lithium batteries, bulk float absorb time? end or return amps?

[yewsuck]

Hold the bus Sir, that is an Oxymoron. To do any one of those functions requires a BMS. Well at least the LVD and HVD
**** I agree! I should have added a eyes roll icon when I wrote that. haha ****
LVD and HVD for LFP work differently than PB. For Pb batteries you monitor the pack voltage. So if you have a Pb 48 volt system you set LVD at 42 volts, and HVD at say 60 volts. For LFP you monitor every cell and if any one single cell hits it set point operates the associated disconnect. The only thing in common is the location of the disconnects. Th eHVD is between charge and battery, and LVD is between battery and load.

As for balancing there are a few different ways to go about it and can be done manually, but is a real PIA to do manually and could take a few days.

The issue right now battery management for Lithium batteries is still in development. There is NO DIFFINITION of what a BMS is, thus no standard protocol. Each manufacture is fighting right now saying their way should be the standard. In the EV world some common standards are coming about called CANBUS which is a data bus protocol so all the parts of an EV can communicate with each other like the motor controller, pumps for battery cooling and hydraulic braking, steering motors, and BMS. Bu twhen you get to the BMS there is no common method. Commercial manufactures use Active Balancing that is a Bottom Balance method which involves taking power from the higher SOC cells to the lower SOC cells. They also incorporate Thermal Management.

Botton line is for Solar it just needs to be a KISS system using passive management. Right now that comes from the DIY EV product line. There is really nothing useful for Solar right now because there is NO MARKET using LFP batteries for solar. Today lithium is not ready and cannot compete with Pb systems. I think that will change, but not today. Right now the only thing out there is toy stuff like GenSun made for a boat 12 volt marine LFP battery.
**** I totally agree on this one. I have looked at that GenSun controller before and failed to how it is for LFP. It seems to be just the same old solar controller with the voltages lowered. It still keeps trying to charge already full batteries. Tsk Tsk, I just shake my head while it is lowered. ****
2 things have to change for solar. 1 prices of LFP needs to come down. Unfortunately they have bottomed out and starting to rise like solar panels. 2 Is the most important thing, test of time. So far the first two generations failed that test. 3rd generation has just been released, so it will be another 10 years to pass the test of time. If 3rd generation fails, gotta wait even longer to prove 4th generation. I think you see where I am going with this. Personally I think th eLFP market will die, and something like Lithium Manganese will take over. All I can say for now is we are not there yet.

**** I agree with all you say. I also added a little in your quote. It would be nice if the message editor had a button to change color. I am too lazy to go google how to do it by hand. ****

---Mod Note: The message editor (as well as the message composer) has a color change button. It is the capital letter A in the middle of the tool bar which drops down a color palette. But to get it you may need to hit the "Go Advanced" button next to the "Save" button.

-YS

[Living Large]

**** I agree with all you say. I also added a little in your quote. It would be nice if the message editor had a button to change color. I am too lazy to go google how to do it by hand. ****

-YS

Yes, I think it is a question of semantics. I am thinking back to someone (else) here earlier who said that shunting would not normally be used, and (I believe I remember) is not needed at all. Dereck from the start I believe recommended it. In any event, we've moved the ball farther down the field now. What I am hearing is that an occasional rebalance can readily be done by using BMS shunting - which would easily make it worth the relatively small cost - IMO.

[Mike90250]

Quote Originally Posted by Living Large
Let's say I only needed 1A @ 58V for balancing (58W). Is a generator overkill? How long might be needed for this balancing procedure? Might a storage battery suffice to power the LFP charger? Just asking.

Living Large sorry for taking time to answer, bu tI needed some time to think things through. Talked to a couple of buddies of mine who are way into EV's and LFP batteries, some 8 years into it and on their 3rd and 4th EV's. They confirmed some things I have hinted and hit on in past replies on this thread and others threads. These two individuals claim they do not use BMS, but in fact they do, just not going the commercial solution and mainstream thinking. This will shock you, they both firmly believe the Balance Bleeder boards are not the solution to battery management, but the killer of of LFP batteries. That's right, Balance Boards are the problem, not the solution. I will try to explain as best as I can.

