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

[nebster]

Originally Posted by CatNewBee

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

In this particular case, the rationale Chris put forward is that a BMS is worthwhile in order to help out in the scenario where there is a sudden failure of a cell.

So the discussion we are having already presumes a cell is going to fail, suddenly and for some random reason -- whether there is a BMS or there is not a BMS installed.

The question, then, is: given a scenario where a cell fails rapidly, does a BMS actually add enough value on the margin?

It's an interesting one with no single right answer. Like I've been saying for years on this thread. :-) We have to do the analysis for our specific scenarios.


In my case, I have a 1p16s6p battery. A single cell failure of the type you describe ("become a big resistor") would not take out any neighboring cells. The offending string would start to behave like a string with very little capacity, which means it would neither accept nor deliver much energy during charging and discharging, respectively. The pack would essentially degrade semi-gracefully to a 1p16s5p pack. There is no "blast radius" for my pack for this specific type of failure, so at the end of the day, I have to notice that I've lost 15% of pack capacity -- that would happen, eventually, but I know it might take me a few weeks or months. At that point I have a single cell to replace.

Looked at from the other perspective, if I subscribe to the BMS camp, then I need to buy six 16s BMSes and wire 96 sense wires all through my battery pack. I have no reliable way to attach sense wires, because I am not willing to insert an additional small lug in the power path given how sensitive the string Ri's are to the torque on the power lugs alone. I dread pulling 96 very tiny wires across over 2 meters of dangerous, 53V battery surface area. They would have to be very tiny so that they acted as their own fuses, because I am loathe to wire up 96 fuses.

The wiring would be very fussy and error-prone, and the BMSes alone would substantially increase the cost of the battery pack -- roughly 25-30%, when I did the math three years ago.


In contrast, someone with a 2p4s pack composed of expensive 1000Ah prismatic cells could perhaps obtain a simple 4s BMS with monitoring only for $30 and install it. For that user, a single-cell instant failure gives the BMS the opportunity to shut the pack down (which is a consequence in and of itself, by the way) when the sibling cell struggles. In that scenario, the $30 saves destroying a second expensive cell and at the same time makes it much more likely that carrying a single backup cell would cover more failure modes. The math starts to look quite favorable for having a BMS to help with a sudden cell failure in this case.

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.

I really think you are thinking about the average user when you make a lot of your comments, and I think most of what you have written is exactly right for that user. But when we start scaling up, the tradeoffs creep in quickly. It's probably unfair for me to force the conversation always to include a huge spectrum of battery pack sizes, but on the other hand, I know it is a mistake to simply talk about this as if everyone is just assembling a small 12V pack.

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.

Agreed. I feel like in general the risks are pretty acceptable all around, especially once infant mortality is factored out. (And despite all my blabbering up above, I think for most users in most situations, a non-balancing BMS is completely appropriate.)

[CatNewBee]

Yes, you have a point, a high voltage system (48V and above) built with a many parallel battery strings adds a lot of complexity for monitoring.

My focus is on 12V, since this is very common, 24V systems are also around, but above is quite rare in boats and RVs.

48V, 96V or higher are often either for propulsion or stationary power walls. This systems have different requirements. But even in automotive high voltage propulsion batteries, there are pack monitoring BMS's in place, ok, a Tesla costs 100 grand, and the BMS is a tiny fraction of the cost...

[nebster]

Originally Posted by CatNewBee

My focus is on 12V, since this is very common, 24V systems are also around, but above is quite rare in boats and RVs.

48V is actually the standard selected by the one meaningful standalone vendor in the RV space today. There are a lot of reasons 16s is the sensible choice once packs get above 10 to 15kWh. I predict we will see most packs move to this voltage as density improves, cost goes down, and appetite for mobile electricity goes up.

48V, 96V or higher are often either for propulsion or stationary power walls. This systems have different requirements. But even in automotive high voltage propulsion batteries, there are pack monitoring BMS's in place, ok, a Tesla costs 100 grand, and the BMS is a tiny fraction of the cost...

In automotive packs at any voltage, the performance envelope is dramatically different. A BMS delivers a lot more value, in both protection and performance, for those use cases.

[klaus53123]

HI Folks,

I have 4 Winston 400 Ah, 3,2 V units on my boat.

After six years I am under the impression they need replacement. Unfortunately it seems that I am either too stupid or too blind to find a source for these units.

Are they not sold anymore???? In Europe I can still get them, however they wont ship here.

I would appreciate it a lot if somebody could help me with some suppliers (doesn’t have to be Winston, but need the 400 Ah)

Many thanks
Klaus

[CharlieJ]

@klaus53123: Here is one source that recently popped up in the sidebar adverts. Have absolutely no knowledge of them: Authorized sales representative of Thunder Sky Winston Battery Limited

[newhaul]

Originally Posted by klaus53123

HI Folks,

I have 4 Winston 400 Ah, 3,2 V units on my boat.

After six years I am under the impression they need replacement. Unfortunately it seems that I am either too stupid or too blind to find a source for these units.

Are they not sold anymore???? In Europe I can still get them, however they wont ship here.

