LiFeP04 Batteries for Solar & BMS

[Willy T]

Winston is LFP aka LiFeP04.

LFP resting SOC Voltages.

100% = 3.450
90% = 3.338
80% = 3.331

Note those are RESTING VOLTAGES. But here is what you are missing Willy. When you use a Solar Panel with a conventional off the shelf charge controller, the charge controller never terminates the charge until darkness. Even if you set BULK, ABSORB, and FLOAT to 14.3 volts as you claim you are saturating the batteries to 3.575 volts which is above 100% SOC.

Unlike FLA and other battery chemistries LFP current flow stops when the supply voltage and battery voltage are equal. That is Saturation. When that happens the battery rest voltage is the same as when current stops. It does not relax. The reason is there is no self discharge or surface charge like there is with FLA or any battery that works via chemical reaction. Lithium Ion Batteries are not chemical, they are Ion exchange between anode and cathode. An Ion is at the Atomic level of unequal Electrons and Protons in the molecule.

If you really want to avoid touching 90% or great SOC in a solar system you have to set the voltage to equal or less than 90%. Because as soon as you go into CV modes of Absorb and Float and the current starts to taper, you will saturate the cathode. You want that Saturation voltage to correspond to whatever SOC percentage you want. So if 80% is your target, set Bulk = Absorb = Float = 3.331 x 4 = 13.324 volts. Want 90%, then 3.338 x 4 = 13.352 volt. Want to make life simple? 13.3 to 13.4 volts. Do that and when the batteries saturate, all charge current stops and you are Floating. Any power demanded by your loads will come from the Panels assuming the load demand does not exceed what the panels can deliver. If you exceed what the panels can deliver will come from the batteries and as soon as the demand relaxes the batteries will recharge assuming you have enough daylight left.

There are two ways to charge a Lithium Ion Battery:

1. Is a 2-stage Constant Current / Constant Voltage method which is basically what you are forced to do with Solar Charge Controllers on the Market. But there is only 1 voltage setpoint with a solar charge controller, the end SOC percentage. Set Bulk = Absorb = Float = SOC Target Voltage. This forces your controller to act like a pure voltage source with current limit. Your controller will act as a Constant Current source until the controller impedance matches the batteries impedance IR curve. At that point the controller voltage holds whatever setpoint voltage you programmed it to be and current will taper off to ZERO amps when battery voltage equals controller voltage.

2. The second method is also called Fast Charging and is pure Constant Current until the pack voltage reaches a Set Point Voltage. That will take you up to 90% safely. That set point voltage has to be slightly higher than the desired Resting SOC Voltage. The Setpoint voltage is not a predefined voltage as it depends on the Charge Rate. Bu tit will always be slightly greater than Resting Voltage. For me charging a 100 AH LFP at C/2 (50 amps) is about 3.5 volts on the highest voltage cell being Bottom Balanced. The highest Voltage Cell is the weakest cell in the pack. When the charge is terminated the cell voltage relaxes to about 3.34 volts. Available AH are 98 AH on a 100 AH pack.

Thank you for taking the time to type the above. I have seen it expressed in the many White Paper PDF's available on Google. Again you have to charge the way you want, good Luck to you, I hope you have many years and many cycles.

Want 90%, then 3.338 x 4 = 13.352 volt. Want to make life simple? 13.3 to 13.4 volts. Do that and when the batteries saturate, all charge current stops and you are Floating. Any power demanded by your loads will come from the Panels assuming the load demand does not exceed what the panels can deliver. If you exceed what the panels can deliver will come from the batteries and as soon as the demand relaxes the batteries will recharge assuming you have enough daylight left.

I am certainly going to give this a try, I am not married to any voltage. You and I are only .07v apart per cell.

[Sunking]

When I tested the Balance Boards it only took about 3 minutes to all go solid led and all 8 batteries to balance.

Winston is LFP aka LiFeP04.

LFP resting SOC Voltages.

