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

[Sunking]

In 1981, bought a 5w military solar panel and a 20w panel. Next bought 2 x 80w panels, Then built a 12v x 1500w pure sine wave inverter which is now backup for the cheapo Chinese 12v 3000-6000w


Any one who asks me what sort of system they should have, the first thing I ask them, is to add up the wattage of all the things they want to use.

Just amazing you really have no clue what you are doing. You have no idea what the difference is between a battery panels which is all you have and grid tied panels. You might want to stop now you are really making a fool out of yourself.

[tasman]

Just amazing you really have no clue what you are doing. You have no idea what the difference is between a battery panels which is all you have and grid tied panels. You might want to stop now you are really making a fool out of yourself.

In this country and every other country I've been to, there is no difference between panels for grid connect, or off grid. Because you claim to be some holier than thou know it all technical genius, googled to see what, if any difference there is between off grid solar panels and grid connect solar panels, just for a laugh.

Guess what, nowhere could I find a reference to any difference between solar panels used for grid connect or off grid systems, they are all exactly the same and all you have to do is provide some links showing they use different solar panels for each application.

The facts are, if you read the reply posts to you, it's clear who is making a fool of themselves. Especially when you make such ridiculous claims with nothing to back it up, but your ego. Why would anyone listen to someone who does nothing but insult everyone on this forum and demands you are only one that's right. Seems you have absolutely no personal hands on experience whatsoever with solar and their energy storage system, other than in your head and it shows.

[russ]

In this country and every other country I've been to, there is no difference between panels for grid connect, or off grid. Because you claim to be some holier than thou know it all technical genius, googled to see what, if any difference there is between off grid solar panels and grid connect solar panels, just for a laugh.

Guess what, nowhere could I find a reference to any difference between solar panels used for grid connect or off grid systems, they are all exactly the same and all you have to do is provide some links showing they use different solar panels for each application.

The facts are, if you read the reply posts to you, it's clear who is making a fool of themselves. Especially when you make such ridiculous claims with nothing to back it up, but your ego. Why would anyone listen to someone who does nothing but insult everyone on this forum and demands you are only one that's right. Seems you have absolutely no personal hands on experience whatsoever with solar and their energy storage system, other than in your head and it shows.

tasman - be gone - you are only confusing to people that have no idea.

[Living Large]

tasman - be gone - you are only confusing to people that have no idea.

Whew. Thank you. Just goes to show that Google is not always your friend.

[Living Large]

Uh, but the problem is, interfacing the BMS to the solar charge controller. Most of the BMS have 1-10A shunts, to bypass the full cells, and a special AC mains charger they interface to, to stop the charge when the first 2 or 3 cells fill up.

But solar is a different creature, we charge at much higher rates (I hit +40a now, and can get +60a adding the generator to it). And the 10A shunt on the BMS is going to glow cherry red while 30A continues into the battery. Most solar charge controllers don't have a connection to the BMS, so they are not going to stop till they think they are done. So you have to build a special "one-of" system for your bank, and hope there were no engineering mistakes. Not that it can't be done, but that it's hard to do right.

Mike

One of the few details I did iron out for if I were to add a B-Monitor-S to an LFP bank for the purpose of adding dynamic functionality was that the CC in question (classic 150) has an input that suspends charging until the input changes state. So when the first cell hits a selectable target "charged" voltage in the BMS, charging from PV will cease.

My belief is that I will absolutely need some extra logic and possibly disconnects, in addition to a B-Monitor-S, to build a "one-off" system, but the choices are either that custom design or not use LFP. As Dereck said much earlier, not ready for solar for the average person.

[Sunking]

My belief is that I will absolutely need some extra logic and possibly disconnects, in addition to a B-Monitor-S, to build a "one-off" system, but the choices are either that custom design or not use LFP. As Dereck said much earlier, not ready for solar for the average person.

LL I am not quite following you here. MS and most all CC's have a built in clock, and when they trigger to Float or interrupt will reset and resume charging the next day assuming the signal has reverted and not latched.

In addition as I tried to explain to Northern if you Bottom Balance and want to be a minimalist in terms of equipment, you do not need any additional equipment and can use most all Solar Charge Controllers. Stay with me here and I will explain.

1. When you balance at the top is a Voltage Reference Point only that represents a 100% State of Charge voltage. It does not tell you a lot about capacity. A 10 AH cell has the exact same 100% SOC as a 200 AH cell. In a Top Balanced system the only time the battery voltages are equal is at 100%.

