LiFeP04 Batteries for Solar & BMS

[Sunking]

It sounds like shunts would not be used, and are *not needed*, using the methodology we are now discussing. Is that correct?

Correct if you are Bottom Balancing there is no need for Top Balance Boards. Just remember there methodologies of Top and Bottom Balancing. Today Top Balancing is the direction Manufactures are going. This comes from the commercial EV market. Well most of the commercial EV market. Example the Leaf uses top balancing, and they are having problems which have made the news. Chevy Volt on the other hand uses a different methodology of not allowing their batteries to ever be charged to 100% SOC. Its a whole new field with Lithium and breaking Pb ways of doing things.

Is a "BCU" is a battery control unit?

Yes. Depending on manufacture can be called anything like EMS (Energy Management System) Like I said there is no definition of BMS.

Remember, I'm new to this stuff. When we were talking shunts, I was looking at individual BMS board with shunts and a dumb make/break serial loop to allow for LVD and HVD. I assume the voltage thresholds with the devices I was considering are not user-settable.

I am new too to some extent. My LFP experience is with EV's and I am still evolving. There is a Forum devoted to DIY EV and many of the members are the engineers and designers of commercial EV's. That got me started down the Top Balance road. However their are a few Contrarians in that group with considerable experience of 7 to 10 years I have come to be friends with. They are ole dogs like me that take their experience and have learned to refine and evolve their approach from hard lessons of the past.

I saw something you wrote here recently that described a 3 or 4 wire loop, I believe. That would allow for communication, and is a step above the devices I was looking at. Can you point me to an example of a "BCU" that might fit the bill for what is now being discussed? Thanks again.

Give me a day to do some research. I know of a couple but there is more out there. Some do use individual circuit boards to monitor cell temperatures which i simportant for EV's, not so much for stationary uses like RE. Hum this gives me an idea of a biz. This would be real easy to use something like Arduino controller with 16 DAC's.

In the meantime read this Slide Show. Only takes 5 minutes. Then I think you will see where I am coming from. Bottom Balance, use LVD monitoring every cell, and charge motoring every cell and terminate when first cell reaches 100% or slightly less. Remember lithium cells last longer if not charged to 100%. Some would say the method I am thinking of is not BMS because I am not advocating taking every cell to 100%. For me I want maximum battery life and range is not as important or useful for my application. That thinking can easily be applied to RE.

[Living Large]

No you are right, I am doing an About Face 180 degree turn. There are two methods of Balancing Cells: Top and Bottom. Top Balance is what we have been talking about all this time using Balance Boards. The issue I have been convinced of is to NOT TOP BALANCE because we reference or calibrate to 100% which is meaningless. No two cells have equal capacity. For example if we are running 100 AH cells the lowest might be say 101 AH and highest 115 AH. If we balance at the top the 101 AH cell will go FLAT way before the 115 AH cell. Top Balance also forces you to go to 100% SOC on every cycle, thus causing stress. No need to go to 100% with LFP, you can get around 50 to 100% more cycles only going to 90%.

Solution or a different option is Bottom Balance or reference all cells to 0%. Then charge until the first cell hits a target SOC like 90 or 100%. That will be the weakest cell of say 101 AH. All cells will have the same capacity, just not fully charged. On the discharge side they all discharge and arrive at 0% roughly at the same time. You use your BCU to monitor cell voltages. When the first cell reaches 2.9 volts trips the LVD. On the charge side you terminate charge when th efirst cell approaches 90 to 100%.

To bottom balance you do NOT USE BALANCE BOARDS, you only monitor cell voltages with the BCU and program the BCU to fit your taste and alarm points coupled with a LVD to protect the batteries from over discharge. As for Charging is going to depend on how robust the CC is. If it can take an external signal like a contact closure from the BCU to turn it off, you set the CC to supply Constant Current util told to quit by the BCU. Otherwise you will set BULK = 58.4 volts and Absorb to 0 minutes, and Float to 56 volts. That will put you into Constant Current Mode until the cell average voltage is 3.65 volts (or a bit lower like 56 volts to 90% SOC) then immediately drop down to Float voltage so you can use solar power until the sun gives out instead of batteries.



BCU should do that.



Cannot give you a direct link but Click Here and scroll down to show date Nov 13 2009. Once you start the video and if you are short on time FF to about the 40 minute mark and watch the last 20 minutes. It will recap and visualize what I just covered. Jack is kind of the God Father of DIY EV who started it all. He is one of the men I spoke with and pointed me to the episode to demonstrate his POV on BMS. He goes against the industry which is Top Balance. It shoul dopen your eyes.

Thanks - I'll go check out the video. I understand shunts aren't need for bottom balancing. It sounds like shunts would not be used, and are *not needed*, using the methodology we are now discussing. Is that correct? Edit I'm watching the video now, and the answer to this question appears to be "take off the shunts if you have them." This means I just need to make sure I monitor my cells, have a HVD, and not overcharge. I like this guy's style.