To start they Bottom Balance rather than Top Balance the Bleeders perform. When we Top Balance we are calibrating to 100% SOC. reference level. However any LFP battery even if from the same lot are not equal in capacity. For example one 100 AH cell my have 102 AH, another 106 AH, and at the top end 112 AH. Those are real world measurement range. So if we top balance we are calibrating to the 100% level, but that has little to do with capacity because no two batteries are the same. With me so far?

However there is another reference point, the BOTTOM or 0%. As you should know by now is LFP batteries can tolerate a fair amount of over charge, not a good idea to push to 100% as I have said many times. However like all Lithium batteries cannot tolerate being fully discharged and that is what you need to worry about. So picture this we receive our new set of batteries, wire them all parallel and let them rest Equalize for a day. On day two we discharge them down 2.75 volts let them rest a bit and drain again if needed. So after they rest we end up with a voltage between 2.5 to 2.75 volts. We have now Bottom Balanced to ZERO SOC reference point. Still with me?

Now we are ready to go, and install our new almost dead batteries. (2 volts is death) We now charge the batteries with a Constant Current until the first cell reaches 3.65 to 3.7 volts. As soon as we see the very first cell hit 100% charge is terminated. The cell that gets to 3.65 volts is the weakest cell in the pack. We don't care about the other cells reaching 100% because LFP last longer in the PSOC range of less than 100%. All the cells will be at a slightly different voltage when charged up but that is not a problem because all the batteries now have roughly the same AH capacity which is the capacity of the weakest cell.

Now for the magic. When we discharge the batteries we arrive at 0% reference level on all cells at roughly the same time where we Calibrated. Do you understand?

So if you use that methodology; you want to monitor the cell voltages. When the monitor sees any cell drop to three volts turns on a Yellow Alarm to alert you that you are close to shut down. When any cell reaches 2.75 volts for more than 15 seconds, or any cell touches 2.5 volts with no time delay operates the LVD.

On the charge side there are two approaches. One is use Constant Current and terminate when the first cell hits 3.65 to 3.7 volts then terminate. Second method is just to charge in a Constant Voltage of 35 vpc (about 90% SOC) with no termination. So if you are using 16 cells you would set to Float at 16 x 3.5 volts = 56 volts.

Hope that helps.

Thanks Sunking, saving a copy here, and moving orig to the new LFP feeding thread you started.

[PNjunction]

If this is true, it brings me back to a question I asked a month or two ago. How would balancing occur? Even though trying to avoid 100% SOC, would you let the batteries get charged to 100% only occasionally (when you see one or more cells off by 0.1V), and the shunting would occur at this time? And would the low current charger only be used during this infrequent procedure so as to avoid the condition described - 1A shunting and 29A through the batteries?

That's the point - after an initial sanity charge to approximate equality among cells, you don't need to do that ever again if you run under the bleed-off voltage. In our low-current application, it is very unlikely to have cells go out of balance - from a capacity standpoint - and not necessarily voltage - if they were reasonably closely manufactured that way!

But what *IS* balance? Most assume this means that each cell has equal capacity and internal resistance from the factory. Great if you can obtain perfectly matched cells. In the real world, and with aging, that may change. Thus minor amounts of differing top-charge terminal voltage are not something to freak out over. Over .100 difference is where I'd start to worry, but ideally get them a bit closer but I'm not losing any sleep over it as long as any one cell does not stay under 3.5v, nor go over 3.6v during charge.

Example - I have married a 20ah cell into my normal 40ah (4S) battery, and got a PERFECT top-balance with my hobby charger. But the only thing that means is that the exposure to voltage is the same across those cells. Capacity-balancing is meaningless! My frankenstein setup with perfect top-balance is limited by the cell that is only half the capacity of the other three! If I don't watch it, even under my low-current discharge usage, that bad boy will get abused.

So even if I used cell-balancing boards, it means diddly-squat to this test battery with a cell that is only half the manufactured capacity than the others. It will play nice at the top, but be far from useful limited by the small cell with half the capacity of the others. Once my curiosity was satisfied, I put my batteries back together and did an initial sanity charge to get them close. Now I just do as I normally do with a two-terminal charger. Again- this is me with a very simple 4S 12v setup.