2. So let's say we have a simple 4S 100 AH LFP making a 12 volt battery. We Top Balance with each cell at 3.6 vpc or pack voltage is 4 x 3.6 volts = 14.4 volts. In that pack Cell #1 is the weakest at 90 AH, and the other 3 are strong at 110 AH. So when fully charged the pack capacity is only 90 AH. Now let's discharge say 80 AH. Cell #1 only has 10 AH left in it and the other 3 have 30 AH. Cell #1 SOC voltage is at 10% SOC = 3.02 volts, and the other 3 cells @ 30% SOC = 3.17 volts. Pack voltage = 12.53 volts and if you converted would lead you to believe you still have 27% SOC and are safe when you are on the edge of loosing Cell #1. Point I am making here is when you Top Balance the voltages are only equal at the top. As you discharge the weakest cell will be the lowest voltage of all the cells. In a Top Balanced system you must monitor every cell, not the Pack for your Low Voltage Cut Off

3. In a Bottom Balanced we reference at 0% SOC of 2.5 vpc. On a 12 volt system is 10 volts. Not only do we know the SOC voltage, but we ALSO KNOW THE CAPACITY OF EACH CELL = 0 AH. That is completely different than Top Balance. We now now capacity and SOC are equal at the BOTTOM.

4. So how can we charge a Bottom Balanced system. There are two ways. You already know using a Monitor (call it BMS PLC, or whatever floats your boat) But there is a minimalist approach using a Constant Current Constant Voltage method. Every Solar Charge Controller is a Constant Current Constant Voltage charger. All we have to do is pick the right voltage set points. Back to our example of 4 cells with 1 weak cell and 3 strong cells. We want to set Float at 3.4 volts per cell aka 90% SOC to start. If the charge controller wil permit it set Bulk (constant current) = 13.6, Absorb (constant voltage) = 13.6 volts, and Float (constant voltage) = 13.6 volts.

5. So now we are charging for the first time. We watch the voltage as we charge and we hit 13.6 volts and notice the current starts to taper off and we are holding 13.6 volts. We measure each cell and we are looking for the cell with the highest voltage. That cell is going to be the WEAKEST CELL in the pack. We monitor it until the charge current has tapered down to a trickle indicating we are fully charge dup per VOLTAGE setting.

6. We again check cell voltages and find that weak cell and note the voltage. We see it is at 3.6 volts and the others are at 3.33 volts. Cell #1 is at 100% SOC and the other 3 are at roughly 85% SOC. That tells us we have the voltage set a bit too high. So we back it off by .1 volts.

7. Next day watch again. This time the weak cell is 3.5 volts. Perfect. The other three are at 3.3 volts Pack capacity is around 80 to 85 AH, and every cell has the exact same amount of AH.

8. Now it is time to discharge through your INVERTER. We set the Inverter LVD anywhere between 10.5 to 12 volts. As the batteries discharge the voltages start to become equal near the. At 10.5 volts is 2.625 volts per cell significantly above 2.5 volts where we Bottom Balanced. It would be next to impossible to over discharge the weakest cell.

9. Lastly we monitor things about once a weak when we are charging and keep an eye on that weakest cell at the end of the charge to make sure it is not creeping up to 3.6 volts and make voltage adjustments when necessary. For those rare times we get near the bottom make a simple check and the voltages should be fairly eqaual at the Bottom.

FWIW that is exactly how the majority of the DIY EV guys operate. It is only the guys with BMS that Top Balance that destroy cells. Bottom Balance guys eliminate all that risk. Some like me go that extra step and use a monitor on the charge and discharge side. I do so because I charge at C/2 using only constant current and terminate when my weak cell hits 3.55 volts. It rest and settles at 3.4 volts aka 90% SOC. All the other cells are slightly lower. SO KISS (keep it simple stupid). It is not that complicated. Go top balance you complicate things, and make it more expensive than necessary.

[Living Large]

LL I am not quite following you here. MS and most all CC's have a built in clock, and when they trigger to Float or interrupt will reset and resume charging the next day assuming the signal has reverted and not latched.

I was commenting on Mike's comment, and I'll admit that I may have misspoken - I never did find a description of how the Aux port works on the Classic (which I find very odd - why wouldn't it be in the manual? Rhetorical question), and *assumed* the input would have to be latched. You are telling me new information that a newbie with no equipment doesn't know - the CC resumes charging the *next day* after being triggered.