Is a "BCU" is a battery control unit? Remember, I'm new to this stuff. When we were talking shunts, I was looking at individual BMS board with shunts and a dumb make/break serial loop to allow for LVD and HVD. I assume the voltage thresholds with the devices I was considering are not user-settable. I saw something you wrote here recently that described a 3 or 4 wire loop, I believe. That would allow for communication, and is a step above the devices I was looking at. Can you point me to an example of a "BCU" that might fit the bill for what is now being discussed? Thanks again.

ps In dream world, I would have a screen on maybe this BCU that shows the individual cell voltages so I don't have to break out the Fluke constantly. But I need to walk before I can run.

[Sunking]

From your reply here and the post that begins this (new) thread, it sounds like you are saying that you are against top balancing LFP for solar, or am I wrong about that?

No you are right, I am doing an About Face 180 degree turn. There are two methods of Balancing Cells: Top and Bottom. Top Balance is what we have been talking about all this time using Balance Boards. The issue I have been convinced of is to NOT TOP BALANCE because we reference or calibrate to 100% which is meaningless. No two cells have equal capacity. For example if we are running 100 AH cells the lowest might be say 101 AH and highest 115 AH. If we balance at the top the 101 AH cell will go FLAT way before the 115 AH cell. Top Balance also forces you to go to 100% SOC on every cycle, thus causing stress. No need to go to 100% with LFP, you can get around 50 to 100% more cycles only going to 90%.

Solution or a different option is Bottom Balance or reference all cells to 0%. Then charge until the first cell hits a target SOC like 90 or 100%. That will be the weakest cell of say 101 AH. All cells will have the same capacity, just not fully charged. On the discharge side they all discharge and arrive at 0% roughly at the same time. You use your BCU to monitor cell voltages. When the first cell reaches 2.9 volts trips the LVD. On the charge side you terminate charge when th efirst cell approaches 90 to 100%.

To bottom balance you do NOT USE BALANCE BOARDS, you only monitor cell voltages with the BCU and program the BCU to fit your taste and alarm points coupled with a LVD to protect the batteries from over discharge. As for Charging is going to depend on how robust the CC is. If it can take an external signal like a contact closure from the BCU to turn it off, you set the CC to supply Constant Current util told to quit by the BCU. Otherwise you will set BULK = 58.4 volts and Absorb to 0 minutes, and Float to 56 volts. That will put you into Constant Current Mode until the cell average voltage is 3.65 volts (or a bit lower like 56 volts to 90% SOC) then immediately drop down to Float voltage so you can use solar power until the sun gives out instead of batteries.

My second question is regarding your "yellow" trigger for low voltage - would something off the shelf do what you suggest, or is this another example of "not ready for solar". Do you think I would need to design a custom circuit? I don't mind design; my only concern would be creating a low power-consuming solution.

BCU should do that.

Edit Did you forget to add the link to the video you reference? I don't see one.

Cannot give you a direct link but Click Here and scroll down to show date Nov 13 2009. Once you start the video and if you are short on time FF to about the 40 minute mark and watch the last 20 minutes. It will recap and visualize what I just covered. Jack is kind of the God Father of DIY EV who started it all. He is one of the men I spoke with and pointed me to the episode to demonstrate his POV on BMS. He goes against the industry which is Top Balance. It shoul dopen your eyes.

[Living Large]

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.

Interesting. Yes, I was patiently waiting for a response, but I didn't know it would be this thorough. Thank you very much. I did follow much of what you said where you asked "following me?", but there are still ? marks in my mind.

One of the first things I came across two months ago regarding LFP was a video showing bottom balancing batteries in an EV. Then there was the discussion here regarding top balancing and shunts. I followed most of this, I believe.

From your reply here and the post that begins this (new) thread, it sounds like you are saying that you are against top balancing LFP for solar, or am I wrong about that? Based on what I read, I am now thinking that if a cell got off by 0.1V or some other action threshold I chose, that going back through the bottom balance procedure may be in order - not relying on the shunt system and top balancing. I am now thinking back to someone saying the batteries should never go out of balance if balanced initially, so maybe I am anticipating something that may rarely or never be needed? Have we arrived at shunts aren't desirable for a solar LFP bank?

My second question is regarding your "yellow" trigger for low voltage - would something off the shelf do what you suggest, or is this another example of "not ready for solar". Do you think I would need to design a custom circuit? I don't mind design; my only concern would be creating a low power-consuming solution.

Edit Did you forget to add the link to the video you reference? I don't see one.

[Sunking]

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.

[Sunking]

Thought I should share some thoughts about Lithium Iron Phosphate aka LiFeP04 or LFP in this thread. I am not going into other Lithium chemistry like Cobalt, Manganese, or other types because for Solar LFP and Renewable Energy applications IMO LFP is the only chemistry that works economically and technically.

