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

[Sunking]

The power that isn't burned off in the resistors stays in the cell. This is why you need to cut charger current back during the balance phase--if the balancers can't keep up, the high cell is still getting overcharged. The energy you don't want to keep in the high cell has nowhere to go but the balance resistor, or into the cell itself. The other cells in the series aren't getting any more power than they were before the balancers kicked on.

Now you are beginning to understand. No External Lithium battery charger and NO SOLAR CHARGE CONTROLLERS DOES THAT. You would have to have a integrated BMS/Charger to do that. Most Balance or Vampire Boards are dumb passive devices. Even centralized like my Orion is just a simple Bypass. To do what you describe is to have the Balance Boards communicate with the charger so when th every first Balance Boards Turns On, it signals the charger to cut back current to what only the bypass board can bypass. Otherwise if the current is greater than what the balance board can bypass, the remaining flows through the already fully charged cell. There is no reason to ever go to full charge.

[Sunking]

Sorry cannot edit, so have to double post.

The power that isn't burned off in the resistors stays in the cell. This is why you need to cut charger current back during the balance phase--if the balancers can't keep up, the high cell is still getting overcharged. The energy you don't want to keep in the high cell has nowhere to go but the balance resistor, or into the cell itself. The other cells in the series aren't getting any more power than they were before the balancers kicked on.

Now you are beginning to understand. No External Lithium battery charger and NO SOLAR CHARGE CONTROLLERS DOES THAT. You would have to have a integrated BMS/Charger to do that. Most Balance or Vampire Boards are dumb passive devices. Even centralized like my Orion is just a simple Bypass. To do what you describe is to have the Balance Boards communicate with the charger so when th every first Balance Boards Turns On, it signals the charger to cut back current to what only the bypass board can bypass. Otherwise if the current is greater than what the balance board can bypass, the remaining flows through the already fully charged cell. There is no reason to ever go to full charge.Solar Charge Controllers DO NOT HAVE ANY ABILITY to communicate with a BMS. Even Genasum the only company to make a Solar Charge Controller, is just a Simple Float Charger fixed at 14.4 volts. It has no I/O ports to communicate with a BMS.

[wb9k]

Sorry cannot edit, so have to double post. Now you are beginning to understand. No External Lithium battery charger and NO SOLAR CHARGE CONTROLLERS DOES THAT. You would have to have a integrated BMS/Charger to do that. Most Balance or Vampire Boards are dumb passive devices. Even centralized like my Orion is just a simple Bypass. To do what you describe is to have the Balance Boards communicate with the charger so when th every first Balance Boards Turns On, it signals the charger to cut back current to what only the bypass board can bypass. Otherwise if the current is greater than what the balance board can bypass, the remaining flows through the already fully charged cell. There is no reason to ever go to full charge.Solar Charge Controllers DO NOT HAVE ANY ABILITY to communicate with a BMS. Even Genasum the only company to make a Solar Charge Controller, is just a Simple Float Charger fixed at 14.4 volts. It has no I/O ports to communicate with a BMS.

There are lots of systems that have communication between the charger and balance boards. A123's system for the Hymotion Prius packs was doing that ten years ago. All the automotive OEM's I'm familiar with use similar setups. The charger on my motorcycle goes to 58.2 Volts and just shuts off. Works just fine, no communication needed because balance is reset every charge cycle (and I'm watching the pack voltage like a hawk while riding). Between relaxation and the little balancing I get at that pack voltage, I have a perfectly balanced pack after every charge cycle. There are crappy BMS's and there are good ones. I don't recommend the crappy ones...but the existence of bad BMS's isn't a reason to ditch control altogether.

A 14.4 Volt charger is fine too. If the setpoint of the charger cannot overcharge a balanced pack, you're good unless you've got severe imbalance prior to charge, which would indicate that something else is wrong [that's why you need cell-level monitoring to shut things down if the situation becomes dangerous for any single cell]. As you approach 100% SOC, the voltage delta between the cells and the charger goes down, so current becomes self-limiting. Balancers should never have a problem dealing with the small imbalance that develops between charge cycles in such a system. 14.4 Volts = 3.6 Volts per cell, when your charger current falls to zero (or very near to it), you should have a perfectly balanced pack at 100% SOC, every time. With no automatic balancers, I'd rather turn that down charger down to 14.0-14.2 and manually balance periodically. I've used this technique with no problems on several of my smaller packs for years now---but it's a hassle and I've gotten tired of it. To me it's much better and safer to have automatic balancing every charge cycle. This insures you won't run into the problem you describe here. Cell-level monitoring and shutoff control take care of the times where something has gone truly awry...and you want this whether you have automatic balancing or not.

