Mechanisms that decrease the Lifespan of Lithium-Ion batteries and how to avoid them

[Mike90250]

Hi all,

I'm not a regular poster here, but after similar encounters with Sunking on two other forums, I was approached by Simon to ask ask if he could quote one of my posts (which can be found on the battery technology forum at endless-sphere in a thread called "lithium balance--is it top, bottom, or middle?"). I said he could post with or without attribution, which is good because the moderators here have apparently forbidden attribution, which I find difficult to understand...........

Welcome wb9k

I think it's not "attribution" but active links to other forums that is the issue.

I personally don't have the time to do line by line comparisons of posts to check for errors or data conflicts and then rule which is correct. I think a forum is a great venue to compare techniques and results. We do have to be careful because neophytes can come along, take a post out of context, and having no other knowledge, set their <noun> on fire.

No user serviceable parts inside

[wb9k]

Reality check.

Hi all,

I'm not a regular poster here, but after similar encounters with Sunking on two other forums, I was approached by Simon to ask ask if he could quote one of my posts (which can be found on the battery technology forum at endless-sphere in a thread called "lithium balance--is it top, bottom, or middle?"). I said he could post with or without attribution, which is good because the moderators here have apparently forbidden attribution, which I find difficult to understand. The lengthy quote above contains my words, not Simon's. More importantly, they are not incorrect at SK asserts, they are hard facts which I can report confidently, since I run the warranty lab at A123 Systems in Livonia, MI and these are all statements about customer practices with which I am familiar. SK never even attempted to address any of these statements in the thread at E-S, or any other point that challenges his many unsupported (and incorrect) assumptions about Li ion cells and practices in using them. Like me, Simon finds this trend to be disturbing and potentially dangerous and has sought to correct the misinformation being persistently disseminated by SK. Also disturbing is his parallel refusal to actually debate the facts. He simply makes unsupported, untrue statements and then ridicules anyone who dares to disagree, no matter who they are or their level of knowledge on the subject. No actual data is ever given by SK in any of the threads where we have had this..."discussion". I don't want to spend a lot of time here repeating the work I've already done on two other forums, but after seeing my words put here and disregarded out of hand yet again by SK, who knows damn good and well that these are not Simon's words, but those of an industry insider who knows better than he does, I feel it's my responsibility to set the record straight--yet again--for the benefit of readers who are at the mercy of what they read online and just want to proceed in this field correctly and with confidence.

I hope to spend the bulk of the rest of my day working on my electric truck project. Anyone who is interested in knowing more about me and my knowledge base can peruse my many posts at ES and a few at DIY Electric Car. Good luck to all!

dh

[Sunking]

We must have been looking at different videos. There is only discussion of the different types of Lithium cells used in cars up to around 27 minutes. This was all about temperature, nothing about how SOC affects the lifespan.

You do not listen very well huh? The whole lecture was about lifespan.

[Sunking]

Sad that you deleted the paragraph with links that show Sunking at his best. Below is the text from one of the posts in that thread that has some important and useful information in it. I don't usually like posting without attribution but looks like I have to.

You can quickly tell Simon is incorrect because no EV manufacture takes their batteries to 100% SOC. If they did none could offer the warranties they offer.

What else did you expect a manufacture for Bicycle Battery BMS to say? Did you expect them to say: you do not want or need my product so don't buy it? He has no other choice but to tow the company line, otherwise he is a Conflict of Interest. Could it be you just hate to admit there is another way that bypasses your vested interest. That most Lithium batteries out there do not use any BMS or do not top balance.

Personally I think you should Top Balance because you do not know how to do anything else or a lot about lithium batteries. I think every consumer should use a BMS and Top Balance if we are talking about commercial mass market. But here is what is really going to suk for you and your company. Companies are coming out with manufactured Bottom and Mid Balanced Systems. Consumers will have a real choice. Those that know what they are doing can Bottom Balance and get more out of their battery. You cannot do that because your eyes are closed and is a conflict of interest.

[karrak]

Hi Simon pete here, I removed the links to the other forum. I will be doing this whenever I see it. I am sick of bleeding page rank to other sites.

Sad that you deleted the paragraph with links that show Sunking at his best. Below is the text from one of the posts in that thread that has some important and useful information in it. I don't usually like posting without attribution but looks like I have to.

I started to go through the OP, planning to knock down points one by one, but I just don't have the patience...not today.

The problem here Sunking, and with all the other guys over at DIY Electric Car who think they can bottom balance and not bother with any cell level monitoring, is a plethora of incorrect assumptions upon which is built a house of cards.