So what are some of the known facts about LFP, and how do we use those facts to get the most out of them:

• Today LFP has the longest Cycle Life of all the battery chemistries from 100% to 20% State of Charge aka SOC. Side note Pb or lead acid has longer life in a narrow range of 100 to 75% SOC of about 30%.
• 100 wh/Kg specific Energy Density or roughly 3 times more than Lead Acid making them lighter and additionally smaller volume which is why they are used in Electric Vehicles aka EV.
• Very low internal resistance aka Ri. This means they can be charged and discharged very fast, very high charge/discharge efficiency and for RE use eliminates Peukert Law Effect.
• Negative coefficient RI meaning their Ri is highest at full charge, and lowest at completely discharged. That means a very flat charge/discharge cure, and will die trying to give everything they have to provide power. When a LFP fails it fails Short Circuit whereas all others fail open circuit. That means you still work at reduced voltage assuming the other batteries in the string have something left to give.
• Of all the Lithium batteries has the highest over charge tolerance of about .7 volts per cell compared to .1 volts of other Lithium chemistry.
• Lowest watt hour capacity cost of lithium chemistry. On the order of $0.30 to $0.45 per wh. For reference a Pb cost $0.09 on the low end for a 1 year battery and $0.24 per Kwh for a 5 year Pb battery.

So how does one best manage LFP to get the most out of it or most bang for the buck? Well first thing to look at is we never want to mess with the ends of the discharge curve. That being completely Discharged or Fully Charged, especially the Completely Discharged portion as that is DEATH for all Lithium batteries. Fully charged LFP offers us some room to play with which I will get to in a minute discussing Battery Management Systems aka BMS.

Lithium batteries unlike Pb operate best in a charge range of 90% SOC and down to 10% SOC. Operating in that range increases cycle life about 50% or 2000 up to 3000 cycles. So how do we do that? Answer is with some form of BMS. Trick is there is no definition of what BMS actually is. It can be something as simple as volt meter to monitor pack voltage, to managing each cell voltage under charge and discharge, and everything in between.

BMS basically falls into two categories of; Top Balance and Bottom Balance. Which is better? IMHO something in-between with bias to Bottom Balance with Low Voltage Disconnect aka LVD.

Today the manufactures of LFP BMS are Top Balance systems, but that has a flaw that comes from Pb mentality of keeping Pb batteries at 100% SOC to maximize cycle life. Pb batteries do not like being discharge and operated in Partial State of Charge aka PSOC. For Pb anything less than 100% SOC is a slow death. All Lithium batteries are best operated in PSOC range less than 100 % and greater that 0%. At 100 % stresses lithium, and at 0% is instant death or a brick. For maximum cycle life 90% or less, and 10% or higher. So as you can see there is already one flaw with Top Balance going to 100% or 4.2 volts per cell.

To go one step further using Top Balance is assuming Best Case that all batteries are created equal when in fact they are NOT. What I mean is when you buy say a 100 AH LFP cell, not every cell is 100 AH. Fortunately after many thousands of test by third parties when you buy a 100 AH cell none lest less than 100 AH, but none test the same. Realistic range is 102 up to 115 AH which is a good thing. Problem is if we use Top Balance we calibrate for Best performance. Imagine we have a car with 4 gas tanks that are supposed to be 10 gallons. But in reality range 9 to 11 gallons and we only monitor the 11 gallon tank level. See a problem with that idea? When we fill up all four tanks are full, but have different capacities. We go driving and monitor the 11 gallon tank, when we see or assume that tank has two gallons left we shut down. Problem is we completely drained the 9 gallon tanks to never be able to hold any more fuel. We turned it into a BRICK. Why because we Referenced to BEST CASE which is a huge No-No in design.

What if we referenced to Worse Case or empty tank of 0% plus a little more so we never flirt with empty. That is BOTTOM BALANCE my friends. How do we do that? Real simple it is done initially by discharging the batteries when we put them into service. How? When we receive the batteries we connect them all in parallel and let them sit for a day. Then we put a load on them and discharge to 2.75 volts. Let them sit for a while and keep discharging a little until they come to rest at between 2.5 to 2.75 volts. We have now referenced the DO NOT CROSS LINE on the bottom or instant DEATH. FWIW 2 volts is the mile high cliff in which you fall to death If you reach it.

So now let’s put that to practice in a BMS. We initially receive our cells from the manufacture and BOTTOM BALANCE. We look for a BMS that monitors all cell voltages. We install a LVD that will disconnect the battery pack when any cell discharges to about 10% SOC or 2.9 volts. We charge at as high of a current until the first cell hits 3.75 volts that means setting your charge controller for 15 volts for every 12 volts to force it into CONSTANT CURRENT. When the first cell reaches 3.75 volts (weakest cell) we go to FLOAT of 3.2 volts per cell or 12.8 volts for every 12 volts of battery. Going to float stops charging the batteries, but leaves the panels available for any load after the batteries are fully charged.

Watch this video and it should explain what I may have missed.