But my point was this--when balancers are turned on, all other cells in the pack are still getting the same amount of energy they were getting before. They don't get the "surplus power" from the high cell as some kind of extra bonus that brings them up faster than if the balancers weren't there at all. Saying "balance current gets shunted to lower cells" is, to me, a very misleading way of putting things, especially for beginners.

[Sunking]

There are lots of systems that have communication between the charger and balance boards. A123's system for the Hymotion Prius packs was doing that ten years ago. All the automotive OEM's I'm familiar with use similar setups.

Who gives a damn about an EV on a Solar Forum? Solar Charger, even the one MPPT model out there do not have any means to communicate with a Balance Board. This is not a EV forum using AC powered EV chargers. This is a Solar Forum, we use Solar Charge Controllers. Your world does not apply. That is what you do not understand. It would be a rare event to ever get a battery fully charged on solar, does not matter what kind of battery be it lithium or Pb. Solar is a Soft Source with unknown very limited power. With Solar and LFP there are very few things you can add on to it. You can use Balance Boards but they are worthless because they cannot communicate with the Charge Controller to cut back on current when the first board comes on. Nor would you ever want to do that in a Solar System. You want to utilize every bit of solar power you can. So if by chance your batteries were to fully charge by noon, last thing you want to do is turn the solar off and run on batteries when you still have plenty of daylight left. That would be just plain STUPID. You want to Float at less than 100%. So if you got to say 80% when th ebatteries quit ccharging, any loads can use Solar power when demanded so you can save your battery power when there is no sun light. Bottom line is with Solar you basically have a 4 hour window to get your batteries charge between 10 am and 2 pm. As for DIY EV, almost none of the chargers made for lithium DIY EV have the ability to communicate with the charger. They are all CC/CV set to 3.6 vpc and rely on Vampire Boards to bleed off energy. If you really did hang out of DIY EV forum you would know that, and you would also know there are hundreds who use no BMS and Bottom Balance. Heck there is a thread there running right now no BMS is required.

[wb9k]

Who gives a damn about an EV on a Solar Forum? Solar Charger, even the one MPPT model out there do not have any means to communicate with a Balance Board. This is not a EV forum using AC powered EV chargers. This is a Solar Forum, we use Solar Charge Controllers. Your world does not apply. That is what you do not understand. It would be a rare event to ever get a battery fully charged on solar, does not matter what kind of battery be it lithium or Pb. Solar is a Soft Source with unknown very limited power. With Solar and LFP there are very few things you can add on to it. You can use Balance Boards but they are worthless because they cannot communicate with the Charge Controller to cut back on current when the first board comes on. Nor would you ever want to do that in a Solar System. You want to utilize every bit of solar power you can. So if by chance your batteries were to fully charge by noon, last thing you want to do is turn the solar off and run on batteries when you still have plenty of daylight left. That would be just plain STUPID. You want to Float at less than 100%. So if you got to say 80% when th ebatteries quit ccharging, any loads can use Solar power when demanded so you can save your battery power when there is no sun light. Bottom line is with Solar you basically have a 4 hour window to get your batteries charge between 10 am and 2 pm. As for DIY EV, almost none of the chargers made for lithium DIY EV have the ability to communicate with the charger. They are all CC/CV set to 3.6 vpc and rely on Vampire Boards to bleed off energy. If you really did hang out of DIY EV forum you would know that, and you would also know there are hundreds who use no BMS and Bottom Balance. Heck there is a thread there running right now no BMS is required.

You're making some broad generalizations/oversimplifications there, which may or may not be true based on circumstances. I've worked with small solar rigs for nearly 20 years, so I do have some idea of how they work and what they do, but thanks.

[Sunking]

TA 14.4 Volt charger is fine too. If the setpoint of the charger cannot overcharge a balanced pack, you're good unless you've got severe imbalance prior to charge, which would indicate that something else is wrong [that's why you need cell-level monitoring to shut things down if the situation becomes dangerous for any single cell]. As you approach 100% SOC, the voltage delta between the cells and the charger goes down, so current becomes self-limiting. Balancers should never have a problem dealing with the small imbalance that develops between charge cycles in such a system. 14.4 Volts = 3.6 Volts per cell, when your charger current falls to zero (or very near to it), you should have a perfectly balanced pack at 100% SOC, every time. With no automatic balancers, I'd rather turn that down charger down to 14.0-14.2 and manually balance periodically. I've used this technique with no problems on several of my smaller packs for years now---.

