Last fire.. :-(

[Gazoo]

What maximum discharge levels are recommended for being ‘gentle’ and extending cycle-life?

I would not worry about that too much. You paid for the cells....use them based on what you need and don't stress over it. You already know a cell that's fully cycled each day in ideal temps will last for at least 5 years. I think keeping the cells between 3.0 and 3.4 volts is the norm for extending cycle life. We need more data concerning this.

I’ve been discharging my 8S pack to 25.3V (or ~3.16V on my well-bottom-balanced pack) at a discharge rate of ~20A or less than 0.04C and the voltage pretty quickly snaps back to 25.65V or just over 3.2V per cell soon after low voltage disconnect…

A cell discharged to 2.5 volts will bounce back to higher voltages fairly quickly.

I think EVE recommends replacing the cell when the capacity drops to around 70%. Some manufacturers recommend replacing them at 80%. I wonder if this is because they are concerned about possible internal shorting if going beyond that?

 

[A.Justice]

Yes, this has been confirmed in the literature:

ShieldSquare Captcha

iopscience.iop.org

Full article from behind the paywall.

Iron dendrites - JustPaste.it

justpaste.it

 

[Luthj]

possible internal shorting if going beyond that?

I am sure that's parts of the consideration. I think the major driver is divergent IR between cells. An engineer that worked on telecom backup power systems indicated that self discharge going very high was a main driver for their banks which spent lots of time floating. Eventually the self discharge variance between cells exceeded the systems ability to balance.

 

[fafrd]

I am sure that's parts of the consideration. I think the major driver is divergent IR between cells.

Can you be a bit more specific about what you are referring to? Is this for parallel sets of cells only (a string formed from more than one cell in parallel?

An engineer that worked on telecom backup power systems indicated that self discharge going very high was a main driver for their banks which spent lots of time floating. Eventually the self discharge variance between cells exceeded the systems ability to balance.

I’m still unclear exactly what this is a reference to.

Parallel cells or also single-cell strings?

‘Balance’ as in top-balance? (Meaning active or passive balance above the knee of the charge curve?)

 

[Luthj]

Can you be a bit more specific about what you are referring to? Is this for parallel sets of cells only (a string formed from more than one cell in parallel?


I’m still unclear exactly what this is a reference to.

Parallel cells or also single-cell strings?

‘Balance’ as in top-balance? (Meaning active or passive balance above the knee of the charge curve?)

When cells are in series, self discharge imbalance results in one cells SOC drifting down compared to the others. This it typically remedied by the BMS balancing at the top of each charge. Even if the bank is in parallel series, each parallel cell group can suffer from high self discharge. If the self discharge exceeds the BMS ability to balance, then that cell will drift downward, eventually getting low enough to cause operational issues. This doesn't appear to be a common issue with good cells, but when you have a cell thats 70, 60, 50% of new capacity, self discharge can be dramatically higher.

 

[fafrd]

When cells are in series, self discharge imbalance results in one cells SOC drifting down compared to the others. This it typically remedied by the BMS balancing at the top of each charge. Even if the bank is in parallel series, each parallel cell group can suffer from high self discharge. If the self discharge exceeds the BMS ability to balance, then that cell will drift downward, eventually getting low enough to cause operational issues. This doesn't appear to be a common issue with good cells, but when you have a cell thats 70, 60, 50% of new capacity, self discharge can be dramatically higher.

Got it - thanks.

Is there any data / papers on how self-discharge rate increases as LiFePO4 cells age / wear?

When I was top-balancing, I was surprised by how much charge it would take to ‘top off’ my battery after letting it settle overnight.

3% of capacity per month (0.1% per day) is the common spec you can find but I was needing to top of with at least double if not triple that amount of charge.

If increased self-discharge is another sign of substandard, used, or non-Grade-A cells then perhaps we should be characterizing self-discharge rate of new cells (in addition to capacity and IR)?

 

[Luthj]

If increased self-discharge is another sign of substandard, used, or non-Grade-A cells then perhaps we should be characterizing self-discharge rate of new cells (in addition to capacity and IR)?

