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

[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]

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]

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]

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.

[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.

[wb9k]

Very incorrect statement, a Shunt is a Shunt period. Balance boards put a fixed resistance across the cell to shunt a fixed amount of current AROUND the cell to be passed onto lower level cells. That is the whole concept of Balance boards.The Flaw in that is it only bypasses a small fraction of the current flowing through the cell. Typical Balance Boards only shunt anywhere from 50 ma up to 2 amps in some units. .5 to 1 amp is typical. That does no good when the charger is still pushing 10 amps. You bypass 1 amp around the cell, bu tstill left with 9 amps flowing through a full charged cell. Does the bypass resistor burn of power, sure they do Ohm's law still applies. If you have a 1 amp shunt you have roughly a 3 Ohm bypass shunt resistor burning off 3 watts as 1 amp of current flows through it, and that 1 amp is fed to all lower cells. It recombines with the current still flowing through the cell. This is basic electronic and DC principles 101 a student knows.

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.

dh

[Sunking]

The second to last sentence above concerns me a bit, and SK has made similar comments to the effect of balancers "shunting off power to weaker cells". This is not how things work. Resistive balances take excess power from a cell and dissipate 100% of it as heat.

Very incorrect statement, a Shunt is a Shunt period. Balance boards put a fixed resistance across the cell to shunt a fixed amount of current AROUND the cell to be passed onto lower level cells. That is the whole concept of Balance boards.The Flaw in that is it only bypasses a small fraction of the current flowing through the cell. Typical Balance Boards only shunt anywhere from 50 ma up to 2 amps in some units. .5 to 1 amp is typical. That does no good when the charger is still pushing 10 amps. You bypass 1 amp around the cell, bu tstill left with 9 amps flowing through a full charged cell. Does the bypass resistor burn of power, sure they do Ohm's law still applies. If you have a 1 amp shunt you have roughly a 3 Ohm bypass shunt resistor burning off 3 watts as 1 amp of current flows through it, and that 1 amp is fed to all lower cells. It recombines with the current still flowing through the cell. This is basic electronic and DC principles 101 a student knows.

[wb9k]

wb9k, I think you have the experience and expertise to answer this question. There are only 3 - 4 people that actually have a functioning solar / solar assisted off-grid system here so it's hard to get facts and not opinions.

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.

Let me know if my reply to PNJunction doesn't answer your question as well....I think it should come close.

dh

[wb9k]

This following might best be put into another thread, but wanted to grab your attention just in case...

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!).

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.

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. When you say the voltage can shoot up to 4.5 Volts per cell, I'm curious where exactly in the system you are measuring the number. When we talk about what voltages are safe for cells, we need to take those numbers RIGHT AT THE CELL with a dedicated v sense line. Measurements made right at the charging supply output terminals are pretty useless as there is typically a significant voltage drop between the supply and the cells. If you're really seeing spikes of 4.5 at the cells, I would worry that the voltage setpoint is too high during the pwm phase. Terminal voltage spikes of about 3.8 are around the upper limit for LFP, and you don't want to stay there any length of time--A123 specs a max of 10 seconds at 3.8 Volts. Do you have any screenshots of the voltage measured at the pack terminals during the PWM phase?

I've done some characterization of commercial chargers and diagnostic stations for professional mechanics/dealerships. The output of the chargers runs the gamut from nice smooth DC power to raw rectified AC. It doesn't seem to matter much. Special techniques for "recovering" LA or NiCd cells don't apply to Li at all as far as I know. If a Li cell has been damaged, it can't really be recovered.

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).

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. Most chargers I'm familiar with (including large cyclers that mimic drive cycles) will CC until one cell hits 3.6 or just above, depending on the CC current level. At that point, current is usually cut in half until somebody again hits 3.6 Volts. This may happen several times before getting to actual CV, it depends on the charger. The ideal CV phase, in my opinion, sources exactly 3.6V for each series element (assuming LFP), and allows balancers to trim voltages until every cell is at exactly 3.6 Volts, and charge current is very near to zero. Sometimes the CV voltage will be turned on and off (much too slowly to be called "PWM") so that the balancers can continue to trim without having to fight the charge current 100% of the time. This speeds the final balancing process a bit.

The big fish run things like Outback or Midnite Solar controllers, where us small fry might use Genasun, or Pb-controlers carefully tweaked (no temp comp etc) for voltage - still pwm is present.

Maybe some of the EV chargers ARE using pwm - I don't know. Just wondering if we are hurting or helping by not using a more linear approach to CC's with lifepo4 in solar applications....

I guess I might as well go deeper here - the "drift together" is due to the current reaching the lower cells by virtue of the repetetive leading edge of the pwm pulse. My observation is that it *helps*, but of course is dependent on both capacity and internal resistance.

The second to last sentence above concerns me a bit, and SK has made similar comments to the effect of balancers "shunting off power to weaker cells". This is not how things work. Resistive balancers take excess power from a cell and dissipate 100% of it as heat. The energy cannot be sent to other cells without a much more complex switching network, something that is rarely done because of the high hardware cost (and very low cost of power thrown away during balancing). Amps into the pack puts equal amps through each series cell...some cells may waste more of that power than others as heat with or without the help of balancers, but that excess energy is never passed on to other cells in a passive/shunt balancing system. There is no return path to facilitate that transfer. I think this is important to understand.