Geez dude you just agreed with me. What the hell do you think I have been talking about all this damn time. Run below 100% and th ecells self balance and if they should ever become unblanced is really simple to be re-balanced. I personally have not had to in 8 moinths, and if you go to DIY EV Forum or EVTV you will find hundreds of people who have not had any balance problems in 3 years. I grant you on a large EV high voltage pack Cell level monitors have benefit. But that is only because of the number of series cells is so much greater 3 to 6 volts is not noticeable. But trust me 3 volts low on a 12 volt system is going to get your attention real damn quick when nothing turns on and you notice your battery is 8 to 11 volts. You do not need a cell monitor to tell you something is wrong. Your Inverter already caught it and shut down.

[wb9k]

Geez dude you just agreed with me. What the hell do you think I have been talking about all this damn time. Run below 100% and th ecells self balance and if they should ever become unblanced is really simple to be re-balanced. I personally have not had to in 8 moinths, and if you go to DIY EV Forum or EVTV you will find hundreds of people who have not had any balance problems in 3 years. I grant you on a large EV high voltage pack Cell level monitors have benefit. But that is only because of the number of series cells is so much greater 3 to 6 volts is not noticeable. But trust me 3 volts low on a 12 volt system is going to get your attention real damn quick when nothing turns on and you notice your battery is 8 to 11 volts. You do not need a cell monitor to tell you something is wrong. Your Inverter already caught it and shut down.

Unless something's wrong with your inverter. I had a solar charge controller fail years ago such that it allowed full panel voltage to the battery all the time. Had I been running unsupervised Li, it could have resulted in a fire. Or say you've got an LFP 12 Volt battery that you're pulling out of storage and see it's at 10 Volts, but you have no way of knowing individual cell voltages and assume everyone is balanced, just low. So you put it on the charger and walk away....but what you really have is three cells at near 100% and one near 0%. Another fire waiting to happen. The problem with these approaches you keep suggesting is that you presume to know all circumstances at all times, but that's not the reality--for anybody. These systems should have some redundancy for robust safety and reliability.

Li cells don't self-balance...you can say it til you're blue in the face but it still won't be true. As I explained above, the illusion of "self balance" is in fact an expression of cell variation--the very thing that should be telling you why you DO need to balance, and the more often the better.

[Living Large]

Li cells don't self-balance...you can say it til you're blue in the face but it still won't be true. As I explained above, the illusion of "self balance" is in fact an expression of cell variation--the very thing that should be telling you why you DO need to balance, and the more often the better.

Dereck never stated in my time here that LFP cells "self balance". There was an extended discussion here 3-4 months ago, in which it was discussed by multiple users that one can monitor the cells and rebalance manually when needed, but for some period of time the cells will tend to stay in balance. That was good enough for me. I knew going in with eyes wide open that I would have to monitor all my cells, and rebalance occasionally. This is for solar. We never discussed motorcycles or cars or any of that, because it isn't what the application is. You guys are now having an argument over balancing that as a novice observer I can tell will never end. You might considering agreeing to disagree. Dereck said months ago that a solar user could choose any balancing method they wanted to - it's whatever "floats" your boat. Everyone's philosophy is a bit different.

[Sunking]

Li cells don't self-balance...you can say it til you're blue in the face but it still won't be true. As I explained above, the illusion of "self balance" is in fact an expression of cell variation--the very thing that should be telling you why you DO need to balance, and the more often the better.

Not able to edit my replies and not what I meant. I couldn't fix it. It should have read " If Floated to less than 100% re-balance should not be a problem even though lithium cells are not self balancing. with no parasitic loads lithium only imbalance of any significance is self discharge which is extremely low so any differences are so small and insignificant can be ignored. Look you may not like it or agree, but Solar is a completely different set of parameters radically different than motive. Solar does not have the extremes unless you are talking RV where the engine alternator will do most of the heavy work. Max charge rate is on the order of C/10 to C/6 only for a few brief hours in a day. Batteries are in open air at room temps and not sealed up in a oven and coffin. Discharge rates typically C/20 and occasional burst of maybe 1C, but on average very slow discharge rates. Couple all that together with LFP batteries that are extremely safe by default, operated in kind conditions, and operate at Low Voltages of 12, 24, and 48 do not need as stringent controls. I know you are talking about cell level, but with 4, 8, and maybe 16S at low discharge rates do not really need it.

[wb9k]

Not able to edit my replies and not what I meant. I couldn't fix it. It should have read " If Floated to less than 100% re-balance should not be a problem even though lithium cells are not self balancing. with no parasitic loads lithium only imbalance of any significance is self discharge which is extremely low so any differences are so small and insignificant can be ignored.