I'm not worried about balancing, which others here are debating, although I realize that the balancing method used plays into system operation. I am assuming I will bottom balance. And for the moment, I am not worried about the actual voltage window you are choosing.

What I am totally confused about at this point is what, if anything, hardware outputs of a B-Monitor-S would be used for. When we first discussed the fact the BMS can detect a cell going below a low or above a high threshold, I assumed I would want to stop charging or disconnect a load based on that - using a BMS output. I did later think, walking my dog one day, about what you are saying here - that based on observations of individual cells, you could tweak your set-points. Maybe I shouldn't have left and stayed in the conversation. So you are fine tuning the system operation by giving it more information than just setting a threshold based on assuming all cells have the same voltage at all times. Is it the case then, that as long as one remains within the desired voltage window for all cells as they charge and discharge daily, that BMS hardware outputs would not be triggered?

Returning to the BMS - for example the the Orion Jr. I will not be using the balancing function, but what else exactly will I be using it for?

1. To track cell voltage information that I read with my eyes, and use to manually tweak the settings in the CC and inverter/charger (passive)
2. Possibly LVD and or HVD (active)?

If I am tracking the performance of the battery bank daily to weekly, it seems like tweaking as you describe will keep things working smoothly. But would (or should) I use BMS outputs of LVD or HVD as a safety backup? If a never to exceed low voltage is detected, I could shut down my inverter/charger using an input it has that shuts it down. If a never to exceed high voltage is detected, I could interrupt my CC and also shut down the inverter/charger using the same input as LVD to account for the possibility it is charging using the generator when the fault condition occurs.

When I assumed I would be using BMS outputs for every day operation to either start or stop charging, it made me think I needed additional hardware (which would be the case). I think I made it too complicated.

[Sunking]

So you are fine tuning the system operation by giving it more information than just setting a threshold based on assuming all cells have the same voltage at all times.

No when did I ever say that? I walked you through it step by step. Cell voltages are only equal at one of two reference points, either at the bottom, or at the top. Anywhere in between they will not be unequal. If you Top Balance you are going to need a BMS to monitor and control both charging, and discharging. If you Bottom Balance you do not need what is known as a BMS. You can use your inverter (built-in LVD) or some type monitor control circuit to disconnect if a cell voltage gets low. For charging it can be done with nothing more than the proper voltage set points in you Solar Charge controller, and tweaking it for the first couple of days to zero in.

As for the Orion Jr it is a full fledged BMS. It is also a PLC. However you do not have to use the Top Balance capabilities. No police are going to come around and force you to use it. What might be throwing you in the Industry there is no definition of BMS. All commercial BMS on the market are tailored to Top Balancing. The Orion Jr is really a Programmable Logic Controller that you can program, and it can be programmed to be used in a Bottom Balanced system.

Again and for the last time there are two ways to use a Bottom Balanced system. Take your pick.

1 Let's call it a Minimalist System. In a Solar application you do not need anything except the batteries, charge controller, and an Inverter with Built-In LVD. Nothing else, same thing any lead acid battery system uses. NOTHING MORE OR NOTHING LESS. You Bottom Balance the batteries, and set the Charge Controllers voltage set point to 54.4 volts. When your controller reaches the Float Mode final stage, you measure all the cell voltages and find the cell with the HIGHEST VOLTAGE and take note. That cell will be the weakest cell. If the weast cell voltage is 3.4 to 3.5 volts is perfect. All other cell voltages will be LOWER. If the weakest cell voltage is above 3.5 volts, lower the Charge Controllers voltage a bit. go to bed and repeat the next day. Keep doing that until you get the weak cell to have a voltage of 3.4 to 3.5 volts. On your Inverter set the LVD voltage to as low as 42 volts or as high as you want. 16 cells x 2.5 volts = 40 volts.. So 42 volts is more than enough to protect you from over discharging. If the cells are Bottom Balance when you reach 40 volts, all cells will be at 2.5 volts where you Balanced them. All 16 cells at 2.5 volts have the exact same capacity of 0 AH. Then about once a week check cell voltages after a charge, and adjust Charge Controllers voltage as if needed.

2. The other way is to use something like an Orion Jr. or what ever you want to call it. If you want the Orion to protect you from over discharge monitor all cells and have it send a signal to the Inverter to shut down when any cell reaches 2.5 volts. On the charge side have the Orion send a signal to the Charge Controller to shut off when any cell reaches 3.6 volts and set the Charge Controllers voltage high enough (59 to 60 volts) so anytime it is on, it is in Constant Current mode. That will charge the batteries fastest time possible.