100% = 3.450
90% = 3.338
80% = 3.331

Note those are RESTING VOLTAGES. But here is what you are missing Willy. When you use a Solar Panel with a conventional off the shelf charge controller, the charge controller never terminates the charge until darkness. Even if you set BULK, ABSORB, and FLOAT to 14.3 volts as you claim you are saturating the batteries to 3.575 volts which is above 100% SOC.

Unlike FLA and other battery chemistries LFP current flow stops when the supply voltage and battery voltage are equal. That is Saturation. When that happens the battery rest voltage is the same as when current stops. It does not relax. The reason is there is no self discharge or surface charge like there is with FLA or any battery that works via chemical reaction. Lithium Ion Batteries are not chemical, they are Ion exchange between anode and cathode. An Ion is at the Atomic level of unequal Electrons and Protons in the molecule.

If you really want to avoid touching 90% or great SOC in a solar system you have to set the voltage to equal or less than 90%. Because as soon as you go into CV modes of Absorb and Float and the current starts to taper, you will saturate the cathode. You want that Saturation voltage to correspond to whatever SOC percentage you want. So if 80% is your target, set Bulk = Absorb = Float = 3.331 x 4 = 13.324 volts. Want 90%, then 3.338 x 4 = 13.352 volt. Want to make life simple? 13.3 to 13.4 volts. Do that and when the batteries saturate, all charge current stops and you are Floating. Any power demanded by your loads will come from the Panels assuming the load demand does not exceed what the panels can deliver. If you exceed what the panels can deliver will come from the batteries and as soon as the demand relaxes the batteries will recharge assuming you have enough daylight left.

There are two ways to charge a Lithium Ion Battery:

1. Is a 2-stage Constant Current / Constant Voltage method which is basically what you are forced to do with Solar Charge Controllers on the Market. But there is only 1 voltage setpoint with a solar charge controller, the end SOC percentage. Set Bulk = Absorb = Float = SOC Target Voltage. This forces your controller to act like a pure voltage source with current limit. Your controller will act as a Constant Current source until the controller impedance matches the batteries impedance IR curve. At that point the controller voltage holds whatever setpoint voltage you programmed it to be and current will taper off to ZERO amps when battery voltage equals controller voltage.

2. The second method is also called Fast Charging and is pure Constant Current until the pack voltage reaches a Set Point Voltage. That will take you up to 90% safely. That set point voltage has to be slightly higher than the desired Resting SOC Voltage. The Setpoint voltage is not a predefined voltage as it depends on the Charge Rate. Bu tit will always be slightly greater than Resting Voltage. For me charging a 100 AH LFP at C/2 (50 amps) is about 3.5 volts on the highest voltage cell being Bottom Balanced. The highest Voltage Cell is the weakest cell in the pack. When the charge is terminated the cell voltage relaxes to about 3.34 volts. Available AH are 98 AH on a 100 AH pack.

[Willy T]

You are changing your story again.



3.575 volts is 99 to 100%. Stick with 14 or 3.5 vpc.

That is balancing voltage, I think your 's was 2.5v when you bottom balance. That is NOT what one would charge to on a daily basis, the balance boards use 3.55v you have to get above that to trigger them. ( 3.575 ) it only needs to be done when and if the cells get out of balance. I only charge to 3.45 on a daily basis and posted so.

Winston uses 3.70 as their upper limit.

When I tested the Balance Boards it only took about 3 minutes to all go solid led and all 8 batteries to balance.

[Sunking]

Wow, I kind of agreed with the general information you posted until I read your last sentence. I guess you really are the King. Since I charge to a lower voltage ( 85% ) and discharge to a higher voltage ( 30% ), I should be good to go.

You are changing your story again.

I don't do it every cycle. Mine is initiated manually and is for 15 minutes at a time to push the voltage up to 3.575v per cell ( 14.3v )

3.575 volts is 99 to 100%. Stick with 14 or 3.5 vpc.

[Willy T]

I am not going to allow you to tell people to destroy their batteries because of your ignorance and incompetence.