OEM's DO NOT balance plug-in vehicles at less than 100% SOC. The 20%-80% thing you describe applies only to HEV's...where accurate balancing is extremely difficult (and thus risky) at the extreme edges of charge, so it is generally (but not completely) avoided. Longer life is an ancillary benefit, and some have learned to manipulate this and take advantage of it, but this is unique to HEV's. PHEVs and full electrics live in a range-crazed world, and, AFAIK, every OEM out there takes their packs to 100% SOC during a complete plug-in cycle. I know for a fact that this is the case with GM (Chevy Spark), the Fisker Karma, and cars built by Chinese OEM SAIC. I don't know for certain what Nissan and Tesla do, but I would be very surprised to find them doing anything different. So, this is one of your cornerstone assumptions blown out of the water--it just isn't true.

Here's another...OEM's DO NOT spend a lot of time sorting cells. If they build the pack themselves, they may do a visual inspection of the cells and check OCV out of the box. They may run tests on a small sample of incoming cells, but they DO NOT hand-sort, match and select every cell for their packs. This would be an enormously expensive endeavor, one that OEM's expect the battery maker is already providing through their own quality test processes. So here is another fundamental assumption in your thinking that is just plain wrong.

You also dramatically overstate the detriment of taking a cell to 100% SOC. Keeping a cell at 100% SOC for 24 hours will destroy it? Nonsense. Let's talk about the mechanisms at work in LFP. If you take a cell over 3.60 Volts (RESTED), you will plate Li onto the cathodes, causing permanent capacity loss. This is not something that happens terribly rapidly...unless you were to do it habitually, you probably would not even notice anything at all, and even then it would take weeks, or more likely months, before the trend became visible. It's a bad thing to do, but nowhere near as severe abuse as driving a cell negative, which is indeed nearly an instant killer. If you don't exceed 3.60 Volts, the only stress you are subjecting the cell to is a slightly accelerated growth of the SEI layer. The rate of this is roughly proportional to SOC. If you want zero SEI layer growth (the main cause of Li cell degradation under normal, proper use), keep your cells at 0% SOC, or hold them below 50 degrees F at all times. These are obviously not practical in a battery you wish to use on short notice, which is why nobody does them. The SEI growth difference between 80% and 100% is not great, and 100% SOC should not be considered an abusive condition.

Speaking of what the OEM's do.....every one of them uses extensive BMS systems with cell-level voltage monitoring and balancing electronics. Why does this not play into your considerations, if you're trying to get to what they do? The fact is, without these things, you are flying blind. A cell defect or other anomalous condition can get past the safeguards you recommend if it is severe enough, or is allowed to fester long enough (and the bigger the pack, the more likely a user is to fall into that trap.) THE PRACTICES YOU AND OTHER ARE ADVOCATING HERE ARE EVENTUALLY GOING TO KILL SOMEBODY, IF THEY HAVEN'T ALREADY . I may seem like an ass when discussing this topic, but that is the bottom line right there. It might happen only in 1 car out of 100 or 1000, but that level of risk is still totally unacceptable and taking that kind of risk would land OEM personnel in jail. It probably seemed like hyperbole when I first stated that at DIY EC, but just a couple weeks later the justice dept. announced they were looking at criminal charges against GM personnel for the ignition switch issue...which has a FAR smaller risk than what you are advocating here.

A small pack on a bike (in plain sight at all times) or similar in the hands of a technically competent individual carries a small risk and still requires vigilant, regular babysitting. The larger a pack gets, the greater the energy potential, the greater the risk. A pack like this tucked away in a car (out of sight, out of mind) in the hands of a technically competent individual is riskier than it should be. In the hands of a lay person (who believes you when you say it's perfectly safe), it's a death trap. The practice is quackery, plain and simple. Do yourself a favor and stop before it's too late.

Simon

[Living Large]

The real truth is that much of it is determined by how fast you charge in the bulk stage. For instance, charging at 1C or more, you CAN take these cells up to the extremes of the spec sheets, like 3.8v - 4.0 because you don't spend much time there. On the other hand, if you are charging at say the C/20 or .05C rate, which happens to be the rate at which is commonly the absorb current cutoff, then 3.45v to maybe 3.5v max would be the ideal. You are already at the absorb rate right from the beginning and TIME spent going any higher than that is bringing out the parasitics. Thus slow charging may be more a factor in degradation than fast charging. BUT, keeping it in the context of a typical solar user, we will never have 1C available for use with an array for a bank appropriately sized for autonomy and the generality of going no higher than 3.5v is a safe value without going too far into detail, like how that might leave off some real desirable usable capacity at the high-C rate.

This is fascinating discourse. I had not considered the relationship of charge currents of the generator vs the array to the optimal voltage threshold to charge an LFP bank to. I'll have to make a note of this in the event I actually pull the trigger. There's a lot to know, that's for sure. I can greater appreciate those who recommend a "starter bank".

