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

[CatNewBee]

Sorry, it was about user taking care of the cells without a bms. The human factor is number one cause of battery failures, for all chemistries LFP, FLA, AGM,...

There usually are signs before a run off, but they are not noticed. On very rare occasions there is a single event, like a breaking plate that causes an internal short cut in a FLA battery. It usually starts slowly and then accelerates until the run off. You can watch the charge voltage while charging, from 3.3V to 3.5V it takes along time, from 3.5 to 3.65 it goes quickly and from there to 4V is a few minutes, the voltage goes up exponentially an the cell blows quite quickly.

From 3.3 to 3.65 volt you will barely notice a temperature rise.

Same for discharge. It is not a human job to look at each cell constantly.

[JmanC]

For monitoring the cells I use those basic digital volt meter displays one at each cell just to get an idea if one is in trouble or any obvious mismatch when going to 100% reseting the battery monitor. My thinking is what can the BMS do if a cell strays off if your are mostly cycling in the 20-80% range? Even if it detects a problem what do I do? re-balance? test cells for capacity & Ri?

Is an expensive fluke worth it? I ask as if they need recalibration over time forgetaboutit! Just more baby sitting. I think just a yearly re-balance with benchtop supply is all I would do. If my cells are well matched that is. Along with basic good fusing, low & high cutouts. Untill they can provide a BMS that has a proven avergage lifespan of 30years I think they are toys...

[TheOffice]

Has anyone heard of/tested Mighty Max batteries? https://www.mightymaxbattery.com/

Website has no physical address. Amazon and Walmart sell the 100 aH for $649. Too good to be true. or is there that much profit in ReLion etc?

[Maine Sail]

Originally Posted by TheOffice

Has anyone heard of/tested Mighty Max batteries? https://www.mightymaxbattery.com/

Website has no physical address. Amazon and Walmart sell the 100 aH for $649. Too good to be true. or is there that much profit in ReLion etc?

There are soooooo many ways for companies to cram pure junk into a sealed plastic box and ship it to you that it is absolutely scary. I would urge you not to buy LiFePO4 batteries based on price as they are absolutely not all created equally.

[newhaul]

Originally Posted by TheOffice

Has anyone heard of/tested Mighty Max batteries? https://www.mightymaxbattery.com/

Website has no physical address. Amazon and Walmart sell the 100 aH for $649. Too good to be true. or is there that much profit in ReLion etc?

I have seen to many issues with the packaged batteries build your own bank then you know the build quality .

But if set on a " drop in" go with a good company that has a good reputation and that honors the warranty they put out .

[TheOffice]

Guys, thanks for confirming my fears! I guess the fact that Walmart sells them only shows volume, not quality.

[nebster]

Originally Posted by CatNewBee

On very rare occasions there is a single event, like a breaking plate that causes an internal short cut in a FLA battery.

Okay, so do you have an example? Documentation? What exactly happens quickly in a prismatic LFP cell?

FLA examples are interesting digressions, but again that's not the question.

[CatNewBee]

In parallel configurationd 4SxP a cell fails and does not get charged, it has high resistance, the cell pack loses capacity, the charge makes the other cell runoff while your pack shows 60% SOC, that kind of things.

Or you run your inverter at SOC 70% and after few minutes the remaing cell is depleted and runs off on deep discharge conditions...

I mean, people who rule out a BMS very likely are also negligent in chosing quality cells, and very often built packs from crappy cells of any size by creating parallel / serial packs. Also I am pretty sure monitoring them is not their strength and balancing once a year with a bench top charger is more of a wishfull thinking. Installing a battery in a tight compartment with all the bridges is a lot of effort, no one rips it once per year apart only to test and balance.

No offense, but I would not bet on such a system. In this case I would even recommend drop in batteries, hoping at least the cells inside are from the same batch.

If you reaaly spent money and effort to get quality cells, matched and tested, I am pretty sure you want to keep them happy, so using a BMS with balancing is just a logical path.

[nebster]

Originally Posted by CatNewBee

In parallel configurationd 4SxP a cell fails and does not get charged, it has high resistance, the cell pack loses capacity, the charge makes the other cell runoff while your pack shows 60% SOC, that kind of things.

Alright, now we have a concrete example; thank you.

I think you meant xP4S, in other words paralleling-first. Let's take what I suspect is the worst-case example, 2P4S.

And, let's make it the absolute worst-case scenario, somehow the prismatic cell magically just becomes a giant resistor and effectively drops itself out of the circuit. All the energy flowing into that parallel pair ends up in the second, non-damaged cell.

So, as you describe, you charge things up and the good cell charges at double, but its normal voltage rise only contributes 1/4 of the 4S voltage, so the pack-level voltage hides the issue for a while.

Right, so in the worst case, what happens? We over-charge the second cell and ruin it. (Well, setting aside the chance that we set it on fire, but we know that's quite hard to do with LFP, and thermal monitoring is a completely separate thing that we should all be doing with PTC fuses or something similar.)