The real way to detect 100% SOC is when you are charging with at least .05C, and have a top voltage of at least 3.45v. More current and higher voltages (no need to go past 3.6) just mean that the absorb-time down to where you just start to go below .05C charge current will be shorter. In either case, you'll achieve 100% SOC - it just depends on how much time you have.

I guess what I'm saying is that you are on track - balance your cells once for sanity based on voltage *for convenience*, with bleed-off or whatever you decide to use (relying on the manufacturer to be close!) and from there on, run a conservative upper voltage. You'll STILL get 100% SOC with conservative voltages if you give the battery enough time to get to .05C. But no need to do even that.

If one is running conservative voltages, and is tripping the bleed-off resistors frequently after the first sanity charge, that usually means CRAP cells, or bad wiring / high-resistance infrastructure or running > 1C charge / discharge for long periods of time.

I'm just saying to beware of going bananas over so-called top-balance voltages indicating anything other than mere exposure to voltage. Yes, it is nice to line all the cells up with the very same voltages to feel good about it, but does a .100v difference actually MEAN anything in the long run when each cell has slightly different characteristics to begin with - as demonstrated with my frankenstein battery test?

And, due to my low-current application <.2C, I can even withstand slight differences near the bottom before the pack-level lvd takes over. EV's different story. Set your LVD conservatively too, like 3.15v per cell. You have a bit of headroom at the bottom too. Of course, I de-rated my capacity by 20%, ie my 40ah is treated like a 32ah capacity when I do my load-calculations and this coincides very nicely with 3.15v or so lvd, which I try to avoid in the first place!

Voltage is only a proxy measurement for guidance. Close is nice, but can be meaningless. Time spent achieving absolute voltage perfection may be damaging your cells rather than performing a so-called balance. Reasonably close, in our application, is workable.

[northerner]

The real way to detect 100% SOC is when you are charging with at least .05C, and have a top voltage of at least 3.45v. More current and higher voltages (no need to go past 3.6) just mean that the absorb-time down to where you just start to go below .05C charge current will be shorter.

The thing I'm having a hard time dealing with is charge conditions during normal operation when using solar as the power source. As we all know solar is variable in nature and charging currents can vary quite wildly at times. I'll explain with 2 cases:

Case #1) Clear sunny skies bring high charge currents all day. As a result the charge voltage will rise higher than on a day with less sun, when close to fully charged.

Case #2) Clear sunny day in the morning brings that pack nearly to full charge, but then high clouds roll in, dropping the charge current to less than half of what they were in full sun.

As one approaches full (or perhaps 90%) charge in case #1, the pack voltage and cell voltages are higher than normal due to the high charging current. As a result, the charge controller will shut off early prior to reaching the desired state of charge, because the trigger voltage has been met.

As one approaches full (or perhaps 90%) charge in case #2, the pack voltage and cell voltages are lower than normal due to the lower charging current. As a result, charging will continue for perhaps the rest of the day,as the trigger voltage of the charge controller is not being met.

In case #1, wouldn't the battery pack be somewhat undercharged? In case #2, would it be possible to overcharge the pack? This could be even worse if there was even a slight cell imbalance at the top?

Here are some typical voltages for example:
Resting voltage = 3.35 volts/cell = 53.6 pack voltage (48v system)
Charge voltage trigger = 3.55 volts/cell = 56.8 pack voltage (48v system)
Therefore the charging current could be anywhere from 0 amps at 53.6v, to max charge current at 56.8 volts.

The small charge current could be viewed as a long duration float condition, where the battery keeps on charging, but the trigger to shut off charging is never met?

[Sunking]

In case #1, wouldn't the battery pack be somewhat undercharged? I

Yes but who cares. The point is to never fully charge a LFP battery.

n case #2, would it be possible to overcharge the pack?

Nope as you select a voltage that only gets you to 90%. The point you keep missing and stuck on is you are using Pb mentality. All that goes out the window with LFP. Everything you thought you know about batteries does not apply to LFP. You never go to 100% SOC, and disconnect when you get down to about 10% SOC.

LFP Ri is so low with the extremely low charging currents do not develop any significant voltage difference between the the charger supply voltage and the LFP terminal voltage. LFP batteries can be stored for a year or more at 50% SOC. They never need to ever be fully charged. They will last 50% longer if only charged to 90%. The only thing you have to guard against is over discharged.