[PNjunction]

The real issue is that everything seems to get taken out of context, including the operating environment. Thus generalities don't always apply in every case, and everyone is searching for the magic-number (voltage or current) to bring out the magic wand.

Lifepo4 voltages are obviously different from other lithium chemistries due to the materials, but the issue of keeping them too long at elevated soc's is a problem, whatever that voltage happens to be.

The real truth is that much of it is determined by how fast you charge in the bulk stage. For instance, charging at 1C or more, you CAN take these cells up to the extremes of the spec sheets, like 3.8v - 4.0 because you don't spend much time there. On the other hand, if you are charging at say the C/20 or .05C rate, which happens to be the rate at which is commonly the absorb current cutoff, then 3.45v to maybe 3.5v max would be the ideal. You are already at the absorb rate right from the beginning and TIME spent going any higher than that is bringing out the parasitics. Thus slow charging may be more a factor in degradation than fast charging. BUT, keeping it in the context of a typical solar user, we will never have 1C available for use with an array for a bank appropriately sized for autonomy and the generality of going no higher than 3.5v is a safe value without going too far into detail, like how that might leave off some real desirable usable capacity at the high-C rate.

Resting voltages are a better way to go, but impractical, since to do it right, you need to wait 12-24 hours, which for solar guys, means we are in the dark waiting.

Ideally, a balancer in the top-balance camp would measure the bulk rate, or perhaps the average if solar driven, and readjust the top balance voltage accordingly. Throw in some temp compensation at the extremes for good measure.

The guy who bottom-balances a variable current solar array system, once again sized properly for autonomy, generally does well with a conservative value for his highest-cell trigger voltage because he can't provide the current that an EV system can - other than using a genny - which brings out yet again a reason to be dynamic in thought...

The beauty of setting up a SMALL system like I did (40ah , 4S) lifepo4 allows me to experiment and watch details for both camps very fast. Some issues like heating electrolyte can be masked by using a very large system because you don't allow for enough time to see it happening - but it is! How do I know? The prismatics are my "large system". My smaller systems comprising of A123 powersports cylindricals allowed me to compare things even faster. (albeit my preaching about cylindricals not being the right tool for a constant-drain job - I got them anyway for scientific monitoring).

Being able to do charge / discharge and capacity tests on my own with these admittedly small systems, allows me to the opportunity to see things BOTH ways and still have great results without a lot of degradation - but always keeping in context my solar relatively low-current autonomy environment. So as the typing drags on and on in the EV forums, I'm happily using my bank and expect many many years of faithful service. I can switch streams at any time - I just choose what's the most practical at the time.

So instead of just slinging words back and forth, I put my Fluke gear to work, measured discharges, purposely overcharged a little to observe phenomenon and just did it for myself. Anyone safety conscious can do the same which cuts through a lot of forum(s) chatter.

[karrak]

Huh? You did not pay a lot of attention. The whole video was about Lico, LiMn, and LFP the three types used in EV's and Energy storage. He talked exclusively about batteries used in Tesla, Nissan, Chevy, and Fiskers. 3.6 volts is 100% SOC for LFP batteries and nominal 3.2 volts. LiCo is 4.2 @ 100% SOC and 3.6 volts nominal. Solar for now is dominated by LFP for 4 very good reasons.

We must have been looking at different videos. There is only discussion of the different types of Lithium cells used in cars up to around 27 minutes. This was all about temperature, nothing about how SOC affects the lifespan. The main thrust of this video is how testing the efficiency of the cells can more quickly determine the impacts of temperature, additives and other factors on the lifespan of the cells rather than using the traditional cyclic testing.

Overheating batteries is not such an issue for stationary applications where you are charging below 0.1C

Incorrect. Charge rates have to be higher than C/10. In fact with Solar you have to charge faster than EV's Minimum for FLA is C/10 and that is with 5 day reserve capacity. With Lithium you can design using 2 or 3 days autonomy which means you are charging at much faster higher rates. For a 1 Kwh with 3 sun Hours requires a 500 watt panel. At say 12 volts requires a 250 AH LFP battery, and a 400 AH FLA battery. The C charge rate for lithium is much higher at C/6 (.17C)than FLA C/10 (.1C) charge rate. C6 is about what a level 2 charger provides to an EV or slightly higher. On the Discharge side depends on what EV you are talking about. Tesla products discharge slowly due to over sized battery. To maintain 60 mph the motor uses roughly 12 Hp or 9000 watts. On a 60 Kwh 400 volt battery is only 22.5 amps on a 150 AH battery is C/7 rate. So your premise that a solar system does not run at the extremes of an EV is false. In fact you can make the case a solar system usin gLithium batteries uses higher C rates than an EV. Proff is in the numbers. You just have not looked at them. and assumed incorrectly.