I would appreciate it a lot if somebody could help me with some suppliers (doesn’t have to be Winston, but need the 400 Ah)

Many thanks
Klaus

I suggest you send a pm to Winston batteries . Sponsoring vendor on here .


https://www.cruisersforum.com/forums...es-223045.html

[Steve_C]

Originally Posted by klaus53123

HI Folks,

I have 4 Winston 400 Ah, 3,2 V units on my boat.

After six years I am under the impression they need replacement. Unfortunately it seems that I am either too stupid or too blind to find a source for these units.

Are they not sold anymore???? In Europe I can still get them, however they wont ship here.

I would appreciate it a lot if somebody could help me with some suppliers (doesn’t have to be Winston, but need the 400 Ah)

Many thanks
Klaus

May we ask why you feel they need replacement? 6 years is not very long, could you provide some specifics for how they are used, charged, discharged etc... Are you using a BMS? I would be very interested as I am sure others would be, to know why they need replacement after only 6 years!

Thanks Klaus!

[bill good]

Klaus,


Were they new when you put them into service?? 6Yrs is way too short of expected life.



Regards Bill

[klaus53123]

Originally Posted by Steve_C

May we ask why you feel they need replacement? 6 years is not very long, could you provide some specifics for how they are used, charged, discharged etc... Are you using a BMS? I would be very interested as I am sure others would be, to know why they need replacement after only 6 years!

Thanks Klaus!

Well, I am finally not too sure wether they really need replacement. Actually it is only one which I suspect to be problematic.

I have a BMS and use them about half the year. During that usage period i charge them to 3,42 Volt (the BMS interprets the end of charge voltage reached as 100% SOC) and use 90% SOC as trigger for recharge to start.

During the other half year when I am not at the boat I set the BMS to stop charging at 3.36V (which is then automatically defined as 100% charge) and again the 90% SOC trigger is used for recharge.

The reason why I suspect one battery (battery no 4) to be at least weak, is the following: the second year when I left the boat, I didn’t charge at all. I thought I had all consumers switched off which I actually hadn’t so something sucked my batteries empty. The battery I suspect to need replacement was the lowest on voltage (2,5 if I remember correctly) and is the one which has the highest internal resistance when full.

Batteries where new when I received them and I top balanced them prior to installation.

Under normal operation voltages only vary a few 1/1000 V. For example: right now I have 93% SOC, 6A consumption and Voltages are: 3.327, 3.326, 3.323 and 3.323)

However when I increase discharge to say 100 A, my forth battery drops faster than the other. However when discharge stops, they come to close V values very soon.

Hope all questions answered.

Best regards
Klaus

[CatNewBee]

Way too low, batteries almost empty. Charge them appropriate from time to time to 3.65V (14.6V to allow balancing and full charge. They starved almost. You may need to manually balance them first before you charge the bank to not run in over voltage protection on a single cell.

It is really ridiculous how many people threat their battery for longevity, it's like vegan low carb moms don't giving milk to their babys to keep them on "healthy cerials and soy milk" organic food, cholesterol, fat, sugar and lactose free. For a healthy life the battery cells need propper feed, as humans too, charge them, use them, they are meant for this.

[klaus53123]

Balancing manually will be an issue. I do have an adjustable power supply on my boat but I it needs 220V input which I don’t have here. Can somebody please advice on a unit to buy which I could use for 110 V?

3,6 V sounds quite high to me are you really sure?

[CatNewBee]

Originally Posted by klaus53123

Balancing manually will be an issue. I do have an adjustable power supply on my boat but I it needs 220V input which I don’t have here. Can somebody please advice on a unit to buy which I could use for 110 V?

3,6 V sounds quite high to me are you really sure?

Cells are full at 3.65V according to the specs, inititial charge is 4.0V, 2.5V is the low voltage cut off for Winston cells.

Operating Voltage for 80%DOD is 2.8...3.8V

I operate mine between 3.0V and 3.55V. But I have configured my BMS to start balancing earlier, so no problem.
Read what your BMS supplier requires.

The cells are resilient, they can cope with higher ranges when cycled regulary, only storage may be an issue on both sides, too low or too high for a long period.

[klaus53123]

Besides the issue with this one cell I have also an issue with my BMS. I want to leave the batteries as they are with no charge during my absence. They will show a pole current of say 3.33 V when I will disconnect them completely and let them sit for 8 months. With no charge.

Assuming a discharge of 1% per month, that would not do them too much harm. Would you agree?

BTW: I start balancing at 3.34 V. Does that make sense?

[Wood]

I'm using a MAISHENG Triple-Output 30V 30A Linear DC Power Supply. Less than $90US.

My cell specs are 3.65V max. I think getting that high is needed occasionally. And needed for capacity testing.

[klaus53123]

Ok, I tried to charge them higher and set the end of charge to 3.5V. Whilst batteries 1-3 reached that threshold with voltages very close together, battery 4 remained at 3.14 V.

Now the question is: does battery 4 just require a manual top balancing or is it damaged? Any idea how to find out.

Once again I would appreciate it a lot if you could advice on a charger suitable to top balance with.

Many thanks
Klaus