[northerner]

Yes but who cares. The point is to never fully charge a LFP battery.

I did not necessarily say to fully charge the LFP battery. Notice in quotes I mentioned going to 90% state of charge, which would be a likely target charge state would it not?

LFP Ri is so low with the extremely low charging currents do not develop any significant voltage difference between the the charger supply voltage and the LFP terminal voltage. LFP batteries can be stored for a year or more at 50% SOC. They never need to ever be fully charged. They will last 50% longer if only charged to 90%. The only thing you have to guard against is over discharged.

I tend to disagree. You can have any charge current between 0 amps in to full amps from your charging source. This can yield a cell voltage anywhere between resting voltage, to charge voltage at your charge rate. This would become even more pronounced as one installs a larger variable charging source (ie solar panel size) versus battery pack size

For example if one charges at 0.05 C with solar power and only goes to 80% state of charge on a regular basis, there may be very little to worry about.

On the other hand if one charges at 0.3C with solar power and goes to 90% state of charge on a regular basis, I would be concerned!

The issue with charging using solar power is that the charge rate is variable and not constant, like it is with EV use and much of the boat world use. It would be interesting to see this backed up with real world data, rather than congecture!

Edit: perhaps that is where having an absorb mode for higher charging currents would be useful, and would prevent premature charging shut down at higher charging currents.

Still though, I don't think one can very accurately predict the charge state of the LFP battery by examining pack voltage alone, and without examining the charge currents that are coming in, and considering the length of time perhaps as well?

[Sunking]

I did not necessarily say to fully charge the LFP battery. Notice in quotes I mentioned going to 90% state of charge, which would be a likely target charge state would it not?



I tend to disagree. You can have any charge current between 0 amps in to full amps from your charging source. This can yield a cell voltage anywhere between resting voltage, to charge voltage at your charge rate. This would become even more pronounced as one installs a larger variable charging source (ie solar panel size) versus battery pack size

For example if one charges at 0.05 C with solar power and only goes to 80% state of charge on a regular basis, there may be very little to worry about.

On the other hand if one charges at 0.3C with solar power and goes to 90% state of charge on a regular basis, I would be concerned!

The issue with charging using solar power is that the charge rate is variable and not constant, like it is with EV use and much of the boat world use. It would be interesting to see this backed up with real world data, rather than congecture!

Well let's see how your thoughts hold up in practice. A 100AH LFP cell has a Ri of .0009 Ohms at 100% SOC. A C/3 charge rate is 33 amps x .0009 Ohms = .03 volts. On a 16S pack is a difference of .47 volts difference. 100% SOC = 57.6 volts, 90% = 55.2 volts. You want to worry about .5 volts when you have 2.4 volts to play with? I would be more worried about an asteroid striking the earth and destroying it before loosing sleep over a .5 volt margin of error.

FWIW there are a few morons who charge there EV's with solar. I know because I helped them set it up initially and where I learned it from.

[northerner]

Well let's see how your thoughts hold up in practice. A 100AH LFP cell has a Ri of .0009 Ohms at 100% SOC. A C/3 charge rate is 33 amps x .0009 Ohms = .03 volts. On a 16S pack is a difference of .47 volts difference. 100% SOC = 57.6 volts, 90% = 55.2 volts. You want to worry about .5 volts when you have 2.4 volts to play with? I would be more worried about an asteroid striking the earth and destroying it before loosing sleep over a .5 volt margin of error.

FWIW there are a few morons who charge there EV's with solar. I know because I helped them set it up initially and where I learned it from.

So are you saying that if you got close to your charge cut off voltage early in the day, and then continued charging the cell all day just below that level (let's say 3.55 v for example), that the cell would not be over charged by the end of the day?

I see quite a difference in cell voltage between the cutoff at 3.55 volts (say at 0.3c for example) and the resting voltage of 3.35 volts (when no current is coming in). Doesn't the Ri of the cell increase near the knee?

[Sunking]

So are you saying that if you got close to your charge cut off voltage early in the day, and then continued charging the cell all day just below that level (let's say 3.55 v for example), that the cell would not be over charged by the end of the day?