I don't have to assume, as I have said before on multiple occasions I have an off-grid system with LFP batteries with about three days of autonomy. My maximum measured charge was 41.2 amps (0.11C), the average daily maximum would have to be somewhat less than this, probably around 30 amps which is less than 0.1C. My highest measured discharge current from my battery is 245.7 amps (0.68C), the average daily maximum would be less than 0.5C.

What you and most are missing the point is a Bottom Balanced system is Passively Safe by design. In a Bottom Balanced system you completely eliminate the risk of an Over Discharge to Polarity Reversal. It takes absolutely Zero Equipment or Investment Cost to get it. It takes ZERO AUTOMATION and completely Passive Fail-safe. When you Top Balance you set up the Perfect Storm conditions to set up a fatal Over Discharge condition. You have engineered a failure point and more frequent battery replacement with shorter cycle life. You also now have to rely on a high degree of error prone automation to protect yourself from a deadly Over Discharge. That automation is expensive and not fool proof. You gain nothing but risk and shorter battery cycle life.

That is what the video clearly told you.

Maybe there is some subliminal message, I didn't get any of this from this video.


Simon

Hi Simon pete here, I removed the links to the other forum. I will be doing this whenever I see it. I am sick of bleeding page rank to other sites.

[PNjunction]

Top Balance comes from Pb mentality and the Disposable Consumer Electronics market with Laptops, Cell Phones. Tablets, Power Tools, Gizmos, and Gadgets where the consumers demand the longest possible run times possible operating from 0% to 100% SOC.

That is true, and consider that in the field of power tool manufacturers like Dewalt (who used to use lifepo4) may have also switched to bottom balance I believe. I can't recall the exact manufacturer who does this, but the point here is that in a high-current discharge application, to protect the cells from user-abuse in high-current situations when nearing total discharge, bottom balance made better sense than the previous top balance routine.

However, if you remove the high-current drill application, and if driving nothing more than a couple of leds from a small lighting attachment, you *CAN* get away with *conservative* top balance routines, since a low-current application relative to the large capacity of the battery itself, (aka a solar housebank sized properly) can be stopped in time without going into super deep discharge or cell reversal like EV'ers can. Start out with high-quality cells manufactured well for both capacity and internal resistance - along with good wiring infrastructure of course. If you are using JUNK, like recycled or used lifepo4, then all bets are off despite your best efforts.

Yes, bottom balance is the preferred choice. However for practical reasons, if you play your cards right with conservatism and education, use high quality componentry, and are in a relatively low current application like a solar housebank is, in the right hands, top-balance (not necessarily at 100% all the time mind you!) can be a work-around for bottom balancing if you don't use junk.

I suppose we'll never come to a conclusion really. And while what is presented here is great "best practices" advice, unfortunately it gives the impression that lifepo4 is unstable and impossible to use unless you do things only ONE way. They aren't THAT delicate!

What I've been saying for a long time is not to lose track of what application you use the battery with, which here is NOT Tesla, NOT EV, NOT RC, NOT one-off diy junk, NOT trying to be a manufacturer for the next door neighbor, and not trying to use a solar housebank the size of a lunchbox. Tunnel-vision leads to missed opportunites that work, even if it doesn't agree with everyone.

We have the luxury of doing it either way with great results *in our low-current application niche* as long as the normal routines for battery installation procedures, safety, and tlc are followed.

[Sunking]

4871] Why do I think I have seen this before, maybe here somewhere in this thread http://www.solarpaneltalk.com/showth...-Solar-amp-BMS. I could do Middle Balance with Li-Ion batteries using bleed resistors (aka Vampire Boards) like Tesla does, does this mean I am not a DIY?

You or I cannot do Middle Balance or even come close to duplicating what the manufactures do. EV manufactures buy batteries by the millions at a time. They have testing labs and battery analyzers that allows them to analyze every battery and precision match them within less than 1% tolerance. Does not matter if they Balance at the Top, Bottom, or somewhere in the Middle. As long as they start from any reference point all their cells in a pack will always be Balanced. Then when we charge or discharge capacity is equal in every cell. We cannot Middle Balance, but we can mimic Middle Balance via Bottom Balance. We do not have to give up 1 AH of capacity, just remove the risk and double cycle life for our trouble. I am clueless why you are against that.

You and I cannot possible even come close to matching our cells. We have to play the cards we are dealt with. We buy our 4, 8, or 16 100 AH cells and our range is 99 to 120 AH tolerance. The only possible way we can Balance the cells with equal capacity is at the Bottom.

4871]Now me, I want to cycle as much power as possible through as small a battery as possible for as long as possible so I get the best possible economic return from the battery, and don't want to put it in an air-conditioned room if possible.

That is a recipe for the shortest cycle life, and most expensive possible route you can go. You also set yourself up for the perfect storm of Over Discharge Failure.

