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Old 06-04-2018, 15:09   #46
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Re: LifePo4.. Yes AGAIN!

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Originally Posted by alctel View Post
My thoughts as well.
Its no more dangerous than top balancing.. Not sure why both of you have this perception? Just like top balancing, you stay within the limits and the batteries are fine.

The risky part is if you don't have the proper equipment and are relying on yourself to ensure you don't discharge too low.
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Old 06-04-2018, 15:49   #47
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Re: LifePo4.. Yes AGAIN!

I have been running my diy 6p4s 600ah lfp bank for 16 months. I have a house bms that I have never tested so I have no idea if it actually works. Solar is primary charge source supplemented you generally. I hawkishly monitor cell and pack voltages via a celllog8.

The one time I pushed the upper knee wall (at about 13.85v) the low cell, by 20mv, suddenly became the high cell by 150mv. That was enough for evidence for me to stay away from knee walls.

My only concern with my lfp bank is leaving it unattended for a week or more with the fridge and freezer running in the winter- risk of long cloudy stretches with short solar days running down Batts..

Ymmv
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Old 07-04-2018, 07:54   #48
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Re: LifePo4.. Yes AGAIN!

I have a zillion questions, but will start with this one since it's most relevant to the current topic of bottom vs top balancing.

Can the top and bottom SOC be reliably determined by voltage alone?

Or does the voltage cut-off point need to be adjusted based on charge or discharge current? The charts I have seen show the knee voltages at different points based on charge or discharge current.

Or is Ah counting required? In the middle band of SOC where voltage is pretty constant, it would seem that Ah counting might be the only way to get a good estimate of SOC.

BTW, if you prefer I ask these as separate threads, just say so. I'm happy to do that.
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Old 07-04-2018, 08:49   #49
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Re: LifePo4.. Yes AGAIN!

Quote:
Originally Posted by kcj View Post
I have been running my diy 6p4s 600ah lfp bank for 16 months. I have a house bms that I have never tested so I have no idea if it actually works. Solar is primary charge source supplemented you generally. I hawkishly monitor cell and pack voltages via a celllog8.

The one time I pushed the upper knee wall (at about 13.85v) the low cell, by 20mv, suddenly became the high cell by 150mv. That was enough for evidence for me to stay away from knee walls.

My only concern with my lfp bank is leaving it unattended for a week or more with the fridge and freezer running in the winter- risk of long cloudy stretches with short solar days running down Batts..

Ymmv
Cheers
Awesome.. This is the kind of response I was hoping to get.

Do you mind me asking what voltage settings you have on your solar controllers? what voltage do you charge to, what is your float voltage?
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Old 07-04-2018, 09:06   #50
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Re: LifePo4.. Yes AGAIN!

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Originally Posted by tanglewood View Post
I have a zillion questions, but will start with this one since it's most relevant to the current topic of bottom vs top balancing.

Can the top and bottom SOC be reliably determined by voltage alone?

Or does the voltage cut-off point need to be adjusted based on charge or discharge current? The charts I have seen show the knee voltages at different points based on charge or discharge current.

Or is Ah counting required? In the middle band of SOC where voltage is pretty constant, it would seem that Ah counting might be the only way to get a good estimate of SOC.

BTW, if you prefer I ask these as separate threads, just say so. I'm happy to do that.
I would say that the bottom SOC can be fairly reliably determined by voltage alone.

The top SOC is more complex and really requires a monitoring of current and amperage if your charge sources are still putting watts in. It is possible to fully charge a LFP battery at 13.6V. So depending on what your charge sources are set at, 13.6, 13.7, 13.8 could all be the 100% SOC voltage. Its really the voltage you have set and the tapering current that tell you the battery has hit %100.

As to a mid point SOC. My understanding is yes a AH counter is probably the only way to get a somewhat reliable number.

You might be interested in this set of videos on Youtube. I think its quite excellent for beginners as the vlogger is learning as he makes the videos. He is very open with what he gets right and what he gets wrong.
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Old 07-04-2018, 12:48   #51
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Re: LifePo4.. Yes AGAIN!

Agree with the above.

Top: pick a voltage to call 100%, when resetting your AH counting BM, get precise by stopping when trailing amps falls to .02C 2A per 100AH.

In daily usage cycling, just stop when you hit voltage, no Absorb. Yes whether that is 99.7% or 98.6% will depend on charge rate.

AH discharged give you SoC on the way down.

Set a voltage floor at the bottom for protective gear. Yes SoC depends on rate, no matter, never go near there IRL, alarms and load buss LVDs on the way down
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Old 07-04-2018, 13:59   #52
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Re: LifePo4.. Yes AGAIN!

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Originally Posted by travellerw View Post
Awesome.. This is the kind of response I was hoping to get.



Do you mind me asking what voltage settings you have on your solar controllers? what voltage do you charge to, what is your float voltage?