No that is not what I am saying. You are still in your Pb box mentality. There are two approaches to charging LFP batteries. One is Top Balance and the Pb mentality to charge every cell to 100% SOC. That is what the battery charger guys want you to do because it takes some expensive smart equipment to help you destroy your batteries faster and have to buy new batteries. It makes them a lot more money initially and recurring more frequently.

When you buy say 100 AH LFP cells, not a single cell has 100 AH. In a lot of 20 the range will be no lower than 100 AH and up to 120 AH. So let's say my weakest cell is 100 AH and my strongest if 120 AH. I wire them all in series what is my capacity? The answer is the weakest cell of 100 AH. If I top Balance does not mean equal capacity, it means equal voltage and the reference point voltage is 100% capacity of 3.6 volts per cell. That means my weakest cell has 100 AH and the strongest is 120 AH. On discharge is by some chance I use more than 100 AH I can go to 120 AH, but I destroy a lot of weaker cells doing so. To Top Balance requires Vampire Boards, BMS, and a charger that can communicate with the BMS. I also shorten the life going to 100% SOC. Why on earth would I want to do any of that?

Well I will not do that anymore. The vampire boards are gone. I have Bottom Balanced my batteries. Now the reference is at 0% SOC (2.5 volts or 0 capacity. All the batteries are at equal capacity at 0% SOC. I now charge at 50 amps until the first cell (weakest cell) reaches 3.5 volts and the charger turns off. After it rest for an hour or two settles at 3.35 volts or around 90 to 95% on that one cell. All the stronger cells are at a slightly lower voltage when charged. The magic is all the cells have the exact same AH capacity of the weakest cell of around 95 yo 98 AH. So when I discharge they all arrive at 2.5 volts at the same time making it almost impossible to ever over charge or over discharge any one cell.

That algorithm can easily be done with Solar Charge Controllers today. There is no CC made to Top Balance or that can be made to work. But to go with LFP you gotta get out of the Pb box or you will fail and destroy your batteries.

[northerner]

I wasn't talking about top balancing with my last post. Just regular charging and getting to say a 90% charge state. If your charge cut off voltage is 3.55 volts per cell or 56.8 volt pack voltage, that is when you would want to terminate charge.

I was referring to an instance where you are charging fast under full sun and almost at the termination voltage of 56.8 volts, and then the clouds start rolling in. Charging continues because the pack voltage drops below 56.8 volts and may in fact stay below that level all day, continuing to charge. Could you not overcharge the cell in such an instance? And would be even more likely to happen if the cells weren't balanced at the top.

[Sunking]

I wasn't talking about top balancing with my last post. Just regular charging and getting to say a 90% charge state. If your charge cut off voltage is 3.55 volts per cell or 56.8 volt pack voltage

No Sir, you are still stuck in a Pb box. You do not terminate the charge when the pack reaches 56.8 volts as you would severely over charge some of the weaker cells. That is a Pb mentality and you gotta get that out of your head. It does not apply to LFP.

In a bottom Balanced battery you apply a constant current until the first cell reaches 3.55 volts and stop. Only 1 or two cells will get up to 3.55 volts. All the rest of the cells will be at a lower voltage. Example lets say we have 2 cells at 95 AH capacity, and all the rest are 105 AH. The two 95 AH cells reach 3.55 volts, and the other 14 cells in a 16S pack will be down around 3.45 volts for a total pack voltage of 55.4 volts.

If you are going to use a Solar Charge Controller to charge LFP batteries you set the Bulk voltage as high as you can like 60 volts so the controller stays in Constant Current charge mode any time it turns on. When the first cell hits the target voltage you turn the charge source off.

[northerner]

Ok, my mistake with the voltage of 3.55v for a 90% charge state, as it should have been closer to 3.45v. My apologies. But that doesn't change the question about potentially overcharging cell(s) at a lower charge rate.

In a bottom Balanced battery you apply a constant current until the first cell reaches 3.55 volts and stop. Only 1 or two cells will get up to 3.55 volts.

What if the charge current continued for quite some time and your lowest capacity cell never quite reached the 3.55v trigger point? Is it not possible to overcharge a cell when the current flowing in is lower, but for a long duration?

