Best way to manage this LFP house battery

[PNjunction]

nabster ..

Anecdotal evidence gleaned from others with large long-term LFP installs syggests that the relative capacities between cells remain very stable over time. My theory is that, if these cells did drift a lot, we would hear a lot more about outright pack failures in conversation about it on various forums. But... who knows?

I think we're all typing at the same time.

Heed my warnings above about your idea of string wiring. That is lead-acid think. Not applicable here, and you'll never *maintain* balance.

Most of these mistakes were already made and discussed on the EV forums about 10 years ago. That is why you don't see much of them today.

[nebster]

I think the point is when monitoring string voltage of 16 cells the whole 'swing' is 0.6V x 16 = 9.6V which can bury multiple cell voltages in it- you can have 10 x 3.5 + 6 x 3.2 vs 16 x 3.39V and many other combinations and they will be indistinguishable from the total string voltage point of view. You probably can mitigate this by measuring parts of 16 cell string, like every 4 cells. Supposedly LFPs don't drift too far from each other but again this has been reported by only handful of people. jflorey2 implies the opposite- his bank got weaker cell (#13 naturally) he needs to watch over closely.

Yeah, I think that's right -- one individual cell will always spike higher than the others. It reaches 100% SOC faster and puts the string at risk if charging continues.

The antidote is to identify the string voltage at that high point and commit to never raising the string above that (combined) voltage.

I believe the questions, then, are:

1) Is the least-capacity cell, when full, still big enough to allow for an acceptable total charge? (Certainly yes, in my test pack's case.)
2) Will the least-capacity cell retain its same capacity relative to the rest of the cells in the string over time? If yes, then the pack-level voltage stopping point will work fine. If no, then we need to monitor and/or actively rebalance and/or replace the troublemaker cell.

Anecdotal evidence gleaned from others with large long-term LFP installs syggests that the relative capacities between cells remain very stable over time. My theory is that, if these cells did drift a lot, we would hear a lot more about outright pack failures in conversation about it on various forums. But... who knows?

[max2k]

Your graph is showing up now.

Now my graph is showing up as the broken picture icon if I am not logged in and I clear my web browser cache. Shows up OK if I am logged in???

Edit: Deleted graph and reloaded and it is no longer showing the broken picture icon. Just back to showing the name of the picture. This seems to be the way the Solar Panel forum website is set up. Very annoying that you have to be logged in to see the pictures.

Simon

your pic was never showing up for me, I just didn't mention it.

[PNjunction]

I'm assembling a new LFP for my RV out of Chinese cells. These are 100Ah prismatics, and I have 112 of them for 16s7p. My plan is to add isolators for each 16s string so they can be pulled out for service or disconnected automatically if something is amiss.

STOP RIGHT NOW.

That's not how you do it, or you will *never* maintain balance, either passive or actively.

With LFP, here's how you do it:

1) Parallel cells to build up your desired capacity first. Cells should be matched close to capacity and internal resistance.
2) Series connect these paralleled-cell groups to achieve your desired voltage

The next problem I see is that you are bottom balancing, yet you are talking about absorb and end-currents. With bottom balancing, you stop charge when the first cell reaches your pre-defined target voltage. Are you set up to monitor 112 cells to act as this trigger?

What you are describing now is a system that is trying to do BOTH top and bottom balancing and you'll get the worst of both worlds.

You have to choose only one method, top or bottom. Unless you have the skills and gear to manage it properly so that you don't inadvertently end up doing both top and bottom, I recommend buying 4 more cells and get in some badly needed practice:

https://www.solarpaneltalk.com/forum...prismatic-bank

Please get some hands-on with a learner-battery first.

Since those cells may be random shelf-picks, you are looking at spending some loving quality time measuring them for both capacity and internal resistance, to make sure your paralleled groups are closest to each other. A Revolectrix PL-8, or other high-end quality charger/analyzer is called for.

[karrak]

Okay, here's another attempt at the same attachment. Let's see if it works better.

Your graph is showing up now.

Now my graph is showing up as the broken picture icon if I am not logged in and I clear my web browser cache. Shows up OK if I am logged in???