For determining if a cell will perform long term, this could be a useful metric. I think that most systems which see regularly cycling, won't encounter issues from normal range self discharge. I would also bet that IR and capacity variance would also correlate strongly with self discharge, as the mechanical factors which affect one, would probably affect the others.

 

[toms]

3% of capacity per month (0.1% per day) is the common spec you can find but I was needing to top of with at least double if not triple that amount of charge.

You were probably lithium plating rather than charging with the extra amp-hours

 

[eabyrd]

A question about "relative" risk between inverter chargers on the one hand, pre-made (factory) batteries on the other, and DIY batteries on the third hand. From what I've read the DIY battery packs are the primary fire risk factor; if I'm misunderstanding that someone will certainly correct me. What about the factory batteries like the Big Battery Husky (48V 100AH LIfePO4) or the Chins-Plus (12V 100AH LifePO4). How would you grade the risk of catastrophic fire from those units as compared to the DIY banks?

What about the inverter itself like the MPP LV6548 (UL Certified, but not listed) or the LVX6048 (No UL Listing) or the various Growatt inverters. From what I've read a catastrophic failure in the inverter generally results in much sound & fury (big bang, stinky smoke), but no lasting fire risk, but again please correct my understanding if I'm wrong.

Thanks
E

 

[fafrd]

You were probably lithium plating rather than charging with the extra amp-hours

Details appreciated. Is there some minimum time you want to let LiFePO4 cells settle / self-discharge before topping-off the SOC?

 

[upnorthandpersonal]

Is there some minimum time you want to let LiFePO4 cells settle / self-discharge before topping-off the SOC?

If you can measure state of charge accurately, 90% SoC cut-off. If you can not, 3.375V per cell (same as the float voltage in Victron charge controllers) seems pretty decent.

 

[upnorthandpersonal]

This thread and others in this section of the forum jump-started the idea of a 'best practices handbook' for DIY battery construction that has the main goal of eliminating the root causes that lead to situations like this. The initial outline and draft is here:

DIY Battery Handbook

With the increasing amount of users on the DIY Solar Forum and more people exploring the options to build their own battery packs we've seen common mistakes and dangerous situations occur more frequently - sometimes with disastrous outcomes. While many of the issues we come across have been...

diysolarforum.com

If you're interested in contributing, feel free to do so in our discussion thread:

DIY Battery Handbook Initiative

Considering the amount of users this site is seeing and the amount of people that are embarking on building their own battery (and the repeating issues we see), we thought it would be a good idea to compose a clear and concise recommended (best practices) document. This document would explicitly...

diysolarforum.com

 

[fafrd]

If you can measure state of charge accurately, 90% SoC cut-off. If you can not, 3.375V per cell (same as the float voltage in Victron charge controllers) seems pretty decent.

I think we’re talking about different things.

When charging, aiming for 90% SOC or 3.375V makes total sense (and currently, I pretty much never get close to that, typically reaching no more than 3.36V before beginning discharge).

But reference was made to ‘lithium playing’ when I referred to topping off my bank during top-balance.

I only top balanced to 3.5V and after building my 8S pack, ‘checked’ my top balance by discharging by ~10% and charging back to 28V.

I also let the fully-charged 8S battery settle overnight and once over the course of a full week before topping off (recharging to 28V / 3.5V).

I know that in general it is not recommended to store LiFePO4 cells / batteries at 100% SOC.

Who knows if I’ll ever be building / balancing another LiFePO4 battery ever again, but just in case, I was interested to understand under what conditions ‘Lithium Plating’ can occur and whether there is generally a maximum SOC (such as 90% or 95%) above which you should avoid charging to avoid causing Lithium Plating.

 

[eabyrd]

I would like to express this gently as I know that people try their best. The idea that DIY lifepo4 batteries are a fire risk is an unfair generalization. It is more fair to say that these projects give people the opportunity to make bad choices with bad consequences. This thread and the earlier thread of fhorst's first fire event are good examples of this. Look at the pictures of the build. Go back and read the build thread. If someone can read these threads and aren't appalled at some of the choices made and decisions made then this may not be an activity that is suitable for that person.