It is well know if you charge any of the Lithium batteries near or to 100%, you cut your cycle life in half. Taking batteries to 100% is a lead acid thing and uninformed consumer demand thing. Lithium batteries thrive in the Partial State of Charge World (PSOC), and suffer being charged above 80 to 90% SOC. Lead acid batteries suffer in a PSOC environment or anything less than 100% SOC.

Top Balance comes from Pb mentality and the Disposable Consumer Electronics market with Laptops, Cell Phones. Tablets, Power Tools, Gizmos, and Gadgets where the consumers demand the longest possible run times possible operating from 0% to 100% SOC. In two or three years our Gizmos are so Yesterday, need upgraded and replaced which comes with new batteries. The manufactures sacrifice cycle life for maximum run times just like you are doing.

For EV's and Energy storage markets there is huge investment in battery cost as they make up 1/3 to 1/2 the total investment cost. It is not a $30 to $50 battery we can afford to replace once a year. A 2 to 3 year battery is unacceptable. EV manufactures know this. They run Between the Sheets or Middle Balance 20/80% SOC. That is the sole reason they can offer up to 8 year warranties. If you had watched the video those points were made very Loud and Clear. Top Balance for Consumer Electronics. Middle Balance for long term investments.

4871]What I would like to know is how much difference will it make to the lifespan of my LFP battery in my off-grid system that is charged from solar energy if I charge to 3.4 volts/cell at an end current of C/50 rather that say 3.35 volts/cell at C/50. If the extra .05 volts makes little difference to the lifespan but may give me another 10% more charge it may be worth my while. The other issue is how much difference will it make to the lifespan if I float it at 3.35 volts rather than terminate the charge until the next day.

You can Float Lithium batteries. In fact all high end Lithium Battery Chargers are just very Simple Float Chargers. No stages or anything, just FLOAT Chargers. The only SMARTS they have is they shut off when current tapers down to a prescribed setting of usually .03 to 05C of C. Of course this assumes Top Balanced to 100% SOC.

However you can Float Lithium batteries indefinitely just like you can Pb. Only trick is to do it at equal to or less than 80% SOC voltage. When and if your batteries reach 80%, the panels can now power opportunity loads or really anything just like you would a standard Pb system. Last thing you want to do is turn the solar off when the batteries are charged. You still want utilize power from the panels if available. It would be ignorant to have your solar system shut down just because your batteries are charged up.

It is a pity that the majority of the video was about Li-Ion batteries and not LFP batteries. I often wonder if the fact that we only ever charge an LFP battery to around 3.4-3.5 volts whereas we charge Li-Ion batteries to around 4 volts for 80%SOC or 4.2 volts for 100%SOC is why LFP batteries are supposed to last longer than Li-Ion batteries.

Huh? You did not pay a lot of attention. The whole video was about Lico, LiMn, and LFP the three types used in EV's and Energy storage. He talked exclusively about batteries used in Tesla, Nissan, Chevy, and Fiskers. 3.6 volts is 100% SOC for LFP batteries and nominal 3.2 volts. LiCo is 4.2 @ 100% SOC and 3.6 volts nominal. Solar for now is dominated by LFP for 4 very good reasons.

1. Lowest price per unit
2. One of the highest cycle lifes
3. Direct drop in compatible replacement with Pb voltage range. All others voltage is not compatible.
4. Safety.

LiCo, LiMn, and LFP are three different elements. Each has its own unique voltage signature. LiCo was chosen by pure luck at the time by Tesla. Back when Tesla made the roadster, all there was available was Laptop 18650 cells made from LiCo. But Licoo is about one of the poorest choices you can make from a Safety and cycle life POV

Overheating batteries is not such an issue for stationary applications where you are charging below 0.1C [/QUOTE]Incorrect. Charge rates have to be higher than C/10. In fact with Solar you have to charge faster than EV's Minimum for FLA is C/10 and that is with 5 day reserve capacity. With Lithium you can design using 2 or 3 days autonomy which means you are charging at much faster higher rates. For a 1 Kwh with 3 sun Hours requires a 500 watt panel. At say 12 volts requires a 250 AH LFP battery, and a 400 AH FLA battery. The C charge rate for lithium is much higher at C/6 (.17C)than FLA C/10 (.1C) charge rate. C6 is about what a level 2 charger provides to an EV or slightly higher. On the Discharge side depends on what EV you are talking about. Tesla products discharge slowly due to over sized battery. To maintain 60 mph the motor uses roughly 12 Hp or 9000 watts. On a 60 Kwh 400 volt battery is only 22.5 amps on a 150 AH battery is C/7 rate. So your premise that a solar system does not run at the extremes of an EV is false. In fact you can make the case a solar system usin gLithium batteries uses higher C rates than an EV. Proff is in the numbers. You just have not looked at them. and assumed incorrectly.

