LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[BjarneK]

Originally Posted by Oceanride007

I've got a height limit of .24m (9.5") and I would like to install some where around 540Ah, one provider suggested laying his CALB 180 on edge. Not sure if this would work cos I would think access would be required for monitoring or whatever. Question to this group is access to battery tops required.

I would worry about putting the batteries on the side in case they start leaking. I once left some no-name 20 Ah LiFePO4 batteries on the side for an extended period and they started leaking which was quite a mess. Calb is of course much better quality, but who knows? Having easier access to the connections would also be nice in case there is a problem. How about using the 100 Ah Calb batteries? They are 216 mm tall and could stand up, leaving a little space for connections above them.

[tanglewood]

CALB says they need to be upright, terminals facing the sky.

[newhaul]

Originally Posted by Oceanride007

I've got a height limit of .24m (9.5") and I would like to install some where around 540Ah, one provider suggested laying his CALB 180 on edge. Not sure if this would work cos I would think access would be required for monitoring or whatever. Question to this group is access to battery tops required.

my 240ah cells are only 8.25 inches (209 mm) tall.
So if you are comfortable with 8 cells to make a 480ah bank your fine .
This is the spec sheet on them

[Checkswrecks]

Originally Posted by Oceanride007

I've got a height limit of .24m (9.5") and I would like to install some where around 540Ah, one provider suggested laying his CALB 180 on edge. Not sure if this would work cos I would think access would be required for monitoring or whatever. Question to this group is access to battery tops required.

The terminals have a hermetic seal which is supposed to be good for any orientation and generally the seals will be for a while. In real life the seals do slightly "breath" with charge/discharge cycles, meaning that the electrolyte will ultimately find or create a leakage path.

Long way of saying that I agree with the others about upright installations.

[OceanPlanet]

Originally Posted by Oceanride007

I've got a height limit of .24m (9.5") and I would like to install some where around 540Ah, one provider suggested laying his CALB 180 on edge. Not sure if this would work cos I would think access would be required for monitoring or whatever. Question to this group is access to battery tops required.

Expensive, however 600Ah and only 8" tall...see attached. External BMS module required (plug & play)

[newhaul]

This pages video from Will Prowse on active balancing .
https://youtu.be/FRzrO2SAqSA
Places a whole new light on the subject.

[CatNewBee]

Originally Posted by newhaul

This pages video from Will Prowse on active balancing .
https://youtu.be/FRzrO2SAqSA
Places a whole new light on the subject.

Not really, stupid test. It is about bottom balancing to squieeze some more Ah when cells are in use. That is not the main reason for active balancing at all.

First, active balancing wastes less energy, you not burn excess charge to heat to slow down one cell, instead you recuperate the charge to the pack and increase efficiency and reduce heat, especially important with direct mounted cell modules, where you heat up the cell while top balancing. The overal performance depends on the balance current of course, it is almost the same if you burn one Amp or recuperate one Amp, what makes your charge current to go up 0.25A. It's not much of a gain in capacity but a gain in less heat.

Second, on bottom balancing of one empty cell, main purpose is to keep that cell alive as long as possible on small discharges, when you have your battery on storage and something drains it slowly. This active balancing may extend your battery life by weeks or months depending on the energy in the other cells until you notice and recharge.

Will is a nice guy, and plays around with LFP for a long time, but sometimes he is simply wrong.

[CharlieJ]

Regarding balancing of LFPs used in partial C discharge house banks, I have found no reason to balance as the large prismatic cells remain balanced. It is my opinion that non-active top balancing overly stresses LFP cells and that balancing should not be used until and unless the cells show that they are becoming unbalanced.

[newhaul]

Originally Posted by CharlieJ

Regarding balancing of LFPs used in partial C discharge house banks, I have found no reason to balance as the large prismatic cells remain balanced. It is my opinion that non-active top balancing overly stresses LFP cells and that balancing should not be used until and unless the cells show that they are becoming unbalanced.

that is the takeaway I got from his video is that top balancing is just about useless .

