. I can't tell the SOC on the flat in the middle, but I can see everything else and it is far more dependable than a Coulomb counter.

Not long ago you were reporting having to do a major overhaul, rebalancing and capacity recovery exercise every 3 years or so. I have systems getting close to this milestone and there is absolutely nothing to do. We sometimes noticed a hint of a capacity shortage coming out of winter when they very seldom (or never) had the chance to get charged properly, but it returns back to normal by itself. Having a bit of absorption and sufficient voltage is very important for long term performance. The strategy minimises full recharge cycles, but when they happen we charge properly.
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Old 29-04-2017, 06:25   #5756
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

This image may help too..

Cycle Test #772 Discharge Graph

http://www.pbase.com/mainecruising/i...5/original.jpg



This discharge curve may be helpful to some. The data is the same data in the video of a discharge test I made last year at .075C, only in a graphical format.

Of interesting note is how little capacity is stored at loaded voltages, even at a low .075C discharge rate, or 30A for a 400Ah rated pack.


In the graph the 400Ah rated battery has only delivered -2.83Ah's by the time the voltage curve levels out at 3.32V under the 30A load. When these cells are at 100% SOC they can have a resting voltage of about 3.38VPC to 3.40VPC or 13.52V to 13.60V, for a 12V nominal pack, but there is really very little stored energy between 3.4VPC and 3.32VPC, about 0.65% of Ah capacity to be exact for this pack.


On this pack the stored energy between 3.4VPC and 3.32VPC was 2.83Ah's. This is the rather abrupt, near vertical, portion of the blue voltage line at the very beginning of the curve. Also, at low rate discharges, such as this .075C discharge rate, the curve is more gradual and it's not until about 2.9VPC or 11.6V that the voltage starts to hit the knee and drop rapidly, in a near vertical fashion.


On a 400Ah rated pack, at a 30A or .075C discharge rate, the working voltage range between about 99.3% SOC and 0% SOC is only a 1.66V difference for this 12V nominal pack.


In the image I have highlighted the loaded voltage points as well as every 10% of capacity in delivered energy.
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Old 29-04-2017, 13:52   #5757
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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This image may help too..

Cycle Test #772 Discharge Graph

http://www.pbase.com/mainecruising/i...5/original.jpg

This discharge curve may be helpful to some. The data is the same data in the video of a discharge test I made last year at .075C, only in a graphical format.
When you connect cells in series, the individual cell curves get summed together with small horizontal offsets due to cell capacity differences, so it blurs the features a little. Unless the purpose is characterising a specific pack, measuring the voltage of a single cell leads to a better generalisation afterwards.

The other thing is that 30A would be a huge average discharge current for most marine house bank installations where 1A < Ibat < 10A most of the time and peak loads are intermittent.
30A (C/13) is too much current to get a good reference curve. Texas Instruments did quite a lot of work on developing lithium "fuel gauges" and they hint that if the current exceeds C/20, you haven't got much of a chance with an OCV curve.

The transition is quite smeared, but the upper and lower voltage shelves are still noticeable.

Discussing incremental capacity vs voltage when the bank is in fact under quite significant load is not correct or meaningful, because if you removed the load and allowed it to settle for a few hours, the results would be completely different, especially at the top.
Charge and discharge are not identical either, so you need two curves. The cells settle at different voltages for the same SOC depending on whether they were previously charged or discharged.


There is plenty of warning and excellent voltage/SOC correlation from 40%SOC down. This is how I get people to assess whether they might want to think about charging: 13.0V = 1/4 left, 12.8V = might want to do something before the LV alarm goes off.
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Old 01-05-2017, 03:14   #5758
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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The system is capacity tested on install to cover any possible issues that may occur in the future. Hard to argue the lost capacity case if you don't know the capacity was there I the first place.
Calendar age and use loss of 4% per yr or simply age loss at 2.5% per yr eh..... Just goes to show the way they are first condition charged and the charging regime used has a long term effect if the lab boys lost 4% per yr over a 2 yr test yet our systems in full time use for 3.5 yrs haven't suffered any loss at all.
The quality of the cells used also has a very big bearing on the capacity loss over time so there are a lot of variables that can effect the capacity retention, over voltage charging is the single biggest killer, holding the cells at a voltage high enough to cause electrolyte heating is the next biggest killer and all the early type cell balancers certainly fall into that cell killer category by holding the cells at 3.8v for long periods.
Understanding just what the manufacturer is tell you in the specs they supply is still the biggest hurdle, that 4v per cell has some very important criteria attached like the charge rate and acceptance rate, just using that 4v figure as a general safe max is very deceiving and will result in serious cell damage and loss of capacity very quickly.

