Charging efficiency LifePO4

[createthis]

I wrote:

I have zero interest in top balancing. I'll charge with my Midnite classic to 13.6v. Then I'll remove the battery and use the pl8 to discharge at a 10 amp rate to 10v, recording the ah draw. I'll then reinstall the battery, charge to 13.8v and repeat. Sound like a reasonable test?

and you replied:

well I did not imply you should Top Balance on your working system. But if you want to know what the capacity of each cell is requires a Top Balance and full discharge. From that you can determine what the AH capacity is of the weakest cell and its 90% SOC is. Example let's say your cell ranges from 101 on the lowest, and 115 AH on the highest. You re-balance at the Bottom, then charge and put in 92 AH. Disconnect the batteries from the controller and inverter, and let it rest several hours. Then measure OCV and you have found the magic Float voltage number specifically for your batteries. That will be around 13.6 volts. Maybe as low as 13.5 or as high as 13.7.

I don't understand why you replied with this. Is it dangerous to discharge my pack to 10v in series, having bottom balanced? Why is the test procedure I outlined above not adequate to determine pack capacity?

[karrak]

Victron recommends 1.05 for the Peukert exponent for LifePO4. That's what I have mine set to. They also recommend charge efficiency of 95%, which I also have set.

I had a look at the manual. It doesn't say what type of battery the 95% is for.

From my logged data 99% would be much closer. This agrees with literature that I have read. I don't use any Peukert compensation.

Simon

[createthis]

I had a look at the manual. It doesn't say what type of battery the 95% is for.

From my logged data 99% would be much closer. This agrees with literature that I have read. I don't use any Peukert compensation.

Simon

You're right. It actually says 99% in the LifePO4 section:

"The charge efficiency of Li-ion batteries is much higher than of lead acid batteries: We recommend to set the charge efficiency at 99%. When subjected to high discharge rates, LiFePO4 batteries perform much better than lead-acid batteries. Unless the battery supplier advizes otherwise, we recommend setting Peukert’s exponent at 1.05. "

I lied before when I said I set it to 95%. I just checked. It's at 99%. Pretty sure I filmed myself setting that too in one of those videos.

[Sunking]

I wrote:



and you replied:



I don't understand why you replied with this. Is it dangerous to discharge my pack to 10v in series, having bottom balanced? Why is the test procedure I outlined above not adequate to determine pack capacity?

Because you want to know what the capacity of each cell first. Only way you can do that is find the capacity of each cell first to determine a base line. No two cells will have equal capacity You have no clue what 90 to 100% is of the weakest cell is. You need to know that. Otherwise you are just guessing.

Example lets say your cell capacity ranges from 99 to 115 AH. You now know the 100% capacity of your pack is 99 AH. The weakest cell determines the pack capacity. Re-balance at the bottom. then use your PL8 to charge and stop when it reaches 91 to 92 AH in. Disconnect everything from the batteries. wait a few hours for the OCV to come to a Rest. Measure the OCV and you have your 90% SOC voltage for your specific batteries. Enter that voltage into the Controller, and set you AH meter to 90 AH. When you want to know the SOC, DO NOT LOOK at the voltage, look at the AH meter.

For the Last time I will repeat myself. Quit tripping yourself up on exact voltages. Voltage is not an exact science on a fully rested and OCV measurement. And even then on a LFP battery is a Ball Park indicator. 3.4 can be 85 to 99%. 3.3 volts is roughly 30%. So how in the world are you going to tell much about voltage when just .1 volt difference is 50-60% change from 3.4 to 3.3 vpc.

The only way you are going to find out what voltage you need to Float at is by measuring AMP HOURS and allowing the batteries to REST and OCV. That is all you are looking for. Quit tripping yourself up with voltages when charging and discharging. You cannot do that with any battery type. With Lithium you measure AMP HOURS In and Out. I can take any fully charged Lithium battery and make the voltage of the cells less than two volts by adding a load and barely touch the capacity. I can take a fully discharged lithium battery at 2 volts and make it read 3.6 volts without hardly adding any meaningful capacity.

