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Old 30-01-2014, 15:18   #3526
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Why does a float voltage (OK 13.8V might be high) harm a LiFe pack? If the float voltage stays well below the HVC level and the cells stay balanced there should be no current flow once the pack terminal voltage reaches the float voltage.
WHat the research indicates is that the establishment of an electric field by the presence of an external voltage cause Li plating. The effect is most pronounced when the bank is full, so if you are stoping at a nominal 80 or 90%, then it would seem that floating such a battery would allow "room" for the float.

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Old 30-01-2014, 15:42   #3527
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by goboatingnow View Post
The research indicates that what typically happens is, the slow plating , is only partially removed during discharge. Hence the indications are that a percentage of capacity is lost.

But capacity is very different to profile accurately in Li, especially in high c banks. so while Ive seen some loss in 18650 cells, I have no figures for large prismatics.

WHat I do know, is that the research indicates that that floating causes loss of capacity and if one examines the various Li charger designs , they all disconnect and reconnect.

Dave
We've "discussed" this before, and I have no doubt that holding a pack at a float voltage that would correspond to a relatively high state of charge would be detrimental. I would suspect that is where most of your "research" is concentrated? I've asked the charger manufacturer just what his charger is doing in this float mode at a low voltage, so let's see what they say.
I still find it difficult to see how a charger providing no voltage and no current can affect a pack, just from a not so common, common sense approach. I don't see how the charger could even support some kind of "voltage field" as previously mentioned when the bank is at a higher V.
Most nonadjustable chargers had a float value built in to compensate for self discharge of LA, basically a "trickle"charge. That would not be good for LI.
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Old 30-01-2014, 15:47   #3528
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I think most of the info I have seen from sites like House Power are geared to the EV folks where the vehicle batteries are either loaded while running or sitting at a charger station, not having loads and charging at the same time.


Steve
Yes, that's why we're arguing, whoops Discussing these things here. Kind of feeling our way though it.
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Old 30-01-2014, 15:51   #3529
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Why does a float voltage (OK 13.8V might be high) harm a LiFe pack? If the float voltage stays well below the HVC level and the cells stay balanced there should be no current flow once the pack terminal voltage reaches the float voltage.
The research indicates that cell degradation (which is greater in pouch prismatics) then 18650 cells, does occur where CC filled by CV ( i.e. float) is carried out

the Journal of Power sources is a good research reference guide. for example

the article in 2002 "Factors that affect cycle-life and possible degradation mechanisms of a Li-ion cell based on LiCoO2" by Soo Seok Choi, Hong S Lim.

discusses such effects on a variety of cathode materials.

I quote the abstract

"An extensive test has been conducted on a lithium-ion (Li-ion) cell under a variety of cycling conditions in order to evaluate factors that affect its cycle-life. The study is performed on a 900-mAh wound prismatic cell with contains a LiCoO2 positive electrode (cathode) and a synthetic graphite negative electrode (anode). Cycle-life is greatly influenced by the charge conditions, but is relatively insensitive to the discharge conditions. High charge cut-off voltages and a long float-charge period at 4.2 V or above have the most severe effects on cycle-life. Another damaging condition is high charge rates above the 1C rate, but reduction in the depth-of-discharge does not improve the cycle-life unlike the situation with other rechargeable batteries such as alkaline and leadľacid batteries. The dependence of the degradation rate on the charge voltage and the period of high charge voltage suggests that an electrochemical oxidation may be the cause"

Bear in mind LiFepo4 is similar but that the voltages are of course lower and there are other differences.


without substantial testing its impossible to say if the effect is noticeable, especially in a large C bank, thats only being little used normally , If you suffer a 10% loss in capacity its unlikely you can even measure or notice that loss. So it "may" or may not be a moto point.

Dave
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Old 30-01-2014, 16:07   #3530
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Dave
Your paper even says it.

" High charge cut-off voltages and a long float-charge period at 4.2 V or above have the most severe effects on cycle-life. "

Obviously this is LiCO, but it is referring to a finishing charge at float levels. Makes total sense, hold a cell at a relatively high voltage for prolonged periods as opposed to a higher voltage for a reduced time would cause more plating.