My cells are all with .001 of each other when charged to 3.650 so I commissioned the bank and discharged to 60% in till I can find the time to install on the boat.

[nebster]

Originally Posted by CatNewBee

Not really, stupid test. It is about bottom balancing to squieeze some more Ah when cells are in use. That is not the main reason for active balancing at all.

Can you explain more what you mean?

First, active balancing wastes less energy, you not burn excess charge to heat to slow down one cell, instead you recuperate the charge to the pack and increase efficiency and reduce heat, especially important with direct mounted cell modules, where you heat up the cell while top balancing.

What does "direct mounted cell modules" mean?

The overal performance depends on the balance current of course, it is almost the same if you burn one Amp or recuperate one Amp, what makes your charge current to go up 0.25A. It's not much of a gain in capacity but a gain in less heat.

I think maybe you're saying, when in the balance regime, an active transfer strategy eliminates bleed resistors sitting there making heat. Is that right?

Second, on bottom balancing of one empty cell, main purpose is to keep that cell alive as long as possible on small discharges, when you have your battery on storage and something drains it slowly. This active balancing may extend your battery life by weeks or months depending on the energy in the other cells until you notice and recharge.

I'm not sure I understand what you mean, can you make it simpler for me and maybe with numbers?

Perhaps, could you be saying that with a bottom-balanced battery pack with one weak cell, an active rebalancer will extend the amount of time that cell can avoid being overdischarged under a suitably low load?

If so, does it follow that a top-balanced battery would not be able to benefit from this active balancing advantage?

Will is a nice guy, and plays around with LFP for a long time, but sometimes he is simply wrong.

I've seen him say some wrong stuff, too. But I didn't hear anything wrong in what he said in the first half of that video.

In a battery with n cells, most of which have capacity x but one of which has lessened capacity x - d, the nominal "wasted" capacity is d * (n - 1). The larger the string, the more that weak cell hurts.

With active balancing, the maximum amount of additional energy that can be stored is simply a function of rate at which the balancer can move energy out of the weak cell times the amount of time we have before our load or charge brings us to empty or full.

I believe Will's point is that the balancer's power transfer rate is too low to make a meaningful difference in reclaimed capacity at normal pack charge or discharge rates.

That seems right to me. You would either need a wicked fast shunt rate (using pretty expensive hardware) or a very slow load or charge rate. DIY house banks won't have either of those.

(Am I missing something?)

[Cpt Pat]

Originally Posted by CharlieJ

Regarding balancing of LFPs used in partial C discharge house banks, I have found no reason to balance as the large prismatic cells remain balanced. It is my opinion that non-active top balancing overly stresses LFP cells and that balancing should not be used until and unless the cells show that they are becoming unbalanced.

That's what I've been doing for the past three years. Every 30 or so charge/discharge cycles, I'll charge to 100% SOC (charge current taper to 0.03C) and measure each cell voltage at the end of charge with my Fluke 117 (that has a NIST-traceable calibration). If I see more than a 0.05 volt mismatch, I connect a 2.7 ohm 10 watt resistor across the high voltage cell(s) until I achieve balance. That resistor cost me $0.27.

And really, it just makes me feel good. I doubt my manual balancing is really needed since I've come to realize I'm playing "whack-a-mole" with "balancing" the cells. Each time, it's a different cell that shows a little high on the few occasions I've actually seen a 0.05 volt mismatch. And besides, at the top of the charge curve, a 0.05 volt difference is a minuscule mismatch in SOC.

With a 0.2C charge current and 14.2 volts open-circuit charge voltage, my cells top out at 3.50 volts, +/- 0.05 volts. If I used cell "top" balancers with a threshold voltage of 3.55 volts (or higher) they would never become active. There'd be no benefit to having balancers. Only wasteful risk.

Now I'm not pushing the cells hard at all. My maximum charge rate is 0.2C and maximum discharge is 0.05C, my normal cutoffs are 20% SOC and 80% SOC, and my ambient temps are never outside +10C to +30C. Someone pushing their cells harder may get different results.

My prismatics were very closely matched from the start, with sequential serial numbers.