Absorb is set at 13.8. Float is 13.1. But for solar, the float is really not a big deal for me because I rarely get out of bulk--and the sun sets every night, the panels are not always pointed to the sun...May be an issue if I shut my fridge and freezers off.. solar is the closest charge source call "to set it and forget it."

The BIG issue in my experience is running the chargers. Aside from the algorithm inadequacies there is the voltage drop issues between the bank and the charger. Not such a big deal on the genset, as I never run it more than an hour or two. I rarely charge off shore power and never leave the charger on unattended or overnight, it's just not necessary and risky. And you have to watch the alternators on long motors. You need a way to shut them off or disconnect them from the LFPs.

Did I say I watch the voltages, cell and pack ?
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Old 08-04-2018, 03:21   #53
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Re: LifePo4.. Yes AGAIN!

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Originally Posted by travellerw View Post
OK... So first.. Change my mind.. Am I crazy to lean towards a bottom balance?
Nope, you're not crazy. It works great!

It's trivial to implement with no special gear, and anywhere in 2.5-2.9Vpc is basically empty, so you have some wiggle room. Use a computer to manage that initial discharge balance.

With 4 cells in your pack, you can afford to check and adjust frequently, until you feel comfortable that the system is stable. You'll likely get bored of this after a few weeks, especially if you are okay charging to 90% SOC instead of trying to push up into the knee. There's just not that much to do.

p.s. Your HVD level is a bit too high. Get something that will warn and disconnect earlier.
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Old 08-04-2018, 03:54   #54
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Re: LifePo4.. Yes AGAIN!

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Originally Posted by tanglewood View Post
Can the top and bottom SOC be reliably determined by voltage alone?
With LFP chemistry at the SOC extrema, resting open-circuit voltage is a good way to estimate SOC. It can be impossible to get to a true "rest" in a real system, because the voltage asymptote is reached very slowly. It is always necessary to measure at the cell level. There is a small thermal impact.

Quote:
Or does the voltage cut-off point need to be adjusted based on charge or discharge current? The charts I have seen show the knee voltages at different points based on charge or discharge current.
This is also true, but it's not mutually exclusive with the answer to your previous question.

In practice, most people have a capped charge rate that is also small enough that voltage elevation due to charging at that rate is predictable, and that makes it easy to set an HVD level for runaway charging.

In practice, in house bank settings, discharge can be quite a bit more variable. In a conservative, low-demand system, again the max discharge rate is low enough that a reasonable LVD level can be established that won't nuisance-trip. In more "aggressive" systems, it becomes helpful to use a dynamic LVD that takes into account current. Some BMSes can do this, and some loads can do it as well (e.g., Victron inverters).

In practice, in EV settings, discharge peaks can be so high that it is mandatory to compensate for current.

Quote:
Or is Ah counting required? In the middle band of SOC where voltage is pretty constant, it would seem that Ah counting might be the only way to get a good estimate of SOC.
The practical way to estimate state in the middle (which is where you will be a lot of the time, out of necessity or perhaps even by design) is to "count coulombs" by measuring current and integrating over time, yes. This approach is simple and reasonably accurate over the time scales that we care about for house banks.

If you care to charge well past the linear regime, cell voltage and/or CV absorption rates become the metrics used to stop charging, at which point most counting estimators "resync" to 100% and go from there. If you care to discharge past the linear regime, cell voltage becomes the metric for making a shutdown determination.

With a large enough series cell count, plus a good voltage measurement, plus a stable enough charge or discharge current for a long enough period of time, I believe it is also possible to get pretty close state estimation with just voltage. In experimenting with my house bank on the bench, prior to installing it and using it for real, I was able to correlate millivolts to % SOC pretty well at a given, stable test current. The cells only reach those predictable voltages after some time, though, so the test current has to be pretty small lest the pack move outside of the linear regime (we're talking hours). In real life, you never get this kind of synthetic, super-stable current, so this is impractical.
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Old 08-04-2018, 07:12   #55
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Re: LifePo4.. Yes AGAIN!

Thanks Nebster, very helpful.

Let me move on to the next question. Back when I first started reading about LFP batteries, it was thought to be important to encase the batteries in some sort of structure that would support the cases and resist bulging. Without such a corset, the batteries could bulge and fail.

Now reading about CALB and other vendors, I'm not seeing that any more. Is this a thing of the past? Was it never really needed? Or have the molded cases been improved to provide sufficient self-support for their contents?

I am envisioning four CALB 180ah cells strapped in parallel as the base building block for my battery bank. Will those need some sort of corset for support, or can I simply focus on the best way to physically secure them?

Thanks
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Old 08-04-2018, 09:38   #56
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Re: LifePo4.. Yes AGAIN!

Quote:
Originally Posted by kcj View Post
Absorb is set at 13.8. Float is 13.1. But for solar, the float is really not a big deal for me because I rarely get out of bulk--and the sun sets every night, the panels are not always pointed to the sun...May be an issue if I shut my fridge and freezers off.. solar is the closest charge source call "to set it and forget it."