This is not what we're talking about, or a least not what I was talking about. We're talking about low voltage float settings (well below the nominal cell voltage), not a float charge used as a "finishing charge" to fill a cell/pack. That's the problem extrapolating all of these papers on to the things people are doing in "house bank" applications. IMHO
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Old 30-01-2014, 17:25   #3531
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LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Dave
Your paper even says it.

" High charge cut-off voltages and a long float-charge period at 4.2 V or above have the most severe effects on cycle-life. "

Obviously this is LiCO, but it is referring to a finishing charge at float levels. Makes total sense, hold a cell at a relatively high voltage for prolonged periods as opposed to a higher voltage for a reduced time would cause more plating.

This is not what we're talking about, or a least not what I was talking about. We're talking about low voltage float settings (well below the nominal cell voltage), not a float charge used as a "finishing charge" to fill a cell/pack. That's the problem extrapolating all of these papers on to the things people are doing in "house bank" applications. IMHO
Dwain
Yes I know and I alluded to that. But of you read the article and others you will see the effect describes also occurs at lower voltage , but less. The effect of the constant voltage is rather like a slow plating process.

Dave
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Old 30-01-2014, 21:54   #3532
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

The BIG question is

How much effect and at what voltage per cell?

Is it related to any voltage above the knee and if so, at what rate?
Exponential, linear or nonlinear, exactly what?

Personally, I'm not going to worry about keeping my home's bank at 3.4 v / cell from grid (with a 24/7 very minor load) and my boat's system at a bulk 3.45 v cell / float 3.4 v /cell from solar.

If either or both take a crap, I'll let everyone know.
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Old 31-01-2014, 06:51   #3533
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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The BIG question is

Personally, I'm not going to worry about keeping my home's bank at 3.4 v / cell from grid (with a 24/7 very minor load) and my boat's system at a bulk 3.45 v cell / float 3.4 v /cell from solar.

If either or both take a crap, I'll let everyone know.
+1 13.8 Bulk/absorb 13.2v float here. and No worries!
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Old 31-01-2014, 07:34   #3534
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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+1 13.8 Bulk/absorb 13.2v float here. and No worries!
We have ours set up basically the same, float essentially turns off the source because it is below the nominal resting or working voltage, but we use it differently.

When the bank is full I terminate all charging sources and re-set the Ah counter. I then let the bank fall to a max of 80% DOD. I then recharge to full and start the process all over. This has been amazingly simple and nice and quiet for many days... If we get back to the mooring and the bank is at 60% SOC, who cares, I leave it at 60% and go home. Can't do that for long with LA.. This really began because I was trying to do accelerated cycle testing to 80% DOD but I found I much prefer the simplicity of charge to full / disconnect charging / discharge to 80% DOD....

This is something you simply can't do with LA and is a new way of thinking about my system. It really took some getting used to but it has proven to be amazing & very, very simple. Also no worries about charge sources potentially running amok when you are not there. We do not charge when not on the boat, no need.

Occasionally the bank won't get dropped all the way to 80% DOD, if we know we have an opportunity to re-charge, but for the most part we charge to full, terminate charging then don't turn it back on until 80% DOD. Simple stuff.....

Our solar panels got very little use this year, they were simply not needed because the alt replenishes so quickly...

I was talking with Charlie J. the other day and told him I don't refer to my "float" settings as float but rather my "drop voltage". When the bank drops to 13.1V the current begins flowing again otherwise it is shut off after the bank runs its 13.9V/13.8V cycles....
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Old 31-01-2014, 08:08   #3535
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by SteveSails View Post
...
What I want to avoid is discharging my bank by even 10% because the fridge is running on a sunny day but the BMS says it's above the LVC so don't charge, that's what the solar panels are for, pick up the load when it can. I think most of the info I have seen from sites like House Power are geared to the EV folks where the vehicle batteries are either loaded while running or sitting at a charger station, not having loads and charging at the same time.

So what is the best way to use a resource like solar or wind which might not be available when the bank runs down to a charge now or else level?