Balancing circuitry just seems like dancing with the devil. It's just another thing to go wrong. It only takes one shorted MOSFET or a bad solder joint in a balancing circuit to kill a cell. Plus, it inevitably increases the quiescent current drain on the cells. Even the 8 milliamp drain from my battery monitor equals 1.3 ampere hour per week (168 X 0.008).

[CatNewBee]

What I mean is the difference between active and passive balancing and the reason why active is better.

And yes, in my 2nd paradraph I was referring to passive BMS cell modules, that are directly mounted on the battery poles (like 123BMS, EV Power etc...)

Active balancing is just more sophisticated and has some advantages over passive balancers, some even act on both sides - top and bottom of the charge curve. On the top side they waste less energy and stay cool, on the bottom they protect from deep discharge of a single cell as long as there is enough energy left in the others. It is not primarily about increasing the pack capacity above that of a single cell.

Lets say you have a battery with 100Ah, 100Ah, 100Ah and 90Ah cells, top balanced. You leave the. System unattended and something drains your battery until LVC. The BMS disconnects after 90Ah, 3 cells have 10Ah, one is zero. The load is disconnected but you are at home and still did not noticed. The battery keeps self discharging, while the 10Ah cells are fine, the one that triggered LVC goes further down and eventually dies. With active modules this cell will be kept above 0Ah level as long as the other cells have sufficient remaining energy. It does not necesserily means that the load bus is reactivated to allow further consumption.

Regarding of noticing out of balance cells when partially charging. You won't notice imbalances, but they are there and grow over time. You only can measure them near the shoulders. If you run your battery lets say between 40 and 80% SOC you will not see any voltage difference for a long time, but at some point one cell may surprisingly trigger HVC or LVC and your capacity may be then 50% It is questionable how much capacity you then can restore by balancing. The drifting cell was operating for a long time unnoticed in a different SOC range than the others and has a different wear pattern. It simply shortens the overall pack life expectancy. You won't replace a single cell after 6 years or so, you will throw the whole pack and replace it then.

But it's your choice what to use and how. There is no right nor wrong. You can even buy cheap drop ins without BMS, made of the same batch of cells and factory balanced. They are still good for 1000...2000 cycles.

[nebster]

Originally Posted by CatNewBee

Active balancing is just more sophisticated and has some advantages over passive balancers, some even act on both sides - top and bottom of the charge curve. On the top side they waste less energy and stay cool, on the bottom they protect from deep discharge of a single cell as long as there is enough energy left in the others. It is not primarily about increasing the pack capacity above that of a single cell.

Okay, I am with you now. "Active" meaning balancers that can shuttle energy between cells, versus "passive" balancers that just bleed off extra energy via resistors.

I agree, such a system could preserve a cell that is weaker and has dipped down very low. It could also keep that whole battery online longer if the load is lower than the shuttling rate and if LVD has not triggered yet.

Regarding of noticing out of balance cells when partially charging.

To be clear, in this section of your post you are now talking about a more general problem that you feel is addressed with any balancer -- passive or active. Right?

You won't notice imbalances, but they are there and grow over time.

You state this often, and BMS manufacturers agree with you. But a lot of us have strong evidence that it is negligible.

It is questionable how much capacity you then can restore by balancing. The drifting cell was operating for a long time unnoticed in a different SOC range than the others and has a different wear pattern. It simply shortens the overall pack life expectancy. You won't replace a single cell after 6 years or so, you will throw the whole pack and replace it then.

Why is that? If the other 15 or 95 cells are all nominal, and one has deterioriated in a way that we couldn't detect all of this time, what would be the harm in putting in a fresh cell that exceeds the capacity of the other 15 or 95 (or 3, or whatever)?

I guess I don't buy that argument. But, I do carry several spare cells on board... maybe I'm the outlier?

[newhaul]

This is just an interesting article concerning our batteries

https://phys.org/news/2020-02-scient...tery-life.html

[newhaul]

Some good info here some of which we knew but it is officially confirmed
https://news.umich.edu/tips-for-exte...ion-batteries/