Charging efficiency LifePO4

[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

[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.

[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 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?

[createthis]

I would set the Peukert coefficient on your meter to 1 and adjust the meter accuracy by altering the battery efficiency factor. If you find the SOC counter going above 100% before it reaches the voltage that resets the counter you need to reduce the efficiency coefficient. If it never gets to 100% before it resets you need to increase the coefficient.

I agree with the 11.7V (2.925V/cell) cutoff. I use 2.8 V/cell.

Simon

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.

[karrak]

Hmmm. I can check that with my Fluke 87. It's 4 gauge wire, a 135 amp anderson connector, and a fuse between the battery and the shunt.

Can you expound on why you think there is a voltage drop? What symptoms are you seeing that lead you to believe that? Thanks.

I am not sure if there is a voltage drop or not. It is easy to check. Whenever you have reasonable current flows it is good policy to check the voltages at the source and destination just to make sure.

Simon

[createthis]

Voltage drop is not going to be an issue for you on wiring. It is so small it can be ignored. Wiring and connectors do have voltage drop. It is a function of current flowing through the resistance of the wire and connectors. Wire resistance is determined by the size and length. You are using 4 AWG copper and I assume fairly short in length. 4 AWG copper has a resistance of 0.25 Ohms per 1000 feet. Assuming your total loop length is say 10 feet (5-feet one-way), means you have a resistance of 0.0025 Ohms. Now use the formula Voltage = Current x Resistance. So at 10 amps you loose 10 x .0025 = .025 volts. Splitting hairs again

But I remind you again, again, and again you keep getting tripped up by OCV going off chasing ghost of charge/discharge voltages leading you to wrong conclusion. You gotta stop that or else you will never figure out what is going on.

What is you Shunt Voltage rating, Current Rating, and what currents are you operating at. Shunts are either 20 mv, 50 mv, and 75 mv

The BMV-700 is supplied with a 500a / 50mv shunt.

[karrak]

Anything from 13.8v to 14.6v will result in a fully 100% charged battery, the only difference being that at lower voltages, the CV absorb period takes longer to reach .05C, which is the canonical place to stop *absorb* for a fully charged battery.

PNJunction hits the nail on the head. From my experience and others ( Barba and a number of others on other forums) charging at 3.4 V/cell (13.6V) can take too long when you have limited sunlight. Contrary to what Sunking keeps saying, charging LFP batteries to nearly 100% will not halve the lifespan of the battery. This is true for other variants of Lithium Ion batteries that operate at higher voltages.

To keep an LFP battery close to 100% full you don't have to float it at 3.4V-3.45V(13.6V-13.8V). Around 3.35 V/cell (13.4V) will do the job and may extend the life of an LFP battery.

Don't bother trying to work out SOC using voltage, your Victron power meter measuring current going into and out of the battery is far more accurate.

I would set the Peukert coefficient on your meter to 1 and adjust the meter accuracy by altering the battery efficiency factor. If you find the SOC counter going above 100% before it reaches the voltage that resets the counter you need to reduce the efficiency coefficient. If it never gets to 100% before it resets you need to increase the coefficient.

I agree with the 11.7V (2.925V/cell) cutoff. I use 2.8 V/cell.

Simon

[Sunking]

Hmmm. I can check that with my Fluke 87. It's 4 gauge wire, a 135 amp anderson connector, and a fuse between the battery and the shunt.

Can you expound on why you think there is a voltage drop? What symptoms are you seeing that lead you to believe that? Thanks.

Voltage drop is not going to be an issue for you on wiring. It is so small it can be ignored. Wiring and connectors do have voltage drop. It is a function of current flowing through the resistance of the wire and connectors. Wire resistance is determined by the size and length. You are using 4 AWG copper and I assume fairly short in length. 4 AWG copper has a resistance of 0.25 Ohms per 1000 feet. Assuming your total loop length is say 10 feet (5-feet one-way), means you have a resistance of 0.0025 Ohms. Now use the formula Voltage = Current x Resistance. So at 10 amps you loose 10 x .0025 = .025 volts. Splitting hairs again

But I remind you again, again, and again you keep getting tripped up by OCV going off chasing ghost of charge/discharge voltages leading you to wrong conclusion. You gotta stop that or else you will never figure out what is going on.

What is you Shunt Voltage rating, Current Rating, and what currents are you operating at. Shunts are either 20 mv, 50 mv, and 75 mv.

To measure voltage drop is childs play. You just take voltage measurement when current is flowing. Examples

At the panel terminals and at the Input Terminals of the Controller. That will tell you the loss between the panels and controller where you biggest loss is at.
At the output terminal on the Controller and at the battery term post will tell you how much loss between the Controller and Battery.
At the Battery Term Post and the Input Terminal of the Inverter will tell you how much loss between the Battery and Inverter.

How much loss there will be depends on the current and resistance. If current or resistance goes up, you loose more voltage. Where voltage becaomes important is at the design stage of the build. You limit voltage loss to 2% between panels and controller at maximum current. 1% from Controller to Battery at maximum current. 1% from Battery to Load at maximum current. All that is used for is selecting wire sizes.