What you and most are missing the point is a Bottom Balanced system is Passively Safe by design. In a Bottom Balanced system you completely eliminate the risk of an Over Discharge to Polarity Reversal. It takes absolutely Zero Equipment or Investment Cost to get it. It takes ZERO AUTOMATION and completely Passive Fail-safe. When you Top Balance you set up the Perfect Storm conditions to set up a fatal Over Discharge condition. You have engineered a failure point and more frequent battery replacement with shorter cycle life. You also now have to rely on a high degree of error prone automation to protect yourself from a deadly Over Discharge. That automation is expensive and not fool proof. You gain nothing but risk and shorter battery cycle life.

That is what the video clearly told you.

[PNjunction]

It is a pity that the majority of the video was about Li-Ion batteries and not LFP batteries. I often wonder if the fact that we only ever charge an LFP battery to around 3.4-3.5 volts whereas we charge Li-Ion batteries to around 4 volts for 80%SOC or 4.2 volts for 100%SOC is why LFP batteries are supposed to last longer than Li-Ion batteries.

I'll do a quick jump - about 25 minutes in, the chart shows the measurements for lifepo4 in the 3rd column, but I think the audience was really only familiar with non-lifepo4 batteries, such as their cellphones, laptops and such that they could easily relate to. Maybe. BUT I was glad to see that 3rd column and just adjusted any voltage references down.

The sad thing is that by now, there are probably only 8 people in the world as interested in these batteries as we are, and like all other batteries, will get murdered and replaced by consumers long before they should have been. I've got a feeling that we will go well beyond the consumer-norm in cycle life, given how crazed we are about them. You know everyone probably sees this and thinks we are dancing on the head of a pin, and wondering why we are fighting and bickering instead of cooperating and compromizing when need be.

Myself included of course.

[karrak]

If you really want to know what causes lithium batteries to fail a premature death it is charging them up above 80% DOD, and allowing them to fully discharge. This is what happens if you use Top Balance topologies. Commercial EV manufactures use Middle Balance (aka Between the Sheets) which you the DIY's cannot do, but you can mimic Middle Balance if you understand the subject matter

Why do I think I have seen this before, maybe here somewhere in this thread http://www.solarpaneltalk.com/showth...-Solar-amp-BMS. I could do Middle Balance with Li-Ion batteries using bleed resistors (aka Vampire Boards) like Tesla does, does this mean I am not a DIY?

The take away is this. Do not charge your batteries higher than 80%, do not discharge your batteries more than 80%. Do not operate or ever charge your batteries when the batteries are 30 degree C or higher.

As Dr Jeff Dahn said at the end of the video if you want your Li-Ion battery to last as long as possible keep it stored in the fridge at 20% SOC.

Now me, I want to cycle as much power as possible through as small a battery as possible for as long as possible so I get the best possible economic return from the battery, and don't want to put it in an air-conditioned room if possible.

What I would like to know is how much difference will it make to the lifespan of my LFP battery in my off-grid system that is charged from solar energy if I charge to 3.4 volts/cell at an end current of C/50 rather that say 3.35 volts/cell at C/50. If the extra .05 volts makes little difference to the lifespan but may give me another 10% more charge it may be worth my while. The other issue is how much difference will it make to the lifespan if I float it at 3.35 volts rather than terminate the charge until the next day.

It is a pity that the majority of the video was about Li-Ion batteries and not LFP batteries. I often wonder if the fact that we only ever charge an LFP battery to around 3.4-3.5 volts whereas we charge Li-Ion batteries to around 4 volts for 80%SOC or 4.2 volts for 100%SOC is why LFP batteries are supposed to last longer than Li-Ion batteries.

The three major root cause of Lithium battery failure are.

1. Lithium Ion Plating of the SEI Layer. This is caused by normal recharging cycles and significantly accelerated when batteries are charged above 80% SOC and/or Top Balanced. Manufactures use additives in the electrolyte to combat and slow the problem. but basically with every cycle you loose a capacity. When Jeff talks about shifting to the right.

By Lithium Ion Plating I assume you mean blocking of the carbon anode by the products of the unwanted reactions of Lithium Ions and the electrolyte or other contaminants and other side reactions. Lithium plating occurs when the Lithium Ions cannot diffuse and insert themselves into the carbon lattice fast enough and build up on the surface and plate out. This can happen because of this clogging and can also occur in tandem with or because of high charge rates and charging at low temperatures. Low temperature slows down the diffusion of the Lithium ions. This is discussed in the video around the 42 minute mark. You loose capacity because the Lithium Ions that could provide you energy are locked up in the side reaction debris. As the carbon anode clogs up the resistance will also increase.