[Living Large]

No when did I ever say that? I walked you through it step by step. Cell voltages are only equal at one of two reference points, either at the bottom, or at the top. Anywhere in between they will not be unequal. If you Top Balance you are going to need a BMS to monitor and control both charging, and discharging. If you Bottom Balance you do not need what is known as a BMS. You can use your inverter (built-in LVD) or some type monitor control circuit to disconnect if a cell voltage gets low. For charging it can be done with nothing more than the proper voltage set points in you Solar Charge controller, and tweaking it for the first couple of days to zero in.

As for the Orion Jr it is a full fledged BMS. It is also a PLC. However you do not have to use the Top Balance capabilities. No police are going to come around and force you to use it. What might be throwing you in the Industry there is no definition of BMS. All commercial BMS on the market are tailored to Top Balancing. The Orion Jr is really a Programmable Logic Controller that you can program, and it can be programmed to be used in a Bottom Balanced system.

Again and for the last time there are two ways to use a Bottom Balanced system. Take your pick.

1 Let's call it a Minimalist System. In a Solar application you do not need anything except the batteries, charge controller, and an Inverter with Built-In LVD. Nothing else, same thing any lead acid battery system uses. NOTHING MORE OR NOTHING LESS. You Bottom Balance the batteries, and set the Charge Controllers voltage set point to 54.4 volts. When your controller reaches the Float Mode final stage, you measure all the cell voltages and find the cell with the HIGHEST VOLTAGE and take note. That cell will be the weakest cell. If the weast cell voltage is 3.4 to 3.5 volts is perfect. All other cell voltages will be LOWER. If the weakest cell voltage is above 3.5 volts, lower the Charge Controllers voltage a bit. go to bed and repeat the next day. Keep doing that until you get the weak cell to have a voltage of 3.4 to 3.5 volts. On your Inverter set the LVD voltage to as low as 42 volts or as high as you want. 16 cells x 2.5 volts = 40 volts.. So 42 volts is more than enough to protect you from over discharging. If the cells are Bottom Balance when you reach 40 volts, all cells will be at 2.5 volts where you Balanced them. All 16 cells at 2.5 volts have the exact same capacity of 0 AH. Then about once a week check cell voltages after a charge, and adjust Charge Controllers voltage as if needed.

2. The other way is to use something like an Orion Jr. or what ever you want to call it. If you want the Orion to protect you from over discharge monitor all cells and have it send a signal to the Inverter to shut down when any cell reaches 2.5 volts. On the charge side have the Orion send a signal to the Charge Controller to shut off when any cell reaches 3.6 volts and set the Charge Controllers voltage high enough (59 to 60 volts) so anytime it is on, it is in Constant Current mode. That will charge the batteries fastest time possible.

Now hang in there, don't go all tasman on me - I think I am close to understanding. The first thing to recognize is your thinking about this has changed quite a bit over the past month. You now have this all figured out in your mind, I do not. But it looks like the answer may be at hand. FWIW, as far as what you underlined in the quote, that was a reference to tweaking thresholds as a result of observing one cell may be an outlier relative to others - 6. We again check cell voltages and find that weak cell and note the voltage. We see it is at 3.6 volts and the others are at 3.33 volts. Cell #1 is at 100% SOC and the other 3 are at roughly 85% SOC. That tells us we have the voltage set a bit too high. So we back it off by .1 volts. Moving on to the big picture....

1. Looking back at why I assumed I needed a BMS
What appears to be the case, is what I was envisioning a BMS (such as the Orion Jr) could do, might be done but isn't necessary if one bottom balances the bank, and tweaks thresholds from time to time as you have described below. I didn't realize that when you got the religion of bottom balancing a few weeks ago, the need for a BMS dissolved. Again, confusion on my part - I thought you were advising a (non-shunting) BMS a month ago for LVD and/or HVD, and maybe that was the case, but now I am thinking it was for a condition (which may include LVD) that is a consequence of top balancing, which isn't on the table. In short, I was stuck thinking you still would want an extra safeguard to ensure the batteries are not over discharged or over charged. Your thinking on the BMS moved at 60mph, and I am on foot and have my thumb stuck out looking for a ride. I am just beginning to understand what a BMS can do, and you've already reached a point where you rendered it unneeded, which I didn't realize.