Steve
I'd also like to hear the answer to this.
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Old 31-01-2014, 08:09   #3536
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

"Drop voltage"...I like it. Soon we will have to come up with a whole new terminology for dealing with Li (or at least LiFePO4)...;-)
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Old 31-01-2014, 08:15   #3537
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I'd also like to hear the answer to this.
The new Genasun GLi BMS has an optional input from the charge bus to enable auto-restarting after an LVC. So if/when some solar kicks in the system turns back on.. Do need to have solar controllers that auto-start as well.

Some other BMS's may have similar, I don't know.
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Old 31-01-2014, 09:09   #3538
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by SteveSails View Post
Why does a float voltage (OK 13.8V might be high) harm a LiFe pack? If the float voltage stays well below the HVC level and the cells stay balanced there should be no current flow once the pack terminal voltage reaches the float voltage.

What I want to avoid is discharging my bank by even 10% because the fridge is running on a sunny day but the BMS says it's above the LVC so don't charge, that's what the solar panels are for, pick up the load when it can. I think most of the info I have seen from sites like House Power are geared to the EV folks where the vehicle batteries are either loaded while running or sitting at a charger station, not having loads and charging at the same time.

So what is the best way to use a resource like solar or wind which might not be available when the bank runs down to a charge now or else level?

Steve
As written I can't follow the logic or the question? Can you rewrite that and be more specific? A 90% SOC BANK would not LVC.... If you wire as HVC/LVC and they are separate then charging can still occur even when the LVC contactor is open..
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Old 31-01-2014, 09:45   #3539
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I think the question boils down to:
How should you use low amp alternate charging sources (wind and solar) with Lithium.

The problem is:
Some are suggesting to charge then discharge the batteries in full cycles only.

But:
The low amp alternate charging sources (wind and sun) are not always available when the battery is discharged.

Also:
While these sources are available, we would like to keep the battery at %100 SOC because we don't know when they will be available again.

For example:
On a sunny day solar might top off my battery by 10:00am. If I stop charging (or Floating) at that point the battery will be partially discharged by the end of the day even though the solar was available. Next day might be cloudy and now I may need to run the engine to charge. This might have been unnecessary if the battery had been fully charged at the end of the previous day.
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Old 31-01-2014, 10:02   #3540
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by NahanniV View Post
I think the question boils down to:
How should you use low amp alternate charging sources (wind and solar) with Lithium.

The problem is:
Some are suggesting to charge then discharge the batteries in full cycles only.

But:
The low amp alternate charging sources (wind and sun) are not always available when the battery is discharged.

Also:
While these sources are available, we would like to keep the battery at %100 SOC because we don't know when they will be available again.

For example:
On a sunny day solar might top off my battery by 10:00am. If I stop charging (or Floating) at that point the battery will be partially discharged by the end of the day even though the solar was available. Next day might be cloudy and now I may need to run the engine to charge. This might have been unnecessary if the battery had been fully charged at the end of the previous day.

I was not suggesting charging in full cycles only just that this has worked very well for us. If I know we are going to be on the hook for a while I will charge when I can.

I think it all depends upon how you size your bank and manage it. Ours can run our boat for about 6 days without any charging what so ever but we only use about 50Ah's per day. If we used 100Ah's/day I would likely have an 800Ah bank... Six days is a long time to decide if I might want to put some Ah's back in via engine or solar. If I am getting close to 80% DOD I will kick the solar on to slow my DOD and eek out a few more days of quiet.... If the bank is sized to use all 80% in a day or two then I would certainly consider going larger.

I personally have no issue keeping these sources on if you charge to 13.6 - 13.7V then do a "drop voltage" to 13.1V - 13.2V or so. This drop voltage essentially turns off the charging until the bank hits 13.1V and engages current again. With solar and wind you should have no problems with current taper at the currents provided by wind/solar even at 13.6V to 13.7V...

Keep in mind that not all solar or wind controllers will maintain a drop/float voltage over night and will re-boot into bulk/absorb/float the next day. For this reason our solar is currently set up for 13.7V and 13.1V.. 13.6V & 13.2V will work too..

The new Rogue controllers have full lithium parameters and a trigger sense.
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