LiFeP04 Batteries for Solar & BMS

[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.

[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]

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]

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]

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]

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.

[karrak]

What is your background?

I am intrigued to know why you are asking and what you want to know.

I would rather try to work out how credible a person is by what they post and not what they say their background is.

Experience with the topic you are commenting on is important. Here is my experience with off-grid power systems and LFP batteries.

First off-grid system around ten years ago, simple 64W solar panel connected to NiCd batteries, ex railway locomotives obtained for free from the company I sold my business to.

Around four years ago added 4kW commercially made inverter and designed and built my own MPPT controller, increased number of panels to 800W.

In April 2013 purchased and installed first batch of LFP batteries, total storage 360ah @ nominal 24 volts. Built BMS using modified Cellog8s and JLD404 meter. No battery balancing boards, decided to do balancing manually. Increased number of panels to 1140W

Last November designed, built and programmed Beaglebone Black based battery monitoring and logging system. I have made this an open-source project here https://github.com/simat/BatteryMonitor.

Simon

[karrak]

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 ??

I also loved being called a kid, being 56 and retired. The memory isn't as good as it used to be, but hopefully I am a bit wiser that I used to be.

My off-grid toys are the system I described it the previous post which runs reliably as our home power system with no backup and another system I have designed and built for a friend which has been working reliably since July 2013, only problem with this system being the failure of the commercially built inverter which was replaced under warranty. Have other toys like electric push bikes but they are not so relevant to this discussion.

To date no dead or bulging LFP cells and maximum cell voltage ever attained was around 3.7 volts during a manual balance and minimum 2.75 volts due to cell imbalance when LFP battery was installed. Normal cell operating range 3.45-3.0 volts. As far as I can tell battery storage and operational characteristics are probably not much different from when I installed the LFP battery.

Simon

[russ]

OK - Playing adult games with a kids knowledge and training?

[karrak]

Please share how....

How to manually top Balance an LFP battery using solar power
1. Choose a nice sunny day when you will get reasonably constant power from your solar panels.
2. Set your Solar Charge Controller to Charge to whatever voltage you want to balance to. In the past I have used 3.6 volts, would probably use around 3.5-3.55 volts these days.
3. For me, I look at the realtime output from my battery monitor (if you don't have a battery monitor you can use a multimeter) and as the battery voltage gets close to the cutoff voltage, if any cell is higher that the others connect a 1 ohm resistor across its terminals until its voltage drops down to the others, then disconnect the resistor. Keep doing this until all the cells are as close to each other as you want to take them. If any cell goes above whatever safety cutoff voltage you would chose say 3.75 volts, terminate the charging but continue to bleed the highest cell until its voltage goes down to the other cells.
4. Remember to set your charge controller back to the charge voltage you would usually use.

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.

Yes, just like you never want to take them to 0% SOC to do a bottom balance.

As far as I can make out it is higher cell voltage and higher temperature that will age LFP batteries. I can't see any mechanism that would make SOC per se age an LFP cell. Maybe I am missing something here, maybe you can enlighten me.

I can't see that taking an LFP cell to a voltage of say 3.6 volts for a limited period of time on an infrequent basis would make much difference to battery life. Maybe I am missing something again.

Simon

[Willy T]

OK - Playing adult games with a kids knowledge and training?

Do you mean people that make posts like this ??

The construction method of large prismatic cells is no tan exact machining process. If you open one up it basically looks like a large sheet of metallic plastic film with a gooey gel.

Example : The above poster has never seen the inside of a large prismatic battery, this plain bs. Anyone that has seen the inside of a prismatic battery knows that it is made up of many sleeved / envelope cells all paralleled to the battery posts, their size and amp hr capacity dictate the over all amp hr capacity of the battery. No Plastic film or Gooey gel to be seen.


WinstonThundersky90ampSectionCut07.jpg


There are people that post things on here that are just plain BS. If you even thought about correcting them, it's a stick in their eye and they will bring the powers to be swoop down on you.

[SunEagle]

Example : .... Anyone that has seen the inside of a prismatic battery knows that it is made up of many sleeved / envelope cells all paralleled to the battery posts, their size and amp hr capacity dictate the over all amp hr capacity of the battery. No Plastic film or Gooey gel to be seen.


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I am always interested in the "internals" of things and batteries is one category I am still in the early learning stages.

Is that the only way to make a prismatic battery or do some companies enclose the individual "cells" is a high density solution to minimize movement and possible internal damage?

[Sunking]

Yes, just like you never want to take them to 0% SOC to do a bottom balance.

There is the part you do not understand. You are not taking them to 2.0 volts (The Cliff), you are taking them to 0% SOC at 2.5 to 2.7 volts. At that point the cell capacities are perfectly balanced to a KNOWN REFERENCE POINT of 0 Amp Hours . At Top you have no clue what the capacity is. All you know is 100% SOC, capacity unknown. In a Bottom Balance system you never touch 0 or 100% SOC and know what the capacity is at any given moment. You do not have a clue what is going on in a Top Balanced system. Once you start discharging from the top you have no clue where you are at.

Taking a LFP to even 2 volts is not a problem. What is a problem and the KILLER is when in Top Balanced systems the weaker cells get to 2 volts while the stronger cells still have sufficient capacity to drive the weaker cells into REVERSE POLARITY. When that happens game over, Cell is destroyed. With Bottom Balanced that is impossible because all cells have the same capacity and all cells reach 2.5 volts the same time. Your Low Voltage Disconnects long before you get there leaving you a lot of breathing room. 2.5 volt Lithium cells cannot deliver any meaningful current. It is Passive Protection and requires no operator or automated controls which are prone to failure.

FWIW a 1 ohm resistor can only bypass 3.6 amps, which may or may not work. If the controller is pumping more than 3.6 amps, you are still over charging the battery. With Lithium batteries the discharge curve is so flat, voltage tolerance needs to be within .001 volts which you cannot achieve either manually or automated. Only Bottom Balance can you achieve that kind accuracy both manually and automated.

[Sunking]

No Plastic film or Gooey gel to be seen.

No plastic film electrolyte in Large Prismatic cells, they are not Polymer batteries. It is a gel inside those pouches. Here is what happens when you shoot one with a nail gun. At about 2:15 into the video you will see the electrolyte pouring out. But you can see it still frame below. The gel is translucent purple in the factory when added. Come out scorched blue.

[Willy T]

I am always interested in the "internals" of things and batteries is one category I am still in the early learning stages.

Is that the only way to make a prismatic battery or do some companies enclose the individual "cells" is a high density solution to minimize movement and possible internal damage?

I don't know of any that use loose electrolyte. Some spiral roll the cells up and they look like a coke can. The cells are vented to the atmosphere that I know of ,GBS, CALB, Winston, Hipower , but the pouches I don't know . I sure someone will find some kind of battery that is different.