2. Charging or allowing your batteries to stay in warm conditions of greater than 30 degree C. We are not talking ambient air temps here, we are talking internal battery temps, and battery temps can reach 50 degree C in freezing weather. This is the MYSTERY some of you call PARASITIC LOSSES. It is not and I repeat not a Parasitic Loss, it is Parasitic Reaction. it i snot a loss of heat or energy. It is a contamination and oxidization of the electrolyte that cause lithium plating and prevents or stops lithium ions exchange between Anode and Cathode. Turn up the heat and charge faster, you accelerate the process. This is why some EV's use Thermal Management, and bites everyone of them in the butt. Keep you lithium warmer then 0 and cooler than 25 degree C, and do not charge faster than C/2

Overheating batteries is not such an issue for stationary applications where you are charging below 0.1C and discharging below 0.5C. Being pedantic, any of the Parasitic Reactions that involve Lithium Ions do result in a loss in energy as they are locked up in the debris and no longer available. The Parasitic Reactions can also increase the resistance of the battery and decrease the energy efficiency of the battery.

3. Over discharge. Nothing will kill a Lithium battery faster than an over discharge. If you Top Balance, you are at risk of an over discharge and will take a BMS and automation to help prevent over discharge. Taking it one step further using a BMS can cause an over discharge if the system faults. Bottom Balance and you eliminate over discharge risk, and it does not cost one penny or require any automation outside what your Inverters already supplies.

Sunking, You are like a broken record, see this thread for more on this http://www.solarpaneltalk.com/showth...-Solar-amp-BMS.

Simon

[PNjunction]

You picked my favorite instructional video Sunking!

One thing overlooked by most of us in regards to parasitic reaction (not parasitic loss - thanks for the correction!) is the TIME factor.

While I don't want to rehash the whole top vs bottom balance debate, one additional problem with top-balancing with the common little shunt resistor, is that it extends the TIME that full cells sit at an elevated voltage while waiting for the others to catch up. Over TIME when cycling, if your cells are spending 10 minutes longer than they should top-balancing each cycle from dinky little shunt resistances, that adds to the parasitic reactions over the lifetime of the cell.

The time factor pointed out by Prof. Dahn is how manufacturers who tout cycle-life figures solely from rapid-fire charge/discharge routines from full discharge to full charge are just "beating the clock", and was the real reason that their lab chose to watch and predict cycle life by measuring parasitic reactions with their extensive lab gear, rather than just do simplistic cycle counts. Either that or wait a decade or so for real-time data to accumulate and try again.

Thus it is best to charge and get it over and done within a reasonable amount of time. Bottom balancing where the highest voltage cell is the determining factor to stop the charge is scientifically the best. However, from a practical standpoint, one can top balance with somewhat conservative values, IF they are mindful of the time factor. Thing is, top-balancing is not needed on each and every cycle, especially when conservative values are chosen. And of course the application has to be taken into account, such as EV'ers who can routinely drive cells into the basement, whereas with proper power-budgets, a solar housebank can be caught in time at their relatively low currents compared to overall capacity.

Even though I personally do *sporadic* top-balancing since I monitor things quite closely, it is only so for convenience. But my biggest beef with top balancing as we've discussed before is that we know that voltage alone is only an approximation, and spending too much time trying to line up all your cells to exactly the same voltages is only a feel-good trick. While your voltmeters may all line up nicely, the TIME it took to do so will cost you further down the line from unnecessary parasitic reaction if you do this each and every cycle. Of course quality cells matched for both capacity and internal resistance should be used either way.

In the end, and in the proper hands, it might well be that bottom balancing will give me the commonly touted 2000-cycles, whereas conservative top balancing only when it needs it, nets me 1800.

Also overlooked by some is that lifepo4 (and other lithium chemistries in general) can suffer from SDS, or sudden-death-syndrome without warning due to prolonged parasitic reactions from overcharge / endless top balancing or floating at elevated voltages. Unlike lead where you may get a red-flag from slowly declining output, if lifepo4 is abused long enough, the overgrown SEI layer still functions one day, and is totally closed the next! (another reason that used or "recycled" lithium batteries are a BAD idea for diy'ers.)

I like to point out to newcomers that there is no law that says you have to have those cheap shunt balance boards on all the time, provided you sanely have some other form of monitoring / alarm. Interestingly, even BMS systems like Housepower and Orion come with the option to turn off top-balance routines, and only rely on the rest of the circuitry to monitor and control lvc / hvc alarms, switches and so forth. So that makes it easy to use either top or bottom methods.

I think it wise to note that testing done in the labs and at manufacturer facilities are usually only on a single-cell basis! That is, real-world additional issues like multiple series / parallel cells to actually comprize a usable battery suitable for our application, wiring infrastructure quality, and so forth have to be taken into account at the user level, so we just do the best we can do without throwing caution to the wind.

Of course, procedures on how to bottom balance should one want to do so have been covered elsewhere here.