Wow, I kind of agreed with the general information you posted until I read your last sentence. I guess you really are the King. Since I charge to a lower voltage ( 85% ) and discharge to a higher voltage ( 30% ), I should be good to go.

[Sunking]

Balancing boards are not designed to balance a battery that is badly out of balance, they are there to keep a battery in balance. They should only be working when the charge controller has started to limit the charge current going into the battery. At the end of the balance you have all the cells at the same voltage with the current being supplied by the charge controller equaling the current going through the balancing boards and equal current going into the individual cells until the current from the controller equals the controller cutoff current or the controller charge timeout.

Balance Boards cause the batteries to go out of BALANCE as they are parasitic loads and root cause of battery failures.

[Sunking]

Less expensive? You do not need balance boards to do a top balance. I don't have any balance boards in my system, I do the balancing manually with some simple 5W resistors and my charge controller.

Please share how....

Top balancing has nothing to do with Pb mentality.

It has everything to do with Pb batteries. TOP BALANCE systems are designed to go to 100% SOC. Last thing you want to do is take your batteries to 100%

Two GOLDEN RULES for Lithium Batteries.

1. Never Go ABOVE 90% SOC
2. Never Go Below 10% SOC

If you use a Commercial Manufactured Top Balance System you go to 100% SOC.

[Sunking]

That is not how SK charges, he apparently just cuts the charge off at full charging amps and has no short period of CV and tapering amps.

There is absolutely no need for saturation, that is where you destroy your cycle life going from 90% to 100% saturation. I can charge as slow as C/100 if I wanted and use the exact same algorithm. There is only 1 algorithm used to charge Lithium called CC/CV which is nothing more than fixed bench DC power supply with current limit. I chose current limit to 50 amps and can fully recharge in less than 2 HOURS, typically 1 hour. Depends on how deep discharge.

Point here is going above 90% DESTROYS CYCLE LIFE. TOP BALANCED SYSTEMS are designed to go to 100%, thus why EV manufactures DO NOT ALLOW their batteries to go to 100%, and they use batteries that are matched in Capacity so essentially they are Bottom Balanced. All cells have the exact same capacity making it impossible to drive any cell into reverse polarity. The ONLY way you can have Balanced Cells, meaning equal capacity between all cells in a Solar System or a DIY EV is to BOTTOM BALANCE. The only other possible way to have Balanced Cells is to buy them quantities of 10's of thousands and match them up to get within 1% tolerance.

In a Solar System you are fighting one HUGE BATTLE every day. Time, you don't have enough of it to mess around with any SATURATION. You DO NOT NEED and Saturation as it destroy CYCLE LIFE. Why would any fool want to do that? Lithium batteries are extremely easy to charge. To get to 90% SOC all they require is a Constant Current applied until the cell voltage reaches reaches 3.45 volts. So back to TIME and SOLAR. You do not have TIME on your side. The fastest way to 90% SOC is CONSTANT CURRENT, as much power as your panels can generate.

What is so damn hard you cannot understand Willy. Even a child can understand that. EV manufactures understand that. Most of the DIY EV understands that. Why can't you?. Yes you are stuck in a Pb World because you still use the 4-stage Pb mentality. Lithium Charge is clearly defined, has been for 10 years and you still do not know it. I will tell you what it is, CC/CV. It is that simple. If you only want to go to 90% to maximize battery life, Constant Current to 3.4 to 3.5 volts per cell and you are DONE. Saturate them like you are doing and you cut cycle life 30 to 50%. I am not going to allow you to tell people to destroy their batteries because of your ignorance and incompetence.

[Willy T]

Two kids playing with toys sure do know a lot - NOT!

I love the kid thing, at 68 I don't get that complement often. Thank you. I have yet to succumb to Cerebral atrophy and am still willing to learn. I guess you could call my learning system a toy, a $4,000 coffee making system is a better description, since thats all I have on it. Maybe a golf cart would be a better test of a PV solar system, but how many panels could I install ??

[russ]

Two kids playing with toys sure do know a lot - NOT!