Hopefully that is a good start toward answering your questions. A screenshot of what is happening right at the pack terminals (or discrete cell terminals within the pack) would enable me to give a more complete answer, but the short answer is that I wouldn't worry much about how exactly the charge current is meted out to your LFP pack. PWM should be just fine, within reasonable limits.

dh

[Willy T]

wb9k, I think you have the experience and expertise to answer this question. There are only 3 - 4 people that actually have a functioning solar / solar assisted off-grid system here so it's hard to get facts and not opinions.

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.

[PNjunction]

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.

Ack - quoting myself again. Not a good sign.

I guess I might as well go deeper here - the "drift together" is due to the current reaching the lower cells by virtue of the repetetive leading edge of the pwm pulse. My observation is that it *helps*, but of course is dependent on both capacity and internal resistance.

Egads, I really wanted to avoid balance, so maybe just concentrate on wether pwm is a hurt or helpful charging waveform for lifepo4 ...

[PNjunction]

Sounds like you're safe. .02C is a very low charge rate. I could print the A123 numbers here, but CALB is probably different, so it's tough to make an apples-to-apples comparison, even though both cells in question are LFP. I seem to remember seeing the figures for CALB at one point...may be worth a web search.

This following might best be put into another thread, but wanted to grab your attention just in case...

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!).

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).

The big fish run things like Outback or Midnite Solar controllers, where us small fry might use Genasun, or Pb-controlers carefully tweaked (no temp comp etc) for voltage - still pwm is present.

Maybe some of the EV chargers ARE using pwm - I don't know. Just wondering if we are hurting or helping by not using a more linear approach to CC's with lifepo4 in solar applications....

[reed cundiff]

db

Thanks for response. Googled temperature limits again and found the information you mentioned :"....02C is a very low charge rate..." under Battery University where it noted that "... According to research papers, the allowable charge rate at –30°C (–22°F) is 0.02C..." Always try to investigate before I reply on a thread. I was a research physicist at Army Research Labs on White Sands Missile Range.

Have placed a remote thermometer (Walmart special at about $15) in battery compartment to monitor temperature. Plan to install two 12 V receptacles for 12 V fans (12 W curiously enough = 1 amp at 12 V) to provide cross ventilation. The 4.0 kW Magnum PSWI inverter is in same large compartment as the battery suite which opens to the front under the king pins of the fifth wheel. It can emit a lot of heat when running the 13,500 BTU a/c off solar panels/battery suite (1750 W). Chris Dunphy of Technomadia candidly discussed the mistakes he made with his installment of LFP 4 years ago. He installed his battery in the same cramped and unventilated compartment as his inverter. He compounded this by having his rig parked on tarmac in Phoenix in summer. His mistakes and successes should be studied by anyone planning to install LFP in an RV.

Plan to install two x 30 W halogen lamps in the front compartment as well to keep the temperature above 0 C on cold nights. We can run such for quite a few hours from the battery suite.

The BMS (which came with the Manzanita fabricated batteries - fairly expensive) keeps the 16 individual cells below 3.4 V and balances them every several seconds so we are not worried about over charging. However, I may be a bit OCD and check the three monitors numerous times during the day: TriStar MPPT-45, Magnum inverter, and Manzanita BMS)

Since we have only plugged into line power once in two years, we do not have the problem of keeping the cells at to high a SOC. We generally wake up with a -2 to 3 kW-hr deficit (60 to 70% SOC).

If things finally get worked out with Mexican customs (totaled rig and tow vehicle two years ago between Orizaba and Puebla in 70 vehicle pileup), we shall go back to Yucatan, Belize and Guatemala and not worry one whit about extreme cold
Reed and Elaine

[Mike90250]

Gentlemen,
I am pruning this thread, and editing out name calling. STOP IT.

If it's off-topic, it's going away.

You can disagree, but can't claim better safer bigger longer than the other. You can use ENGINEERING TERMS, as it relates to safety and best practices, but it's got to be true and verifiable.

You can say I Feel 300.54 is an unrealistic number. You can say why you feel, but you can't say someone else is stupid, fat, wears army boots, too tall...... Get it ?

[wb9k]

wb9k

Thanks for response. The solar system on 5th wheel is 1420 W of panels which three panels are in series then both strings are parallel to provide 90 V to MPPT and a 48 V nominal battery suite of 16 CALB cells (Manzanita Micro fabrication and BMS). The capacity is 8.6 kW-hr (48 V nominal and 54.4 actual)

We left rig at son's place two winters when we flew down and spent the winters in Guatemala/Honduras and then Ecuador/Peru. The temperatures went down to a minimum of -20 C and battery suite was not disconnected. Inverter was disconnected since it has a parasitic draw of around 50 W. The other parasitic draws are about 20 W so that the daily discharge would have been around 500 W and perhaps 300 overnight. Early morning charging rates would have been around 200 to 400 W, or about 0.02 C. Son checked on this and battery was fully charged at these low rates by mid-morning We have discerned no loss in capacity.

We have had charging rates of over 1400 W in mid-June and high elevation (2700 m) according to Tri-Star MPPT-45 and Manzanita Micro BMS monitors.

Have always appreciated the responses of Karnak and PNJunction as well.

Reed

Sounds like you're safe. .02C is a very low charge rate. I could print the A123 numbers here, but CALB is probably different, so it's tough to make an apples-to-apples comparison, even though both cells in question are LFP. I seem to remember seeing the figures for CALB at one point...may be worth a web search.

I usually like to state charge/discharge/C rates in Amps rather than Watts....keeps things a whole lot simpler.

I'm envious of your extensive travel...happy trails!

dh