So one picks their poison and does the best they can with it. Also overlooked are the terminal connections themselves! Unless they are snug and clean, one might assume their cells are bad or suffering from parasitic reactions when in fact the wiring infrastructure is the bad actor unbalancing the system with high resistance contacts ...

As always, thanks again for the info.

[Sunking]

If you really want to know what causes lithium batteries to fail a premature death it is charging them up above 80% DOD, and allowing them to fully discharge. This is what happens if you use Top Balance topologies. Commercial EV manufactures use Middle Balance (aka Between the Sheets) which you the DIY's cannot do, but you can mimic Middle Balance if you understand the subject matter

Dr. Jeff Dahn and his university group at Dalhousie University are the authority in Lithium Battery Destructive Testing and the video will teach you what you need to know. All top lithium battery manufactures use the university to test predicted life cycle counts and analysts. The take away is this. Do not charge your batteries higher than 80%, do not discharge your batteries more than 80%. Do not operate or ever charge your batteries when the batteries are 30 degree C or higher.

The three major root cause of Lithium battery failure are.

1. Lithium Ion Plating of the SEI Layer. This is caused by normal recharging cycles and significantly accelerated when batteries are charged above 80% SOC and/or Top Balanced. Manufactures use additives in the electrolyte to combat and slow the problem. but basically with every cycle you loose a capacity. When Jeff talks about shifting to the right.

2. Charging or allowing your batteries to stay in warm conditions of greater than 30 degree C. We are not talking ambient air temps here, we are talking internal battery temps, and battery temps can reach 50 degree C in freezing weather. This is the MYSTERY some of you call PARASITIC LOSSES. It is not and I repeat not a Parasitic Loss, it is Parasitic Reaction. it i snot a loss of heat or energy. It is a contamination and oxidization of the electrolyte that cause lithium plating and prevents or stops lithium ions exchange between Anode and Cathode. Turn up the heat and charge faster, you accelerate the process. This is why some EV's use Thermal Management, and bites everyone of them in the butt. Keep you lithium warmer then 0 and cooler than 25 degree C, and do not charge faster than C/2

3. Over discharge. Nothing will kill a Lithium battery faster than an over discharge. If you Top Balance, you are at risk of an over discharge and will take a BMS and automation to help prevent over discharge. Taking it one step further using a BMS can cause an over discharge if the system faults. Bottom Balance and you eliminate over discharge risk, and it does not cost one penny or require any automation outside what your Inverters already supplies.

Anyway here is a video taped at a conference by Dr Jeff Dahn. 1 hour and 15 minutes long with quite a bit of technical information. But if you really want to know about lithium batteries watch it several times and look up the stuff you do not understand.

[karrak]

Thanks PNjunction, for the background information on the life and work of John Goodenough, it was very interesting.

This first one above contains clues about what happens when you no longer have any lithium to intercalate

I assume your statement comes from this in the article
"but if lithium comprised a large part of the material in the cathode, and all of it left on a journey to the anode, then, Goodenough reasoned, the cathode would be hollowed out and might fall in on itself. So could any of the metal oxides manage to hold up under this abuse? And if so, which one, and what was the magic proportion of lithium that could be pulled out? ... Their answer was that about half of the lithium could be pulled from the cathode at 4 volts before it crumpled"

I am not sure that this is valid anymore as it dates from the late 1970s and is regarding early research on Li-Ion variants not LiFePO4. This is the only time I have seen any mention of this problem in any of the information I have read.

Essentially though, it is my own understanding that overcharging (too high a voltage, or too long a time even at conservative elevated voltages) results in lithium-plating, reducing cell capacity.

My understanding is that lithium plating is more likely to be a problem at high charge rates and low temperatures. I can see this being a problem for the EV brigade but not such a problem in applications with low charge rates and in situations where we have more control over the temperature of the battery. The main problem I see for us is the growth of the SEI layer due to unwanted side reactions.

On the other hand, too deep a discharge, and for too long with nothing left to intercolate, means that the anode and cathode structure tries to intercolate - and obviously does a bad job of it and just contaminates the electrolyte, grows dendrites, which of course is pretty bad.

My understanding is that overdischarging a cell results in the copper cathode being ionised (dissolved). The copper ions can latter redeposit and form dendrites.

For anyone interested there another technical article on Capacity Fade Mechanisms and Side Reactions in Lithium-Ion Batteries here http://scholar.google.com.au/scholar...hUCFqYKHbGuD9s. Although it was written in 1997 and is about Li-Ion batteries, allot of the information I would think is still relevant to LFP batteries.

This is another interesting article on Cell Ageing http://www.researchgate.net/publicat...teLiFePO4_cell

I don't understand some of the detail in these articles but I think/hope I get the gist of what is being said. I am certainly learning all the time.

Simon