[Willy T]

Balancing boards are not designed to balance a battery that is badly out of balance, they are there to keep a battery in balance. They should only be working when the charge controller has started to limit the charge current going into the battery.

That is not how SK charges, he apparently just cuts the charge off at full charging amps and has no short period of CV and tapering amps. So with 50 amps going into the Balancing board I could see where it would do nothing or burn up.

A problem with not doing a short period of tapering amps ( absorb ) is that the charger voltage and the actually battery voltage is different. Battery voltage is a lagging voltage, the charger has to have a over voltage to be able to raise the Battery Voltage and charge. At 50 amps that could be as much as .3v + /- . If you chop 50 amps off main stream you'll end up with a under charge. Using a short period of CV ( absorb ) corrects this and allows the charger and battery voltages to converge.

Since the text Book of Solar LiFeP04 has yet to be written, the terms of Bulk, Absorb, Float, and EQ may be different than the way they are used for pb. Please use the term in the way you choose. Example : EQ one could say " BB " ( Board Balance ), Absorb, " VC " ( Voltage Convergence ) a short period of Tapering amps. Ending Amps ( EA ) could still be ending amps, if in doubt ask, don't assume.

Karrak, thank you for sharing what you'v learned in the 18 + months on PV Solar LiFeP04 in a intelligent way.

[russ]

Ah, I see it is an evil conspiracy.
Simon

What is your background?

[karrak]

Simon I have never said to keep LFP around 50% DOD. 50 to 60% DOD is Storage voltage, not operating voltages. I do understand perfectly not going to 100% which is what I have been saying all along. For maximum cycle life one wants to operate within the range of 10 to 90% SOC.

I and I think many others after their own independent research from many sources including battery manufacturers have arrived at the conclusion that keeping the charge in an LFP battery centred around 50% maximizes the lifespan. How practical it is to do this is another matter

The problem with Vampire Boards discovered by the DIY EV crowd are two fold. The Bleeder Boards can only bypass X amount of current. X = 1 amp or less depending manufacture or model. However you do not charge at 1 amp unless you are talking about a 1 or 2 AH battery. Typically you charge at C/5 to C/2. On a 100 AH battery like I have would be 20 to 50 amps. FWIW I use 50 amps. So say you are charging at 25 amps, first Vampire Board turns on when it is fully charged. That cell now is bypassing 1 amp and still being charged with the remaining 24 amps. Next Vampire board turns on with same 24 amps left flowing. Process continues until the last Vampire Board turns on just as the charger hits the CV mode and stops current. In say a 16S system you now have 15 over charged cells and 1 properly 100% charged cells. That is how cheap passive BMS works using Vampire Boards.

Balancing boards are not designed to balance a battery that is badly out of balance, they are there to keep a battery in balance. They should only be working when the charge controller has started to limit the charge current going into the battery. At the end of the balance you have all the cells at the same voltage with the current being supplied by the charge controller equalling the current going through the balancing boards and equal current going into the individual cells until the current from the controller equals the controller cutoff current or the controller charge timeout.

The other problem and the bigger of the two is Vampire Board Failure/ More times than not they fail shorted which can completely drain a cell. Unless you have a monitor that monitors cell voltages you are not going to know it as the pack voltage still appears to be good and everything still works until you notice that weird burning pine tree tar smell coming from your batteries followed by heavy smoke.

Do you have any stats on how many disasters have been caused by shorted transistors on balance boards compared to other reasons? I doubt it is a big problem. I think that the balance boards should be designed to fail safe. I have a simple circuit design in my head that only has two extra components that would stop the problem of the shorted transistor. Anyway, I thought we had agreed that you should have individual cell monitoring which would detect this problem.

I am not saying your top balance method will not work, I know it will as there are many doing that way. All I am saying is there is another way, less expensive, and a whole lot less risk without having to give up any capacity. Top Balance is a Pb mentality carried over. It is also driven by the manufactures to up sale products as Top Balance is expensive to implement. Today manufactures are now starting to respond to consumer demands who prefer to Bottom Balance.