Edit: Deleted graph and reloaded and it is no longer showing the broken picture icon. Just back to showing the name of the picture. This seems to be the way the Solar Panel forum website is set up. Very annoying that you have to be logged in to see the pictures.

Simon

[nebster]

Okay, here's another attempt at the same attachment. Let's see if it works better.
Screen Shot 2017-09-19 at 1.08.11 AM.jpg

[karrak]

your picture is not showing for me, not sure about others

It is not showing up for me either. This has happened to me before, nebster, I suggest you try to delete and reload your picture onto the Solar Panel website

Simon

[max2k]

You advocated for actively monitoring all 112 cells. I simply posit that such a setup, especially one that has a lot of silly flaws engineered out of it, could be quite expensive and/or complex. (They are certainly hard to find at this scale.)
...

I think the point is when monitoring string voltage of 16 cells the whole 'swing' is 0.6V x 16 = 9.6V which can bury multiple cell voltages in it- you can have 10 x 3.5 + 6 x 3.2 vs 16 x 3.39V and many other combinations and they will be indistinguishable from the total string voltage point of view. You probably can mitigate this by measuring parts of 16 cell string, like every 4 cells. Supposedly LFPs don't drift too far from each other but again this has been reported by only handful of people. jflorey2 implies the opposite- his bank got weaker cell (#13 naturally) he needs to watch over closely.

[nebster]

Well, then don't use sketchy PCB's with unbalanced draw. (Your meter doesn't have a "balanced draw" either - and the odds of you dropping something across a cell if you are in there all the time are nonzero.)

You advocated for actively monitoring all 112 cells. I simply posit that such a setup, especially one that has a lot of silly flaws engineered out of it, could be quite expensive and/or complex. (They are certainly hard to find at this scale.)

My point is that I don't expect my cells to deteriorate fast enough to merit a real-time monitoring system. I suspect simply an occasional, manual check will provide nearly as much insurance against a rogue cell without the downsides of a rats' nest of monitor wiring and yet another black box I have to trust to work well.

I'm not sure why you think my meter has any draw of any significance, but in any event I was referring to the load demanded by a monitoring system.

By the way, the odds of my shorting a cell by hand with my meter are close to zero: my probes are about 2mm long. I would argue no one should be working atop a pile of high-energy devices with any low-resistance conductor that is long with respect to the spacing. I bet you would, too.

Well, cells are stable until they're not. My system is bottom balanced to within .01 volts of each other. Once the average cell reaches about 3.35 volts, cell 13 gets to 3.65 volts very quickly (minutes.) So I keep the max voltage much lower than 3.35*16. That will work until cell 13 declines a little more, then it won't work any more.

Mine exhibits very similar performance so far, and so the question really is: when won't it work any more? The evidence I've seen suggests that it will work for a long time.

So, what have you settled on for your voltage? (And how much decline have you observed in cell 13?)

Simple is great. But there's a reason all EV batteries use BMSes - and it's not because they are cheap or foolproof.

I think you/we need to be careful not to conflate EV chemistries, charge/discharge regimes, and thermal envelope with those of a well-designed house energy storage bank. For the most part, the devil is in the edges, and EVs have to push a lot more of them than my application does. It makes perfect sense that an EV would take a more active approach to pack management vis a vis the risks and downsides.

[jflorey2]

I mean, I see what you're saying, but I think there's ample room to disagree: far easier for me to run my meter down the cells on an appropriate interval than to wire in hundreds of leads and terminals and add Arduinos or sketchy PCBs with unbalanced draw

Well, then don't use sketchy PCB's with unbalanced draw. (Your meter doesn't have a "balanced draw" either - and the odds of you dropping something across a cell if you are in there all the time are nonzero.) Also keep in mind that LiFePO4 batteries run away quickly; their voltages change very slowly for a long time until they start rising (or falling) dramatically. Your chances of seeing that in time to prevent damage, using a meter, are close to zero.

If the cells are stable with respect to each other in a nominal sense, then real-time complete monitoring seems like overkill.

Well, cells are stable until they're not. My system is bottom balanced to within .01 volts of each other. Once the average cell reaches about 3.35 volts, cell 13 gets to 3.65 volts very quickly (minutes.) So I keep the max voltage much lower than 3.35*16. That will work until cell 13 declines a little more, then it won't work any more.