The BIG issue in my experience is running the chargers. Aside from the algorithm inadequacies there is the voltage drop issues between the bank and the charger. Not such a big deal on the genset, as I never run it more than an hour or two. I rarely charge off shore power and never leave the charger on unattended or overnight, it's just not necessary and risky. And you have to watch the alternators on long motors. You need a way to shut them off or disconnect them from the LFPs.

Did I say I watch the voltages, cell and pack ?
Very good information.. Thank you very much.

I don't have to worry about my alternators as they are not connected to my house bank.

I think my biggest issue will be that I actually have too much solar. Even with lead acid batteries I get out of bulk every day. In the winter months I get to float about once a week, in the fall and spring I get to float about twice a week, and come summer I will get to float 4 or 5 times a week. Without the long absorbtion phase, I'm guessing I will be plumb full by early afternoon. At that point I will just be throwing power away.
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Old 08-04-2018, 09:43   #57
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Re: LifePo4.. Yes AGAIN!

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Originally Posted by tanglewood View Post
Thanks Nebster, very helpful.

Let me move on to the next question. Back when I first started reading about LFP batteries, it was thought to be important to encase the batteries in some sort of structure that would support the cases and resist bulging. Without such a corset, the batteries could bulge and fail.

Now reading about CALB and other vendors, I'm not seeing that any more. Is this a thing of the past? Was it never really needed? Or have the molded cases been improved to provide sufficient self-support for their contents?

I am envisioning four CALB 180ah cells strapped in parallel as the base building block for my battery bank. Will those need some sort of corset for support, or can I simply focus on the best way to physically secure them?

Thanks
Again.. I don't think anyone has a definitive answer for this.... But I can tell you that all the cruising boats I have met with LFP batteries only about %20 have thier packs compressed.

Right now we are buddy boating with a boat that has a DIY 800AH CALB bank. Its not compressed and has never been compressed in the 2 years they have been using it. No bulging or ill affects so far.

I will be strapping mine with stainless ties and aluminum plates on the ends.
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Old 08-04-2018, 14:03   #58
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Re: LifePo4.. Yes AGAIN!

Quote:
Originally Posted by travellerw View Post
Very good information.. Thank you very much.



I don't have to worry about my alternators as they are not connected to my house bank.



I think my biggest issue will be that I actually have too much solar. Even with lead acid batteries I get out of bulk every day. In the winter months I get to float about once a week, in the fall and spring I get to float about twice a week, and come summer I will get to float 4 or 5 times a week. Without the long absorbtion phase, I'm guessing I will be plumb full by early afternoon. At that point I will just be throwing power away.


As to getting out of bulk---have you done an energy audit? Do you know, roughly, what your daily ah demand is? Do you know your solar ah generation?
If the later does not exceed the former by more than 10% I doubt it is really getting out of bulk.

If you are off the boat for stretches with no loads I would consider shutting the solar down. The LFPs don't like being stored full, or so I've read

If you really have solar to spare you can do like I did and wire your water maker and hwh to run off the inverter....
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Old 08-04-2018, 14:30   #59
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Re: LifePo4.. Yes AGAIN!

So, with the caveats that (a) I have aluminum shell LFP cells, which cannot be squeezed together at all, and (b) I have no actual experience with compressing cells in real life... I do have a few theories about the compression.

It seems to me that there are two things likely in play that might influence folks to try to tightly pack their cells together.

The first one is simply that the terminal connection points are mechanical points-of-failure, and reducing the amount of stress that the bus bars/cables can place on them is probably just a Good Thing. That argues for securing the cells with respect to each other (which is something you do with aluminum shell prismatics, too) but doesn't really justify bulge-suppression.

My second theory is that early installs tended not to know how to manage the cells as well as we do today. They were less conservative, and lots of people (even manufacturers, really) were adopting charge management strategies that would result in occasional overcharge or mild thermal events. In a prismatic cell, with planar "sheets" of electrodes, I'm guessing outgassing or electrolyte expansion at high SOC/temperature caused the bulging. Perhaps there is some merit to the idea of trying to provide more structural support for the sheets as they deform, keeping things more in check and allowing the cell to damage itself a bit less?

In any event, I am strongly suspicious that it is not necessary in a conservatively managed pack. I would hazard a guess that, if you're in a situation where compression might somehow help manage a bulge situation, you've already basically screwed up. And no one should get an LFP pack to that state in "modern times."

That's my guess! No promises as to veracity.
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Old 08-04-2018, 14:34   #60
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Re: LifePo4.. Yes AGAIN!

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If the later does not exceed the former by more than 10% I doubt it is really getting out of bulk.
Quite so. Moreover, it's possible to stay in bulk all the way to 100% SOC, if the bulk current is low.

Put another way: whether*or how often a charger switches to CV is not in and of itself a good proxy for how much energy surplus (or lack thereof) a system is producing. Cell voltage rise is also a function of charge rate.
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