So if a cell fails magically, instantaneously, the parallel buddy cell also will get cooked pretty fast, either because of under or overcharge.

I mean... yes, this sucks. But I think the BMS only saves us one cell here, right?

So for this example, the cost of not having the BMS is losing a second cell in an 8-cell battery.

I guess I'm not convinced that that's a very big deal. I'm already facing a 7-cell conundrum, so in a remote place I have to drop down to a 4-cell (1p4s) battery. If I have only 6 working cells, I'm still running a 4-cell battery until I can get replacements.

I could also carry two spare cells, I suppose, instead of one, and then I have enough extra cells to survive a no-BMS failure of this type.

(And, of course, running a 1pXs battery avoids this scenario entirely, right?)

I think it's interesting to think through the real failure modes that we might encounter. BMSes are not free, in dollars, time, or in introducing new and less well-understood risks.

I mean, people who rule out a BMS very likely are also negligent in chosing quality cells, and very often built packs from crappy cells of any size by creating parallel / serial packs. Also I am pretty sure monitoring them is not their strength and balancing once a year with a bench top charger is more of a wishfull thinking. Installing a battery in a tight compartment with all the bridges is a lot of effort, no one rips it once per year apart only to test and balance.

I agree, plenty of people who cut corners are going to have problems. But amongst us here, we are a small group who are not normal. We are weird.

No offense, but I would not bet on such a system. In this case I would even recommend drop in batteries, hoping at least the cells inside are from the same batch.

In the same light, I'm hesitant to bet on a balancing BMS. There are so many documented issues with those, and so many ways things can get sideways. These tricky scenarios grow with more complex battery banks, when cell counts and topologies get bigger.

If you reaaly spent money and effort to get quality cells, matched and tested, I am pretty sure you want to keep them happy, so using a BMS with balancing is just a logical path.

I spent a lot more money on my bank than you did on yours, and I did the analysis as best I could and was left unconvinced that a BMS was worth it. I think there could be other builders here that end up somewhere on the spectrum besides "balancing BMS is the GOAT," given what I've seen others describe and do.

I might still be wrong, and I enjoy talking about it since keeping the battery healthy is important to me. It's not even that hard to change my mind and install a BMS, if someone could convince me that it makes sense. I'm still waiting.

[CatNewBee]

Well, I am not going to convice you, so good to have a carefully monitored running system for referrence to the other installs. Im curious how well it will perform over time.

Mine has provided 143000Ah up to now (3rd year of use) , 1942kWh and performs like on day one with a BMS.

[newhaul]

Originally Posted by CatNewBee

Well, I am not going to convice you, so good to have a carefully monitored running system for referrence to the other installs. Im curious how well it will perform over time.

Mine has provided 143000Ah up to now (3rd year of use) , 1942kWh and performs like on day one with a BMS. Attachment 210083

question I have .
What accounts for the difference in kWh in and out . There is a 52kWh difference shown on your monitor.

[CatNewBee]

Originally Posted by newhaul

question I have .
What accounts for the difference in kWh in and out . There is a 52kWh difference shown on your monitor.

Charge efficiency I guess, and synchronizations...

Efficiency woud be 1.0277 or less than 3% loss
Also initial charge of 15kWh, stored in the battery at this SOC so ...

Makes 1942.5/ 1905 = 1.02 efficiency

[newhaul]

Originally Posted by CatNewBee

Charge efficiency I guess, and synchronizations...

Efficiency woud be 1.0277 or less than 3% loss
Also initial charge of 15kWh, stored in the battery at this SOC so ...

Makes 1942.5/ 1905 = 1.02 efficiency

Ok thanks and that makes sense
Hadnt had my coffee yet .

[nebster]

Originally Posted by CatNewBee

Well, I am not going to convice you,

You're welcome to convince me: you can start by responding to my explanation of what I think happens in your example bad cell scenario. Did what I write sound about right to you?

so good to have a carefully monitored running system for referrence to the other installs. Im curious how well it will perform over time.

Me, too. It's been just over two years of use now, and in the next few weeks I'll be pulling the pack and testing all the cells.

[CatNewBee]

So all good, if a cell failure or even cell group failure is acceptable, a bms makes no sense.

It is like a solar mppt controller discussion, pwm controller do just fine for a much lower price, some people even contact the panels directly to the battery and save money even on the pwm controller. They monitor the batteries themselves and are sure, the few solar panels will never charge the house full, so why spent money on a controller? And 18 V aren't that bad for fla, they will boil a little, they call it equalization.

So yes, doable and will work.

But if you dont want a bms, why not buying 12V LFP packs without a BMS inside in the first place, ready to use, matching cells and much cheaper and easier to install.

If cell failures or pack failures are acceptable, and you stay in the middle with the SOC, I mean, what could go wrong, right? You may not reach 5000 cycles, but 2000...3000 are a lot too.