2. Looking forward to not using a BMS (KISS)
Today I am hearing that I can safely operate an off-grid system with an LFP bank with no BMS if I bottom balance and set modes and thresholds as you describe, and monitor battery voltages regularly to see if tweaking of thresholds is needed. For LVD, the inverter (if capable, as the XW is) will take care of that. I assume I don't need a HVD if I have my threshold set right and I rely on the CC (for PV) and charger (for genny) to function properly?

BMS optional?
To avoid having to use a DVM on 16 batteries weekly, I could use something like the Orion Jr to track the battery voltages , right? And if it gave me peace of mind, I could drive the Aux input on the CC to interrupt charging, though if I operate as you suggest below that should never happen.

Thanks for your patience. I am happy to hear that it isn't necessary to have a BMS after all.

[Sunking]

Now hang in there, don't go all tasman on me -

LOL, sorry about that LL, bad day yesterday. Please accept my apology. My patience was running a little thin yesterday.

1. Looking back at why I assumed I needed a BMS
What appears to be the case, is what I was envisioning a BMS (such as the Orion Jr) could do, might be done but isn't necessary if one bottom balances the bank, and tweaks thresholds from time to time as you have described below. I didn't realize that when you got the religion of bottom balancing a few weeks ago, the need for a BMS dissolved.

That is part of the source of confusion. There is no standard definition of BMS. From a manufacture POV it means Top Balanced via passive Distributed (Vampire Board on each cell) or Non Distributed where the Bypass Circuits are contained in the same box as controller. For example on my NEV all the BMS was were the 16 Vampire Boards on each cell. Completely passive and not interconnected to anything. Just 16 independent dumb circuit boards unaware of the world around them. They turned on at 3.5 volts, and turn off at 3.4 volts.

The Orion Jr on the other hand like many others have some smarts built into them. Not only do they have Bleeder Boards built into the box, it also monitors cell voltages and program control functions you can control which is what I am doing. I chose that route because I am using a 50 amp Telecom Rectifier as a constant current source to charge with. Instead of letting the Rectifier terminate the charge based on Total Pack Voltage, I chose to let Orion terminate the charge on cell voltages. That decision was based on I wanted the fastest charge speed possible using what I had on hand when I decided to trash the 16 vampire boards.

Point here is if you Bottom Balance, you have several different options available to you. One of those options is a minimalist approach which came about when Nortern stated he was gong to do everything manually but top balanced with no additional equipment. If you go that route Bottom Balanced is the way to go because LFP batteries will not tolerate over discharge. If you Top Balance and do not use a cell monitor, you do not know where the Bottom is without a cell monitor.

But do not conclude that I am telling you to take the minimalist approach method. All I am saying is that option is available to you. Personally I would not go that route if using solar. Only thing I use my Orion Jr for is to turn off the charger, fuel gauge, and tel the motor controller to go into Limp Home Mode when capacity is low and warn me if that ever happens.

2. Looking forward to not using a BMS (KISS)
Today I am hearing that I can safely operate an off-grid system with an LFP bank with no BMS if I bottom balance and set modes and thresholds as you describe, and monitor battery voltages regularly to see if tweaking of thresholds is needed. For LVD, the inverter (if capable, as the XW is) will take care of that. I assume I don't need a HVD if I have my threshold set right and I rely on the CC (for PV) and charger (for genny) to function properly?

BMS optional?
To avoid having to use a DVM on 16 batteries weekly, I could use something like the Orion Jr to track the battery voltages , right?

Yes you got your thinking straight. But like I just said it is an option available to you.

As you know LFP have two danger zones. Being over charged, and over discharged. Between those two the line we never want to cross is Over Discharge. Only time we want to ever approach that cliff is initially when we Bottom Balance the batteries in a very controlled manner. At the top LFP is far less sensitive, and LFP fortunately is the most tolerant of all the Lithium batteries.

So one of the first choices you have to make is how to guard against over discharges. Are you comfortable with letting you Inverter do that job all by itself? Or do you wear suspenders along with a belt and want redundancy of using something like a Orion or some other PLC as first line of defense?

At the top you have more wiggle room to play with. Again you have to decide what you are comfortable with. Do you want to terminate charge when the first cell reaches say 3.4 volts? That will require some type of monitor control like the Orion. Or are you comfortable with just using the Charge Controller to do that by tweaking it in based on the weakest cell finale charge voltage?