But it is a ghost chasing exercise to determine SOC. You could care less what any voltage is while charging or discharging. It means nothing. You would use voltage drop during trouble shooting. Example loosing 1 volt between battery and Inverter at full load current when it should be no greater than .12 volts. You woul dfind the problem with your hand feeling for the hot connector, or your eyes seeing what is smoking or plastic melting

[createthis]

At a shutoff voltage of 12.1(3.02 V/cell) under load an SOC of around 16% is plausible. The voltage of 12.1 is probably a little low, I think it is possible you have some voltage drop between the battery terminals and the Victron meter. Whenever you are taking voltage readings it is very important to check against the reading taken at the battery terminals with a calibrated multimeter.

Hmmm. I can check that with my Fluke 87. It's 4 gauge wire, a 135 amp anderson connector, and a fuse between the battery and the shunt.

Can you expound on why you think there is a voltage drop? What symptoms are you seeing that lead you to believe that? Thanks.

[createthis]

Createthis, I looked at your testing video and have the following observations and questions.

The Classic going into "resting" mode for a short period of time between bulk charge and float is occurring because the Classic stops charging while the voltage drops from the bulk voltage of 13.6 volts to 13.5 volts.

You mention before you started the discharge test that the float voltage was 13.64 volts. Did you change the setting on the Classic from 13.5 to 13.6 volts?

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.

[createthis]

Your maths would be correct if the cells were in parallel.

We are talking about 0.5% versus 1.5% which could also be because the cell I am testing has done over 1000 cycles, or just errors in measurements or a mix of both or maybe something to do with the phase of the moon

Simon

Ah, yeah, these are series. I knew that.

[karrak]

That's pretty close to what I was seeing in the video. I saw 1.5ah between 13.6v and 13.8v. You're seeing 0.447ah per cell, so in a 4s config that's 1.788ah, which is pretty close to 1.5ah. It's possible the Winstons and GBS cells act a little differently too.

Your maths would be correct if the cells were in parallel.

We are talking about 0.5% versus 1.5% which could also be because the cell I am testing has done over 1000 cycles, or just errors in measurements or a mix of both or maybe something to do with the phase of the moon

Simon

[karrak]

Createthis, I looked at your testing video and have the following observations and questions.

The Classic going into "resting" mode for a short period of time between bulk charge and float is occurring because the Classic stops charging while the voltage drops from the bulk voltage of 13.6 volts to 13.5 volts.

You mention before you started the discharge test that the float voltage was 13.64 volts. Did you change the setting on the Classic from 13.5 to 13.6 volts?

Your initial float voltage of 13.64 volts with virtually no current going into the battery will mean your battery is around 99% full before you started the discharge test.

The power efficiency of your LFP battery is around 95%. The coulomb (current/charge) efficiency is 99%.

At a shutoff voltage of 12.1(3.02 V/cell) under load an SOC of around 16% is plausible. The voltage of 12.1 is probably a little low, I think it is possible you have some voltage drop between the battery terminals and the Victron meter. Whenever you are taking voltage readings it is very important to check against the reading taken at the battery terminals with a calibrated multimeter.

Looking at my log the lowest SOC I have taken my battery to is 10%. This occurred under very little load at a voltage of 25 volts (3.12 V/cell). A few minutes before this the minimum battery voltage was 24.26V (3.03V/cell) with a load of around C/3.

Simon

[createthis]

Now for a reality check.

I ran some tests today using my Turnigy Reaktor 300W on a 90Ah Winston cell to confirm the SOC versus different charge voltage data that I posted in page one of this thread.

In summary using a charge voltage of 3.60 volts at an end current of 0.45A (C/200) as my 100%SOC reference i found that.
3.70V(14.8V)@C/50 end current = 100.15%SOC
3.60V(14.4V)@C/50 end current = 99.94%SOC
3.55V(14.2V)@C/50 end current = 99.83%SOC
3.50V(14.0V)@C/50 end current = 99.68%SOC
3.45V((13.8V)@C/50 end current = 99.41%SOC
3.40V(13.6V)@C/50 end current = 98.91%SOC

Here are the details of my tests for anyone who is interested.
I first discharged the battery to around 80%SOC
I them set the charger to charge the battery to 3.40 volts with a charge current of 18A (C/5) and an end current 1.8A (C/50)
I then boosted the voltage to 3.45 volts. The cell took 0.447Ah (0.50%SOC) to go from 3.397 volts @C/50 end current to 3.448 volts @C/50 end current
I then boosted the voltage to 3.50 volts. The cell took 0.242Ah (0.27%SOC) to go from 3.448 volts @C/50 end current to 3.498 volts @C/50 end current
I then boosted the voltage to 3.55 volts. The cell took 0.135Ah (0.15%SOC) to go from 3.498 volts @C/50 end current to 3.549 volts @C/50 end current
I then boosted the voltage to 3.60 volts. The cell took 0.100Ah (0.11%SOC) to go from 3.549 volts @C/50 end current to 3.599 volts @C/50 end current
I then kept the charge voltage at 3.60 and changed the end current to 0.45A (C/200). The cell only took 0.052Ah (0.06%SOC) to go to the reduced end current
I then boosted the voltage to 3.70 volts and reset the end current to 1.8A (C/50). The cell took 0.135Ah (0.15%SOC) to go from 3.50 volts @C/50 end current to 3.55 volts @C/50 end current

This fits with my previous tests and fills in some holes it the previous data.

Simon

That's pretty close to what I was seeing in the video. I saw 1.5ah between 13.6v and 13.8v. You're seeing 0.447ah per cell, so in a 4s config that's 1.788ah, which is pretty close to 1.5ah. It's possible the Winstons and GBS cells act a little differently too.