There are only two voltages you need to concern yourself with.

1. The rested OCV of the pack with the weakest cell charged to 85 to 95% SOC. You have to find that voltage. It is going to be around 13.6 volts. It could be as low as 13.5 or as high as 13.7, or anywhere in between. When you find that voltage that is what you set your Charge Controller to. But guess what, your controller only has a Resolution of XX.Y not XX.YYY. You will be faced with 13.5, 13.6, and 13.7. Splitting hairs. 13.5 or 13.6 is going to be your number assuming the batteries are room temp.

2. LVD voltage. Whatever you want. 11 is a good number and 12 is a good number or anything in between. Just remember that is a Loaded Circuit number, not OCV.

Edit Note

Reacl AH meter every 5 cycles or so. Each complete charge Discharge cycle will add roughly 1+ AH each time you recharge. When you see it creep up from say 90 to 95, reset it to 90. The error is induced because of charge efficiency. More AH goes in than comes out. At 99% Charge efficiency about 1 AH every full cycle

[inetdog]

I did not. Even with the float voltage set to 13.5v, it floats up to 13.6v. I think that's odd too, but I've just started taking it as a quirk of the charge controller.

Did you leave temperature compensation turned on? You do not want to use LA temp compensation when charging Li.
Some CCs will display not the actual voltage to the battery but the normal temperature equivalent of that voltage when corrected for battery temperature.
An example: If you set a voltage of 13.6 and have temp compensation on, and the battery/ambient temp is high enough that the CC will deliver only 13.5. It may display it as 13.6 since that is the setpoint number that cause the actual voltage to be 13.5.
Other than that, possibly just measurement accuracy problem.

[createthis]

Did you leave temperature compensation turned on? You do not want to use LA temp compensation when charging Li.
Some CCs will display not the actual voltage to the battery but the normal temperature equivalent of that voltage when corrected for battery temperature.
An example: If you set a voltage of 13.6 and have temp compensation on, and the battery/ambient temp is high enough that the CC will deliver only 13.5. It may display it as 13.6 since that is the setpoint number that cause the actual voltage to be 13.5.
Other than that, possibly just measurement accuracy problem.

No, I turned it off in that video.

[createthis]

Because you want to know what the capacity of each cell first. Only way you can do that is find the capacity of each cell first to determine a base line. No two cells will have equal capacity You have no clue what 90 to 100% is of the weakest cell is. You need to know that. Otherwise you are just guessing.

Example lets say your cell capacity ranges from 99 to 115 AH. You now know the 100% capacity of your pack is 99 AH. The weakest cell determines the pack capacity. Re-balance at the bottom. then use your PL8 to charge and stop when it reaches 91 to 92 AH in. Disconnect everything from the batteries. wait a few hours for the OCV to come to a Rest. Measure the OCV and you have your 90% SOC voltage for your specific batteries. Enter that voltage into the Controller, and set you AH meter to 90 AH. When you want to know the SOC, DO NOT LOOK at the voltage, look at the AH meter.

For the Last time I will repeat myself. Quit tripping yourself up on exact voltages. Voltage is not an exact science on a fully rested and OCV measurement. And even then on a LFP battery is a Ball Park indicator. 3.4 can be 85 to 99%. 3.3 volts is roughly 30%. So how in the world are you going to tell much about voltage when just .1 volt difference is 50-60% change from 3.4 to 3.3 vpc.

The only way you are going to find out what voltage you need to Float at is by measuring AMP HOURS and allowing the batteries to REST and OCV. That is all you are looking for. Quit tripping yourself up with voltages when charging and discharging. You cannot do that with any battery type. With Lithium you measure AMP HOURS In and Out. I can take any fully charged Lithium battery and make the voltage of the cells less than two volts by adding a load and barely touch the capacity. I can take a fully discharged lithium battery at 2 volts and make it read 3.6 volts without hardly adding any meaningful capacity.

There are only two voltages you need to concern yourself with.