The choices are up to you based on your comfort levels of budget and complexity level you are willing to go.

[Living Large]

{snip}
Point here is if you Bottom Balance, you have several different options available to you. One of those options is a minimalist approach which came about when Nortern stated he was gong to do everything manually but top balanced with no additional equipment. If you go that route Bottom Balanced is the way to go because LFP batteries will not tolerate over discharge. If you Top Balance and do not use a cell monitor, you do not know where the Bottom is without a cell monitor.

{snip}The choices are up to you based on your comfort levels of budget and complexity level you are willing to go.

OK! Thanks a lot for your patience. Before I left the discussion a few weeks ago, I remember Northerner saying he did everything manually. It sounds like the discussion evolved in that direction - and with the benefits afforded by bottom instead of top balancing, it is possible to operate safely and efficiently using this "minimalist" approach. I like it, and as long as things are typically predictable, and there are safeguards in place to prevent disaster, I would be comfortable with it.

I would consider adding a battery monitor as an option, as I suggested, for the limited functions I mentioned. Pulling out the Fluke weekly and measuring 16 cells might get old.

This sounds like a great baseline approach. I'm psyched. However, I just in the last few hours located an on-grid property for sale

[Sunking]

However, I just in the last few hours located an on-grid property for sale

Great, that is what I suggested when you first joined. I learned something along the way. Good luck.

[Living Large]

Great, that is what I suggested when you first joined. I learned something along the way. Good luck.

Yes, you did mention that straight off. One of the things I like about you is that you've got the big picture and the details in focus at the same time. It's almost amusing that you can discuss a solar solution analytically for weeks, yet when the wisdom of doing it versus not going solar at all comes up, you quickly say "well yeah, it's going to cost you 4 times as much and you'll have a new full-time job." We'll see where things lead - there is no contract on any property yet.

I never suspected it would be as involved as it is, but you are dealing with nature and chemistry. This is a fantastic forum - the interchange of ideas of people with various kinds of PV knowledge and experience (as compared with those with 0, like myself) provides for some very informative discussions. And there is a queen, to boot.

[PNjunction]

Top balance or bottom balance, as long as one keeps there distance from the knees, will work out just fine.

Yes, as long as one relies on the manufacturer to get the cells nearly equal in capacity and internal resistance. Since the mantra with solar is to design in headroom no matter the chemistry, a conservative approach works well for US. EV and RC are different stories.

Sadly, many forum discussions fail to take into account real-world setups, that is the additional internal resistance caused by differences in cell strapping, and bus-bar charging length differences which are not on the manufacturer's spec sheet obviously. Even if each cell is exactly perfect from the manufacturer, these real-world connections with differing resistances, along with variances of the cells themselves, can cause different *rates* of voltage during charge and discharge at the extreme top and bottom, which we know is a poor proxy for capacity. Cell voltage balancing to miniscule differences mean nothing. That's why I'm comfortable with no more than 0.100v delta among the cells, although I strive for 0.050v or better, but don't pull my hair out trying to achieve absolute perfection.

Staying conservative helps here as well.

For example, when my lifepo4 cells are measured on a smart iCharger 306B, the 4S battery pack reveals an internal resistance of:

Cell 1: 1
Cell 2: 1
Cell 3: 2
Cell 4: 3

This was *after* I disassembled the straps, and cleaned the slightly oxidized terminals before reassembly. Prior to that, Cell2 was also at 3. Of course the iCharger 306B is not a lab-grade instrument, but it did provide a baseline for me to watch.

Thing is, even with cell4 having a higher internal resistance than the rest, and charging up faster and settling down quicker than the others during charge and at rest, the battery as a whole provided at least 80% of the stated overall capacity before any one cell went beyond 3.0v.

I'm good. For solar, derate your stated cell capacity by 20%, stay out of the knees, run "Sub-C" and live long and prosper.

[Sunking]

Yes, as long as one relies on the manufacturer to get the cells nearly equal in capacity and internal resistance. Since the mantra with solar is to design in headroom no matter the chemistry, a conservative approach works well for US. EV and RC are different stories.

So what are the differences between say solar and EV application?

EV charge much slower than Solar, and discharge at a higher rate than solar. LFP is not effected by Peukert to any measurable degree in either application, thus capacity does not change in either application. The unique characteristic of LFP, a very desirable characteristic, is LFP has 100% round trip charge efficiency. 1 AH in = 1 AH out. Just don't confuse charge efficiency with power efficiency. LFP is not 100% power efficient because AH goes in at a higher voltage than going out.