OK, this is good, but the last statement assumes that cells never fail. This is not a safe assumption. Please consider that I see virtually every failure the company puts out into the field. It's actually possible that I know the actual field failure modes of LFP battery packs--including HV EV and HEV packs, starter batteries, and commercial grid storage--better than anybody else on earth. I don't know if that's true, but people who know me say it's probably the case somewhat regularly. Why can you not accept that the possibility of cell failure is real?

Look you may not like it or agree, but Solar is a completely different set of parameters radically different than motive.

It doesn't matter if I like it, and I happen to agree that this is pretty much always true, but alone this doesn't really mean much, because there WILL in fact be concerns that carry over across all lines. Energy storage is ALWAYS inherently dangerous, and needs to be treated with elevated respect, especially when catastrophic system failure can burn your house down. Even if it's 1 in 100,000, why would you take that kind of risk when it's so easily avoidable?

Solar does not have the extremes unless you are talking RV where the engine alternator will do most of the heavy work.

You assume...while making an exception for one of the most common solar install scenarios out there.

Max charge rate is on the order of C/10 to C/6 only for a few brief hours in a day. Batteries are in open air at room temps and not sealed up in a oven and coffin. Discharge rates typically C/20 and occasional burst of maybe 1C, but on average very slow discharge rates.

Again, all assumptions that any end user can easily overthrow. I'm getting ready to set my uncle up with a solar rig for his remote cabin that violates a few of your fundamental assumptions. Add that it will be completely unsupervised 95% of the time, and a belt-and-suspenders approach is mandatory--even though the pack is only 12 Volts, about 3-4 kW.

Couple all that together with LFP batteries that are extremely safe by default, operated in kind conditions, and operate at Low Voltages of 12, 24, and 48 do not need as stringent controls. I know you are talking about cell level, but with 4, 8, and maybe 16S at low discharge rates do not really need it.

Based on extensive real-world experience in many areas, including solar, I strenuously disagree. LFP is indeed extremely safe....maybe the safest energy storage medium of its capability on earth. It's still not totally foolproof. Use your FMEA skills....and remember the Titanic.

[PNjunction]

The one area I have interest in is the saturation phase of charging. Since charging is a leading voltage and the battery voltage is a lagging voltage, how long should the saturation phase be or to what level. If it is measured by say, ending amps ?? The higher the charge rate, the more disparity I see with shunt counted amp hrs returned. By only using a termination voltage of the charge controller there is a undercharge that accumulates by the cycle.

I'll let the others chime in, but I think we are all familiar with the ol' charge until current drops to C/20, and then call it quits. Taking absorb down to zero is indeed unnecessary and stressful.

But the question is always, great, but at what voltage? since it really depends upon the application. And should it be measured after hours of rest or during charge? 3.45v *at rest* is considered a fully charged cell. There is some variance, as a GBS prismatic can be from 3.38 to 3.45 or so...

Probably the best thing I've seen is an actual formula for it! It goes something like this for charging:

3.45 + (IR * A)

Where 3.45 is considered the full voltage at rest
IR is your cell internal resistance
A is the charge amperage

Maybe the big guns can comment.

Getting those values can come from a prismatic manufacturer if you ask or perhaps special-order them can be done, so that your cells are individually matched for capacity and internal resistance, and will supply a document sheet matching each one's barcode. Not sure if you can special order small cylindricals this way, or as WB9K points out, impractical to do for commercial projects.

[PNjunction]

This is an interesting question. I don't believe I've ever seen data on pwm-based charging, but I have no reason to believe it makes much if any difference at all to the cells. Many automotive applications drain the cells with high-current PWM at frequencies in the kHz range and nobody considers that a problem. Charging should be no different.

Got it - I didn't think so but wanted to check to see if there were any smoking guns laying in wait. As for the solar charge controller using pwm during absorb, I don't have any screenshots or captures. I was going on the generic assumption that really what we are dealing with is just a mosfet doing very fast on-off connections to the load. In the case of a nominal 12v solar panel, which has an ocv of around 18v, during absorb pwm can operate fast enough and change frequency to simulate a linear CV setpoint. But still, you have the very minimal amount of time during the mosfet switch that a voltage can get up to the panel total.

I think - disclaimer - the above comes from an amateur like myself.

The worst part about this is now you have given me a justification to actually look at something like a Fluke 289! (I'm a fluke nut) Dang it - my wallet is bleeding now...