The truth is that many of the things we embark on whether it be building race cars, jumping out of airplanes or hiking in the back country, require a certain level of caution and awareness that not everyone possesses. That doesn't make them stupid by any means. My wife is a country mile from being able to do something like this and she is one of the smartest people I know. It's just that her abilities and brilliance is in a different part of the human experience than manufacturing mechanical and electronic systems. She is working in the next room and what I hear her doing requires skills I will never have.

We need to go into this with our eyes wide open about our abilities and the requirements of this endeavor. It isn't building batteries that is dangerous but we can certainly drive the risk our of sight if we don't understand materials, methods and safety protocols. Supporting a human's weight by the studs of battery cells is shocking. Crushing (recompressing) expanded cells after they have bloated is certainly risky. Drilling and tapping wallowed out terminals of unknown thickness is a terrible idea. The mess of wiring I have seen may not have started the problem but the repeated acts of carelessness that I have witnessed are indicative of a trend that could only be successful by blind luck. I suspect this isn't rare but documenting it in public probably is and I am grateful for the profound humility that it takes to have done so. Accepting responsibility for our mistakes isn't easy and is sometimes a process.

I have chosen to build my own because I haven't seen a ready made pack that I'm that comfortable with. They are mostly built to a price point that creates compromises. Mostly they don't include proper fusing or top quality BMSs. Mostly they aren't built to be mounted in a way that makes them easy to remove from the system in case of problems. Many are designed in a way that when parallel connection them, each bank is very different from the others in the resistance it sees due to daisy chaining them instead of keeping the wire runs equal. There is a long list of reasons that DIY can be superior without into getting into long term maintenance and repair.

Stay safe y'all. I 100% do not intend to insult anyone. I never judge a fish by its ability to climb a tree. Not everyone should take up this challenge.

Again, I was asking about the relative risks of the 3 classes of objects. I understand that a cell is a cell is a cell (by chemistry and grade that is, mostly), and I am not trying to question anyone’s craftsmanship, but to your own point deciding to construct your own battery from individual cells provides opportunity for failures that are not there if one purchases a fully constructed battery. Thus from a relative risk standpoint the factory battery is less of a risk

then there’s the potential for damage should a catastrophic failure occur,

 

[toms]

Details appreciated. Is there some minimum time you want to let LiFePO4 cells settle / self-discharge before topping-off the SOC?

Any time the battery is fully charged there will be permanent lithium plating.

My battery manufacturer recommended never going into the upper knee once the SEI formation process was complete.

Others on this forum have expresses that they “don’t think it’s doing much damage”.

I would be looking for conformation on that before performing repeated low current / high knee voltage journeys.

Keep in mind that in a series assembled pack, if the variation in self discharge is lower than the available balancing, and the pack has acceptable initial capacity - it is pointless to top balance.

Using an appropriate BMS (ie one that tapers current to match balancing ability) is in my opinion THE most important step in a reliable pack.

 

[fafrd]

Any time the battery is fully charged there will be permanent lithium plating.

My battery manufacturer recommended never going into the upper knee once the SEI formation process was complete.

Others on this forum have expresses that they “don’t think it’s doing much damage”.

Thanks for the added detail. I’ve already completed ~10 of those ‘journeys’ so hopefully I haven’t caused more plating / capacity degradation than I would have had I charged to 100% from 0% 10 times (datasheet).

What is SEI, if I can ask?

I would be looking for conformation on that before performing repeated low current / high knee voltage journeys.

Repeated was only ~10 times when top-balancing the pack. Never get anywhere above 3.36V now that the battery is in use these days…

Keep in mind that in a series assembled pack, if the variation in self discharge is lower than the available balancing, and the pack has acceptable initial capacity - it is pointless to top balance.

Agreed, though most BMSs only activate their balance functions pretty high into the knee (meaning that you may be forced to charge to a higher SOC than you’d otherwise prefer to get any balancing dine by your BMS arc all…

Using an appropriate BMS (ie one that tapers current to match balancing ability) is in my opinion THE most important step in a reliable pack.

I’m not clear on what you mean by ‘tapers current to match balancing ability.’