If you are going to use a Solar Charge Controller to charge LFP batteries you set the Bulk voltage as high as you can like 60 volts so the controller stays in Constant Current charge mode any time it turns on. When the first cell hits the target voltage you turn the charge source off.

So you are recommending to use a BMS that actively controls the charging functions when using solar as a charging source. That is contrary to what the majority of the EV and boating crowd do, as the BMS is just there as a last resort protection, and really not used at all. Except, of course, those that go beyond the boundaries and charge to 100 % perhaps, but that is not what I'm getting at here.

The alternative, to really be safe with these cells, is to charge at an even lower voltage, but then you would be losing out on some of the pack capacity. (ie charge to only 80% or so to be safe)

[Living Large]

No Sir, you are still stuck in a Pb box. You do not terminate the charge when the pack reaches 56.8 volts as you would severely over charge some of the weaker cells. That is a Pb mentality and you gotta get that out of your head. It does not apply to LFP.

In a bottom Balanced battery you apply a constant current until the first cell reaches 3.55 volts and stop. Only 1 or two cells will get up to 3.55 volts. All the rest of the cells will be at a lower voltage. Example lets say we have 2 cells at 95 AH capacity, and all the rest are 105 AH. The two 95 AH cells reach 3.55 volts, and the other 14 cells in a 16S pack will be down around 3.45 volts for a total pack voltage of 55.4 volts.

If you are going to use a Solar Charge Controller to charge LFP batteries you set the Bulk voltage as high as you can like 60 volts so the controller stays in Constant Current charge mode any time it turns on. When the first cell hits the target voltage you turn the charge source off.

Sorry to butt in, but this is a form of dipping my toe back into the previous conversation we were having about BMS and how to set thresholds and trigger disconnects, etc. I got myself twisted around in that one so badly I had to step away.

I believe you are distinguishing between Northerner using the overall pack voltage and built in settings to stop charging at 56.8V, versus using a cell level monitor (Battery Monitoring System) to stop charging when the first cell hits 3.55V. Correct?

This feels like answering the easy question on the test before moving to the hard ones. I am finding the real world issues Northerner is facing educational.

[tasman]

Ok, my mistake with the voltage of 3.55v for a 90% charge state, as it should have been closer to 3.45v. My apologies. But that doesn't change the question about potentially overcharging cell(s) at a lower charge rate.



What if the charge current continued for quite some time and your lowest capacity cell never quite reached the 3.55v trigger point? Is it not possible to overcharge a cell when the current flowing in is lower, but for a long duration?



So you are recommending to use a BMS that actively controls the charging functions when using solar as a charging source. That is contrary to what the majority of the EV and boating crowd do, as the BMS is just there as a last resort protection, and really not used at all. Except, of course, those that go beyond the boundaries and charge to 100 % perhaps, but that is not what I'm getting at here.

The alternative, to really be safe with these cells, is to charge at an even lower voltage, but then you would be losing out on some of the pack capacity. (ie charge to only 80% or so to be safe)

Just about everyone I have run into using lifepo4 in RV or off grid use a BMS of some description, those that don't, use things like light globes to balance their systems and at top charges divert energy into hot water heating or A/C.

It is very hard to get out of the LA mindset, when you've been using it for decades but believe once you do, everything with lifepo4 makes more sense. Even if working it all out still grinds the mind.

The one thing I find many forget and do myself, is off grid /RV is very different to EV usage and that's where many fall back to LA understanding. During my time using lifepo4, they have never reached the upper or lower voltage parameters and believe the same would go for most of grid /RV situations.

Unlike EV in Off grid/RV, energy is being used 24/7, so if your system is set conservatively then you shouldn't have any problems with over charge and only need to worry about low voltages.

When I first set up my system it was balanced at the upper limit and sits between 3.5v and 3.1v all the time. The big difference is in the usable energy available compared to LA and the fact you see very little voltage changes whilst in use.
Reckon I will be fiddling with my system for awhile yet, still learning and reckon it will be a few years before a really good BMS system comes along for off grid RV use. The number of solar charge and BMS manufacturers who still make their products with and LA mind, is probably 95%, or you have to pay huge amounts for brand name ones, which also fall very well short of what is required for Lifepo4 systems. The person or company that comes along with a BMS charge system than relates to individual cell voltages and not pack voltages, will make a killing.