Of course if the whole memory effect is not cleared when charging and discharging (two different problems) then the capacity could still have been available, just not accessed.

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Terry sorry for the late reply, I just don't have time to chime in here sometimes at call. I would be interested to know your discharge rate when your were assessing the capacity of older batts after your top end refresh bearing in mind their original rating depending on manufacturer is say a discharge rate of 0.3 then factor in addional build/voltage extremity capacity, the latter which is zip.

I haven't come across a LFP cell yet that will deliver their original rating after a few years of moderate work at a 0.3 Discharge Capacity Test Rate. What I have found though is reduce the discharge rate to say less than 0.1 or better even half that which is still commensurate with the rate a lot of people use to test capacity or even as a comparison the day to day discharge rate that say sailboat people employ.

I have found capacity goes through the roof at a lessor discharge rate to rated (and not just because of cell drift in older batt banks is amplified enormously at the lower end}, albeit a practical rate, but not really a apples with apples comparison that used for my manufacturer for rating purposes. Or in other words there is oodles of extra capacity to burn in LFP the day you buy and providing you you look after them, that capacity will stick around. Probably the perfect battery for sailboat use.

Hence my question on the discharge rate you have been using to test capacity of old batts with and your claim there is no capacity shrinkage??

My guess is you have not been using a discharge rate of close to 0.3D as rated by many manufactures. that is why your getting them grand ..no capacity shrinkage figures.

BTW if you have...I will slink away and hide in the corner.
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Old 01-05-2017, 14:43   #5759
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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"In doubt always try to charge to full" exclusively referred to the strategy used by LA chargers, which is why they are so problematic to embed in LFP systems. They really do that and it also provides maximum reserve capacity at any time, which is great. When you have to manage LFPs, you have a completely new dilemma on your hands between the available reserve capacity and extending battery life.

For storage, the lower the better, really. I remember a story about cells that were stored for 10 years at 20% SOC in a university before getting pulled out for a look, and they were fine.

The graph attached here is from "Calendar Aging of Lithium-Ion Batteries", Keil et al, Journal of The Electrochemical Society, 163 (9) A1872-A1880 (2016). Quite an interesting paper as they stored them across the full SoC range.

Keep in mind that electrolyte chemistry varies between manufacturers and cells models, so it doesn't mean that every cell is going to lose capacity at that rate. They were testing small cells here, but still LFP.
Ocean I read that paper with interest some time ago and have been pondering storage SOC ever since. Conventional wisdom and supported by the manufactures as delivered OCV's, that around 3.295v/3.3v or 50%+ SOC has been the guide to safe storage voltage.

However that paper seems to indicate we should be aiming at a much lower storage voltage or is it a case at a lower voltage even if say 3.28v, self discharge then becomes an issue? I have no knowledge of that but I have noticed that sitting at around 3.3v self discharge is non existent.
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Old 01-05-2017, 16:58   #5760
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Ocean I read that paper with interest some time ago and have been pondering storage SOC ever since. Conventional wisdom and supported by the manufactures as delivered OCV's, that around 3.295v/3.3v or 50%+ SOC has been the guide to safe storage voltage.

However that paper seems to indicate we should be aiming at a much lower storage voltage or is it a case at a lower voltage even if say 3.28v, self discharge then becomes an issue? I have no knowledge of that but I have noticed that sitting at around 3.3v self discharge is non existent.
You need to keep in mind that self-discharge can only be very apparent with the voltage in the steep upper part of the SOC curve and it is also basically invisible at 3.3V, so...
I think that the self-discharge at the top might also reflect deterioration through electrochemical processes that are favoured by the high SOC and it might happen faster for this reason.

I bought Sinopoly cells from fresh factory stock and they were a little above 40% SOC. Two years later, the supplier was still selling that same stock and I saw cells that were just below 40%.

I think that the 50% SOC storage figure needs to be taken for what it is: no manufacturer ever said that their cells had shelf lives of several years or were conditioned for long-term storage. The data in fact indicates that the lower the SOC the better for storage. The self-discharge rate is certainly very low for cells that were never abused, but I don't subscribe to the zero claims. In order to measure self-discharge, you would need to first store cells at a SOC where the voltage gradient is very clear (i.e. 3.320V or 3.280V, not 3.295V), keep the temperature constant and measure again months later or more with a calibrated and certified lab instrument... anything less can't be deemed conclusive.

We just know that they can be stored for several years without going flat, but even figures of 2 or 3% self-discharge per year would deliver that. I wouldn't have much of an issue storing cells at 3.25V for at least a year or two based on what we know.
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