In my case, I elected to go for a bottom balanced battery rather than a top-balanced battery (because I drain every night and never fill close to Float) and I’m going to eventually rig up some wizardry to only turn in my active balancer after battery has hit LVD and shut down for the night.

 

[Luthj]

Every source I have seen thus far indicates that lithium plating at any significant level requires either very low temperatures, or very high currents (1C in many cases). My understanding of the mechanic is that lithium deposition in LFP requires the ion mobility threshold of the electrolyte and anode to be exceeded. For this to occur near the top of the SOC range would require high charge rates, which saturate the surface of the anode, while there is still plenty of ions in the electrolyte and cathode. A low charge rate by its nature gives the anode lots of time to incorporate ions into its structure.

If instead you are referring to SEI (Solid Electrolyte Interface) disruption, that would be a different mechanic and discussion altogether.

As always I am interested in new research on the subject.

 

[fafrd]

Every source I have seen thus far indicates that lithium plating at any significant level requires either very low temperatures, or very high currents (1C in many cases). My understanding of the mechanic is that lithium deposition in LFP requires the ion mobility threshold of the electrolyte and anode to be exceeded. For this to occur near the top of the SOC range would require high charge rates, which saturate the surface of the anode, while there is still plenty of ions in the electrolyte and cathode. A low charge rate by its nature gives the anode lots of time to incorporate ions into its structure.

If instead you are referring to SEI (Solid Electrolyte Interface) disruption, that would be a different mechanic and discussion altogether.

As always I am interested in new research on the subject.

Well, when I was tipping off, we’re talking about a charge rate of less than 0.02C, so sounds as though lithium plating is at least one thing I shouldn’t need to worry about too much…

 

[Luthj]

Well, when I was tipping off, we’re talking about a charge rate of less than 0.02C, so sounds as though lithium plating is at least one thing I shouldn’t need to worry about too much…

That is my understanding. At rates that low, you should deplete the free ions before you saturate the anode enough to cause plating. Obvious you are operating within the envelope described by the manufacturer. Its hard to believe any reputable cell maker would have such a serious flaw as plating at low charge rates, and none of us know about it. That being said, a badly made cell could have too much lithium ion material, and thus be capable of plating near full charge. That a manufacturing flaw though, and beyond the scope of this discussion. I can't find the reference, but from memory, there is a continuous curve which describes the conditions required to plate the anode. It drops to zero current near freezing, and rapidly ramps up as the cell warms. At some point the current requires way in excess of 3.6V for most cells. If I recall the heating from charging at this rate near room temperature would cause most cells to overtemp before the plating threshold is reached.

 

[toms]

Every source I have seen thus far indicates that lithium plating at any significant level requires either very low temperatures, or very high currents (1C in many cases). My understanding of the mechanic is that lithium deposition in LFP requires the ion mobility threshold of the electrolyte and anode to be exceeded. For this to occur near the top of the SOC range would require high charge rates, which saturate the surface of the anode, while there is still plenty of ions in the electrolyte and cathode. A low charge rate by its nature gives the anode lots of time to incorporate ions into its structure.

If instead you are referring to SEI (Solid Electrolyte Interface) disruption, that would be a different mechanic and discussion altogether.

As always I am interested in new research on the subject.

Our understanding of lithium plating (dendrite growth) and SEI (solid electrolyte interface/interphase) are different.

While most online sources i have read state the SEI is formed from electrolyte decomposition, there is also solid lithium deposits within the SEI. The initial formation process is vital to ensure that there are no “seeds” of solid lithium that will grow into dendrites.

Most university undergrad (ie almost every online paper) use high current charge/discharge and correctly observe lithium plating. They need to do this in order to have a paper to submit. If they did very slow current charging at cell saturation they would also observe lithium plating. My cell manufacturer has tests in this area that have been ongoing for over a decade.

In another decade the evidence (one way or another) will be conclusive.

It’s interesting that some of the leading battery chemists cannot come to an agreement on exactly what reaction is occurring at the anode of a lithium cell - us mere mortals have no chance.

I’m not sure how you are separating SEI formation from lithium plating, the two are closely related.

Like you, i’m always interested to get as much information and different views on these batteries as possible.