Less expensive? You do not need balance boards to do a top balance. I don't have any balance boards in my system, I do the balancing manually with some simple 5W resistors and my charge controller.

Top balancing has nothing to do with Pb mentality. If you want to balance on a regular basis it is far easier and I think less dangerous that bottom balancing.

Ah, I see it is an evil conspiracy.

Simon

[Sunking]

I would love to keep my off-grid power supply battery centred at an SOC of 50% to maximize its life but unfortunately I don't have control over the weather. As the old saying goes "make hay while the sun shines". To keep my battery centred on 50%SOC would probably mean doubling its size and of course doubling the cost without a doubling in the lifespan.

Simon I have never said to keep LFP around 50% DOD. 50 to 60% DOD is Storage voltage, not operating voltages. I do understand perfectly not going to 100% which is what I have been saying all along. For maximum cycle life one wants to operate within the range of 10 to 90% SOC.

One of the unique characteristics of a LFP battery is they use very simple charging algorithm. You can charge with a constant current charger until the voltage the cell voltage reaches 3.4 to 3.5 volts and terminate the charge. That gets you to 90% quickly because that constant current can be as high as 1C or whatever current your panels can supply. In a Solar system you want to get the batteries charged as fast as possible. If you use a conventional off the shelf controller it has to be one that allows you to set all 3 stages to one voltage of; 14 ,28, or 56 volts to force the controller into a 1 stage CC/CV charger algorithm required for Lithium batteries.

The problem with Vampire Boards discovered by the DIY EV crowd are two fold. The Bleeder Boards can only bypass X amount of current. X = 1 amp or less depending manufacture or model. However you do not charge at 1 amp unless you are talking about a 1 or 2 AH battery. Typically you charge at C/5 to C/2. On a 100 AH battery like I have would be 20 to 50 amps. FWIW I use 50 amps. So say you are charging at 25 amps, first Vampire Board turns on when it is fully charged. That cell now is bypassing 1 amp and still being charged with the remaining 24 amps. Next Vampire board turns on with same 24 amps left flowing. Process continues until the last Vampire Board turns on just as the charger hits the CV mode and stops current. In say a 16S system you now have 15 over charged cells and 1 properly 100% charged cells. That is how cheap passive BMS works using Vampire Boards.

The other problem and the bigger of the two is Vampire Board Failure/ More times than not they fail shorted which can completely drain a cell. Unless you have a monitor that monitors cell voltages you are not going to know it as the pack voltage still appears to be good and everything still works until you notice that weird burning pine tree tar smell coming from your batteries followed by heavy smoke.

I am not saying your top balance method will not work, I know it will as there are many doing that way. All I am saying is there is another way, less expensive, and a whole lot less risk without having to give up any capacity. Top Balance is a Pb mentality carried over. It is also driven by the manufactures to up sale products as Top Balance is expensive to implement. Today manufactures are now starting to respond to consumer demands who prefer to Bottom Balance.

Now to answer one of your earlier questions when you asked how do you perform Bottom Balance in an operational system. Same way you Top Balance with a Vampire Board. You Bleed it once a year if needed.

[Willy T]

Busted dude. You got caught in a big fat lie. You cannot keep up with your lies. In another thread you made and is a direct quote from you:



You are so busted and a liar.

Do you know how many systems I have ?? I thought not. The LiFep04 is a system I am playing with to see if I want to switch from FLA in the future. If you took the time to read you'd have seen it's connected to a outback GFX 1312 Inverter, hardly a house sized system. You know what you can do with the name calling.

Back to your Golf Cart

Did I mention I am not financially challenged and my boat is 78 feet long and I have a big house and a sports car in the garage ??

Wow, the most dangerous man with LiFep04. I daily charge to 3.45v per cell. I discharge to 3.15v. I have a BMS and boards on every cell, I balance at 3.575 per cell. I have a Tyco relay that engages at 2.6v to cut the bank off. I have a back up charger on a VCS that is set at 2.7 v and disconnects at 3.1 volts. I think I am safer than others.