1. The rested OCV of the pack with the weakest cell charged to 85 to 95% SOC. You have to find that voltage. It is going to be around 13.6 volts. It could be as low as 13.5 or as high as 13.7, or anywhere in between. When you find that voltage that is what you set your Charge Controller to. But guess what, your controller only has a Resolution of XX.Y not XX.YYY. You will be faced with 13.5, 13.6, and 13.7. Splitting hairs. 13.5 or 13.6 is going to be your number assuming the batteries are room temp.

2. LVD voltage. Whatever you want. 11 is a good number and 12 is a good number or anything in between. Just remember that is a Loaded Circuit number, not OCV.

Edit Note

Reacl AH meter every 5 cycles or so. Each complete charge Discharge cycle will add roughly 1+ AH each time you recharge. When you see it creep up from say 90 to 95, reset it to 90. The error is induced because of charge efficiency. More AH goes in than comes out. At 99% Charge efficiency about 1 AH every full cycle

I still don't understand why the test procedure I outlined above is not adequate for determining pack capacity to the given charge voltage. I feel like you're just re-iterating what you previously said without answering my question.

[Sunking]

I still don't understand why the test procedure I outlined above is not adequate for determining pack capacity to the given charge voltage. I feel like you're just re-iterating what you previously said without answering my question.

OK you are not getting it. That is why I keep repeating myself you are stuck/hung up on EXACT VOLTAGE and trying to relate it to capacity. You want to know AMP Hours, not voltage. The voltage will be whatever it is. Now time for you to answer a question or two.

Q1. What is the capacity of your weakest cell.
Q2. What is the 90% capacity or your weakest cell?

I know you do not know. So if you do not know, do the test your way and get 80 AH what does that mean? Same answer you do not know. You need a baseline. What if you find out the weakest cell is say 107 AH and with your test you only get 80 AH fooling yourself into think it is exactly 13,6 volts is 90% SOC. When in fact it is only 74%.

Sure you can do the test your way, but what does it tell you? You don;t know because you have nothing to base it on. If you are shooting for 90% capacity, 90% of what capacity. You do not know what 90% of something is if you do not know what 100% is.

Tell you what I will give you 90% of the money in my pocket. All you gotta do is tell me how much money is in my pocket. How much do I have. I will ggive you a dime, is it ten cents or ten dollar, or even a Benji.

Point here is you do not know what the 90% pack voltagge is going to be. I can tell you it will be around 13.6 volts. But don't full yourself into thinking the cell voltages will be equal. If Bottom Balanced the weakest cell witll be the highest voltage, and the others will be lower by a few mv.

So do the test your way. It will work, but does not tell you what you want to know. Makes me no difference what the results are. I don't care, it is not my system.

If it were my system I would first Top Balance them in series at a high charge rate so I don't spend days charging all the cells individually. Then I would discharge them fast until almost completely discharge and spot the cell with the Lowest voltage and count the Amp Hours the pack discharged. Then I would take the lowest voltage cell out and finish discharging it to 2.5 volts and count the Amp Hours left in it. Add the two AH and I got the Weakest Cell capacity. From that calculate 90% AH capacity. Bottom Balance the cell, and pump in 91% of the Amp Hours. Let it rest disconnected and measure the resting OCV. Then set my chargger to that voltage and calibrate my fuel guage to 90 AH so I know exactly what the SOC is at any time.

Bottom line if I buy a 100 AH battery, how do I know I got what I paid for without testing it? What if your strongest cell turns out to be 90 AH and weakest is 80 AH. You paid for 100 AH/

I am trying to help.

[createthis]

OK you are not getting it. That is why I keep repeating myself you are stuck/hung up on EXACT VOLTAGE and trying to relate it to capacity. You want to know AMP Hours, not voltage. The voltage will be whatever it is. Now time for you to answer a question or two.

Q1. What is the capacity of your weakest cell.
Q2. What is the 90% capacity or your weakest cell?