I really like simple with critical systems.

Simple is great. But there's a reason all EV batteries use BMSes - and it's not because they are cheap or foolproof.

[max2k]

If it's okay with you, I'm just going to stop right here on your first sentence, because I don't understand it. I also don't understand much of the rest of your paragraph, but let's go one step at a time to make it easier on me.

The charger starts in CC: it feeds in energy at a fixed rate and observes voltage. Since potential monotonically increases with charge, we can set an arbitrary voltage where we ask the charger to switch to CV.

When the cells reach that specified voltage, the charger then decreases current to maintain the requested voltage. As the pack continues to charge, its ability to accept current at the given voltage goes down, and the charger continuously adjusts its output, down.

I'm not aware of a scenario where the charger would ever be increasing its current in any meaningful way during a CV charge, as long as the battery is behaving normally.

Does that sound about right to you all?

It does to me- lack of my practical experience. I thought switch to CV happens based on time not voltage but it turns out the other way around and it makes sense since it's safer.

[nebster]

When Charge Controller operates in CV mode it tries to maintain constant voltage by increasing current which goes into battery.

If it's okay with you, I'm just going to stop right here on your first sentence, because I don't understand it. I also don't understand much of the rest of your paragraph, but let's go one step at a time to make it easier on me.

The charger starts in CC: it feeds in energy at a fixed rate and observes voltage. Since potential monotonically increases with charge, we can set an arbitrary voltage where we ask the charger to switch to CV.

When the cells reach that specified voltage, the charger then decreases current to maintain the requested voltage. As the pack continues to charge, its ability to accept current at the given voltage goes down, and the charger continuously adjusts its output, down.

I'm not aware of a scenario where the charger would ever be increasing its current in any meaningful way during a CV charge, as long as the battery is behaving normally.

Does that sound about right to you all?

[max2k]

Well, you can do that, but then you need 7 BMSes. Seems cheaper/easier to avoid that.

the problem is the safety of the 700Ah LFP resulting 'cells' - anything goes wrong with one of them and the whole 2.1 kWh they store will find its way out. When connected in parallel external circuit can't turn them off and due to low internal resistance + high energy density + low thermal capacity you simply might get an explosion there, depending how short is the short.

LFP cells don't seem to drift out of balance easily, there was no evidence of that happening if one doesn't try to push them to >95% C capacity swing. On the contrary owners seem to have their systems operational for years without doing anything to get them in balance.

as OP also noted having one 1/7 of the system down is better than having all of it there.

[nebster]

I would suggest that if you want to observe and monitor cells so you can replace them "on the fly" - you need to monitor them all.

I mean, I see what you're saying, but I think there's ample room to disagree: far easier for me to run my meter down the cells on an appropriate interval than to wire in hundreds of leads and terminals and add Arduinos or sketchy PCBs with unbalanced draw (I'm looking at you, celllog8). If the cells are stable with respect to each other in a nominal sense, then real-time complete monitoring seems like overkill. I really like simple with critical systems.

And then, I make that case here, but I reserve the right to be proven wrong!

[max2k]

All I'm saying when putting them in CV the current will be defined by CC output CV voltage and battery voltage difference divided by those 23 mOhm. Or your charge controller max output current making it actually CC . If controller is powerful enough it is very easy to screw up unless the cells are already on their upper knee where they can increase their voltage to compensate.


I don't quite follow this paragraph. Can you explain for me?

When Charge Controller operates in CV mode it tries to maintain constant voltage by increasing current which goes into battery. If it has infinite power it will succeed . If your cells are still on their 'flat' part of the curve they won't easily let go the voltage up either so the current in the circuit will be defined by (Ucc_out - Ubatt) / Rinternal. In your case (54V - 48V) / 0.023 = 260A due to very low resistance of the cells. Either your fuse will blow or CC will limit its output current to its rated max (and consequently decreasing its voltage) making it essentially Constant Current at max rated value but something has to give. I slightly exaggerated values (by lowering Ubatt) to illustrate the point. If you improve your connections 23mOhm can become 16x0.3 + say 5 mOhm of wires = 10mOhm making these voltage mismatches even more pronounced as in the same example it would lead to 600A current.