Best way to manage this LFP house battery

[jflorey2]

I really want to avoid a single 16s pack so that I can isolate individual strings or cells without shutting down the system. With 100 cells, the odds of a cell (or bus bar, or lug, or whatever) needing work are pretty high.

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

[nebster]

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

If I end up having to wire up 112 cells of monitoring, that's preferable to having a system fully down while trying to live in it on the road. However, my premise is that I don't need much monitoring. So far I see no indication that I need that, but it will take time and imbalance observation to know for sure.

[nebster]

With reference to the following graph I think the best way to charge to say 90% if you using a generator or some other charger that has a CC output is to set the CV point to the voltage where the voltage reaches 90%SOC at whatever current you are charging at and terminate the charge immediately it reaches the set charge voltage with no absorb/CV charging. With these cells and a charge rate of ~0.2C this would equate to a CV point of ~3.45V/cell. This method will not work with any consistently if you are charging from solar or the charger doesn't charge at a constant current or for low charge rates. One good reason for not having any CV charging when charging from a generator is the extra generator runtime necessary to charge during the absorb phase.

Thanks, I haven't considered going with a lower CC rate, but that's mostly because I anticipate using the genset to charge underway, and I'd like to run that at as high a load for as short a time as possible. That is in conflict with my desire to extend absorb times past 1 hour, though. I don't yet have a mechanism to stop CV beyond a crude timer with a minimum of 1 hour. I've definitely got to work that out!

I am not a fan of running an LFP battery without an individual cell BMS mainly because of unforeseen stuffups and the very real chance of a fault within a cell reducing its capacity which could easily lead to it being overcharged and outside its safe operating zone.

What do you think about only monitoring a handful of the "troublemaker" cells? So far it seems completely consistent: the same couple cells are the ones with presumably the least capacity, so they spike up much earlier than the rest. I'd really like to avoid rigging up 100+ monitoring circuits if I can help it.

How often do you intend to redo the bottom balance?

I was planning to observe the system and deduce when a rebalance was necessary. It would be unacceptable to have to do it every few months, but once or twice a year would be okay. I expect we will cycle the pack roughly twice a week in real life.

I know there are (unmonitored) bottom-balanced LFP packs staying very well balanced after several years in the field, so I have been assuming (hoping) that mine would behave similarly.

[nebster]

Your coulomb counter will loose its accuracy over time unless you reset it on a regular basis as the battery does not have a coulomb efficiency of 100% (I have logged the coulomb efficiency of my battery at ~99.6%). How did you plan to reset the counter?

Yeah, max also brought this up. Coulomb-counting is notorious for wandering off the path, but fortunately:

a) I'm aware of that inclination.
b) My counter can reset on any number of criteria, so I'll just have it detect the end of a generator- or shore-based CC charge cycle.
c) I'm really only using the SOC as a convenience value. I believe I will tie all the real decisioning to voltage (and temperature, in an emergency).

[jflorey2]

If I end up having to wire up 112 cells of monitoring, that's preferable to having a system fully down while trying to live in it on the road. However, my premise is that I don't need much monitoring. So far I see no indication that I need that, but it will take time and imbalance observation to know for sure.

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.

[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.

[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]

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]

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]

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.

[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.

[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.

[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.

[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

[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