I know you do not know. So if you do not know, do the test your way and get 80 AH what does that mean? Same answer you do not know. You need a baseline. What if you find out the weakest cell is say 107 AH and with your test you only get 80 AH fooling yourself into think it is exactly 13,6 volts is 90% SOC. When in fact it is only 74%.

Sure you can do the test your way, but what does it tell you? You don;t know because you have nothing to base it on. If you are shooting for 90% capacity, 90% of what capacity. You do not know what 90% of something is if you do not know what 100% is.

It's a 100ah battery. Is there really THAT much variance? Could it be a 110ah battery or a 90ah battery? I guess I assumed that variance was like +/- 1ah. If I get 90ah out of a 100ah battery +/- 1ah then I know I get 90ah between my charging set point and whatever I'm drawing the battery down to at the given rate I'm drawing. I don't understand how that is meaningless. It seems pretty damn meaningful as that's how I'll actually be using the thing.

[inetdog]

Although you may not get that much variation within production batch, yes, a tolerance of 10% would be within the published specifications of many batteries. If nothing else, they could change over time as the production process for the same battery type is updated. And the capacity will slowly decrease over the life of the battery, not necessarily identically for all cells.

[karrak]

I have zero interest in top balancing. I'll charge with my Midnite classic to 13.6v. Then I'll remove the battery and use the pl8 to discharge at a 10 amp rate to 10v, recording the ah draw. I'll then reinstall the battery, charge to 13.8v and repeat. Sound like a reasonable test?

This sounds reasonable to me. As long as all the individual cells start above 3.4V with an end charge current less than C/50 we know that at 13.6V that all the cells should be >98% full. Going from there to 2.5V should give you the battery capacity. To be safe I would hook the balance leads to the PL8 during the discharge.

Repeating the process at a start voltage of 13.8V will confirm the results with better accuracy.

I would recommend that you get a Cellog8 so that you can easily monitor the individual cell voltages and see how balanced the cells are. This will also tell you the variation in cell capacities. I wouldn't expect this variation to be more than a couple of % at most. One big advantage of the Cellog8 is that you can calibrate it. I have found that after calibration it will be accurate to a couple of millivolts and will maintain its accuracy over several years.

Simon

[karrak]

I did not. Even with the float voltage set to 13.5v, it floats up to 13.6v. I think that's odd too, but I've just started taking it as a quirk of the charge controller.

Ah, this could be because the Classic is not able to regulate the voltage so well when the battery is not drawing any current. There was a very animated discussion about this in this thread

Simon

[createthis]

Although you may not get that much variation within production batch, yes, a tolerance of 10% would be within the published specifications of many batteries. If nothing else, they could change over time as the production process for the same battery type is updated. And the capacity will slowly decrease over the life of the battery, not necessarily identically for all cells.

huh. ok. well, I guess that makes a case for profiling each individual cell. after that, I'll definitely know pack capacity and I can run my voltage set point discharge tests to see how that compares.

[Sunking]

It's a 100ah battery. Is there really THAT much variance? Could it be a 110ah battery or a 90ah battery? I guess I assumed that variance was like +/- 1ah. If I get 90ah out of a 100ah battery +/- 1ah then I know I get 90ah between my charging set point and whatever I'm drawing the battery down to at the given rate I'm drawing. I don't understand how that is meaningless. It seems pretty damn meaningful as that's how I'll actually be using the thing.

If there is 1% tolerance there is not need to do anything, just middle balance and operate like a Pb battery. NASA, Lockhead Martin, and other DOD contractors are the only ones who can afford 1% tolerance cells. That is what they use in sattelites and spacecraft so they do not have to use any BMS or anything else that takes up weight and space.

You are3 using Chi-Com cells which are the lowest quality out there, and GBS lowest of the bunch. Tolerance is +/- 10%. That is why they are so inexpensive. If you were to buy quality cells would cost you 3 to 5 times what you paid. But even then no where close to 1%. That would be military and aerospace grade.

[Sunking]

This sounds reasonable to me. As long as all the individual cells start above 3.4V with an end charge

That would be Top Balanced to get every cell 100%. I tried he does not get it.