What you are really seeing with cells "drifting together" is the expression of different DC resistances of the series cell-busbar combination. The higher the current, the greater the spread. When you switch from CC to CV(or PWM) the main difference as far as the cell is concerned, is that charge current goes down. The voltage rise (vs. rested voltage) across the whole pack decreases, and so do the differences between cell voltages. Less current, less "Peukert-type" losses, less voltage delta.

Awesome - I'm going to digest that a bit. I kind of knew I shouldn't have brought it up, since self-balance is a total misnomer and gets everyone excited. That's why I prefer drift. I should say that I know (and have proven to myself) that just dumping a solar CC onto a misbalanced pack is not the cheapskates way of balancing!

In my case with the A123 cells, and also my GBS prismatics, is that the cells have to be SANELY close to each other to begin with, and the drift takes MANY cycles, not just a single day's charging. Of course quality cells make the world of difference. Other NEW cells I pulled from other powersport batteries which were a total JOKE inside, were a waste of time and they just drifted immediately to the recycling center.

Also not that my solar usage is for realatively low-voltage (typically 12v / 4S configs), not mobile, and not critical. No bleeder boards, just common sense HVC, LVC and dose of monitoring since I like to do battery maintenance. Wouldn't hand it over to my neighbor though.

Thanks for the feedback. Anything that makes me want to study deeper is allright!

[PNjunction]

A side note about degradation --

We throw voltage settings around like so much candy (here and elsewhere), and while user lifepo4 projects are valuable for data, I take them with a grain of salt and consider them anecdotal when I can't determine if they are using a quality standard for measurement.

Far too many times I've seen guys building mega-buck battery systems, and then calibrated and monitored by a shirt-pocket or throw-away meter that hasn't been vetted for accuracy. Or not even taking the time to check that the cheapo Junsi cell monitor is even in the ballpark!

I use Fluke for out-of-box trust, but this isn't really a multimeter thread and I don't care what one uses, as long as they TRUST it or have calibrated it. And THEN, using that calibrated meter as the standard for everything else.

I just wonder how many systems dutifully follow the experience of others, only to be bagged by using a throw-away meter, and giving us false data in the forums?

Ok - degradation issue about meter rant over ...

[karrak]

Getting back to degradation itself, I have always wondered if any degradation studies have been done for LiFePo4 prismatics (or A123 cells if you prefer) in regards to the fact that as solar users, our charge controllers use PWM in the "absorb" phase (what little there is when fed by decent current!).

This light bedtime reading might be of interest http://dataspace.princeton.edu/jspui...181D_10531.pdf. Go to page 124 for the summary.

In other words, we don't REALLY use CC/CV, but CC/PWM. Typically the pwm is done at about 300hz or so. If looked at on a waveform, this simply means that our controllers just close the circuit during bulk, but once a setpoint has been reached, instead of CV, pwm is actually used. Ie, the voltage can actually shoot up to 4.5 volts per cell! - BUT of course at 300hz, the averaging takes place.

What I noticed when using both prismatics, and my prized A123's from Braille and Antigravity brand batteries was that unlike CV which stops current when the first cell is fully charged, with pwm, they tend to "drift together" - and not an exact balance. We've covered balance enough, but my main interest was how lifepo4 reacts to pwm, since that is what we use in the field. (be it a low-end pwm controller, or an mppt which uses pwm during absorb too actually).

I would be surprised if the commercial MPPT controllers are using PWM for the CV/Absorb phase of the charging unless there is a good reason to do so because it generates EMI (radio interference). I would think that most of the MPPT controllers work with switching frequencies > 10kHz (I use 20kHz) and would filter the switching currents with capacitors. This is an educated guess on my part.

Simon

[karrak]

The one area I have interest in is the saturation phase of charging. Since charging is a leading voltage and the battery voltage is a lagging voltage, how long should the saturation phase be or to what level. If it is measured by say, ending amps ?? The higher the charge rate, the more disparity I see with shunt counted amp hrs returned. By only using a termination voltage of the charge controller there is a undercharge that accumulates by the cycle.

Hi Willy,
This is the major difference between charging with a constant current source and the sun. With constant current you can charge to a predictable SOC by charging with constant current to a set voltage cutoff and then terminating the charge. As the amount of sun reaching our solar panels is variable we do not get the same amount of charge in the battery every time it is charged. I have found that when charging to around 3.4 volts/cell and ending the charge at a charge rate of C/20 that the final SOC achieved can be anything between around 80% and 90+%. If it is sunny for the whole period that the battery is being charged the final SOC will be around 80%, if it is cloudy or it is nearly the end of the day when the charging is nearly finished the final SOC can be greater than 90%. I have reduced this problem by ending the charge at C/50 rather than the C/20.

Simon