I just don't see any real differences in application. Solar charges at much higher rates which means you need to keep a close on on charging. On the discharge side a EV discharges much faster and operates at higher temps so more attention needs to be given on the discharge side.

Sadly, many forum discussions fail to take into account real-world setups, that is the additional internal resistance caused by differences in cell strapping, and bus-bar charging length differences which are not on the manufacturer's spec sheet obviously. Even if each cell is exactly perfect from the manufacturer, these real-world connections with differing resistances, along with variances of the cells themselves, can cause different *rates* of voltage during charge and discharge at the extreme top and bottom, which we know is a poor proxy for capacity.

I disagree with you on most of those points. Intercell connection resistances have absolutely no effect on capacity simple because the cells are wired in series. In a series, current in all cells is equal. It is impossible for the current to be different.

I do agree with you at the top AH capacity is unknown because as you noted no two cells are created equal. When at 100% SOC, all that is known is the battery is at full capacity, but the capacity is unknown and there will be variances as that is the nature of the beast of manufacturing. However at the Bottom not only do we know the SOC voltage of 2.5 volts, we also know the capacity = 0 AH. So we have two known reference points which is a huge advantage. Due to the fact the charge efficiency is 100%, if we start at 0 AH and add say 90 AH to a 100 AH pack, then we know all cells have the exact same 90 AH capacity. Their voltages will not be equal at the top due to variances in total capacity of each cell, but we will know the AH capacity of every cell. The only place cell voltages will be equal is at the bottom at a known reference point.

Cell voltage balancing to miniscule differences mean nothing. That's why I'm comfortable with no more than 0.100v delta among the cells, although I strive for 0.050v or better, but don't pull my hair out trying to achieve absolute perfection.

From 10% to 90% LFP cell voltages are linear and a very narrow range of .3 volts where at 3 volts roughly equal 10% SOC and 3.3 volts equal 90% at OCV. .1 volts is a significant difference in the operating range of interest. The only place we need to be very precise is at the bottom reference point which is extremely easy to do.

Internal resistance exist in all batteries. LFP's just happen to be one of the types with the lowest Ri, and that is very desirable especially in high C Rate application as well as low C Rate applications. The question becomes what effect does it have, and is there anything we need to do about it.

A 100 AH LFP has an Ri of less than 1 mill-ohm and ranges from .9 to .6 mill-ohms. The Ri scales up or down with capacity proportionally. The two most important characteristics of Ri are Voltage Sag/Rise under high Load/Discharge currents. The other is heating and power losses of heavy load or charge currents.

At 1C rates on any size LFP battery ranges in a Sag/Rise voltage up to .1 volts. That makes them extremely effective at very high C Rate applications. Not so important for solar, but extremely important for EV's when accelerating and regen braking where C Rates exceed 1C. The difference in how that is handled is in solar applications is a non issue on the charge or discharge side. In solar you only need simple trigrer tates for HVC and LVD. In a high current application you use the same voltage trigger set points, but add an element of time delay with special attention on the discharge side. Regen braking and charging on the EV side is a a non issue because charging is slow, and after you go a very short distance in an EV there is plenty of room at the top for the regen current to be absorbed.

The other issue of Ri is heating and power losses. Ri is the source of charge/discharge power efficiency and is fairly linear across various C Rates. For example at 1C on a 100 AH cell you are heating the cell with 10 watts while the load is burning 320 watts or an efficiency of 97%. Same cell at .1C is burning 1 watt and supply 32 watts with an efficiency of 97%. From that you can conclude two things. Cell heating is of no real concern at 1C and lower. 2nd is round trip charge/discharge power deficiency is .97 x.97 = 94% which no other battery can match.

PN I do agree with you that it is very possible to use LFP with simple controls and limits. I use to be of the school of Top Balance to maximize usable power. Then when I put that into practice, it did not take me long to realize you do not have Top Balance for maximum usable capacity and Top Balance damages cells and is the root cause of most battery failures. It just made since to Bottom Balance where SOC and capacity are both known. At the Top in the Bottom Balance system the key is the weakest cell and only taking it to 90% capacity. It will have the highest voltage when charged. So in that respect when fully charged up actual cell voltages are irrelevant as they will not be equal. That makes it very easy to charge. At the bottom the cell voltage will become equal and making things easy to protect from over discharging.