Charging efficiency LifePO4

[Sunking]

Your PL8 is intended to be a Top Balance Charger. You can certainly use it for that or if you know how it works can use it another way.

Assuming you already have the initial balance, wire all the cells in series, make a Balance Plug, and Fully charge the Batteries using the Balanced Mode.. This is Top Balance and will charge every cell to 100% SOC. It will take the batteries to 14.4 volts and hold until current tapers to a set value. There are two ways you can do this. Either through the Balance Plug configuration A. Painfully slow. Best is Configuration B using the high current port, and finishes on Balance Plug. However you are going to need a Power Supply either way.

I assume you know how to operate the software. For Top Balance you need to figure out how much charge current your DC Power Supply can supply. Hopefully at least 10 amps or this is going to take a long time. 40 amps is perfect. So you are going to set the Charge rate yto as high as youf DC power supply can go say 20 amps/

Set cell end voltage to 3.6 volts. Set End Charge current at 3-Amps

Charge you rbattery and it will be TOP BALANCED.

When done allow to rest a hour.

Now set up the Discharge to 10 amps and end 2.5 vpc.

Now when it discharged it stops when the first cell reaches 2.5 volts. That is your lowest capacity cell. The other 3 will still have capacity left. That is Top Balance failure. All you are going to know is what the weakest cell capacity is and that dictates full Capacity of the pack. If you use this method, you loose your Bottom Balance.

From there connect all cells in Parallel and reblance.

So if you weak cell is say 95 AH you have your TARGET. Once you Bottom Balance again, connect the cells back in series and use the PL8 only using the High Current function and no Balance plug. Charge until you pump in .either 95 AH of the weakest cell or 95% of the weakest cell should get it to roughly 90%. Disconnect and allow to rest. Measure OCV and see exactly what 90% OCV SOC is for your batteries. It is going to be around 13.6 volt +/- .05 volts.

If you are really curious and want to know every cell capacity charge each one to 3.6 volts current taper to 3 amps. Discharge at 10 amps to 2.5 volts and note Capacity of each cell. Find th elowest capacity cell say 98 AH and not it.

Perform Bottom Balance again and then connect back in series. If 98 AH is th eweakest you want to pump in roughly 90 AH to get to 90% or 88 to 89 AH. Let th epack rest and note OCV. Shoul dbe around 13,6 volts +/- .05 volts.

Those two methods are the only way to find the exact OCV for your batteries. But I can tell you this. It is a waste of time splitting hairs, but will teach you something, but maybe worth it. I have done it and never again. I know Bottom Balance, Float to 13.6 volts is close enough. I do not worry about 1 to 3% inaccuracy splitting hairs.

What you will learn is it is important to just monitor cell voltages from time to time when they near fully discharged and nearly fully charged. Sooner or later a cell is going to crap out. You cannot stop it and will need replaced. Don't sweat the littttle details. When charged up look for 13.6 volts at 0 Amps with all cell voltages roughly 3.4 vpc, the weak one will be ever so slightly higher than the others and you know which one it is. Likewise let them ggo low from time to time down to 2.8 or so. They should all be roughly equal at the Bottom. A bad cell is going to stick out like a Tranvestite

[createthis]

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?

[Sunking]

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?

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.

However that is time consuming and splitting hairs. So if all you want to do is charge to 13.8, go for it. Just remember it is not an exact science and not to get confused if you only end up with say a 5% difference. Your actual Full charge voltage can be as low as 13.7 and as high as 13.9.

Lihium batteries are no different than Pb batteries. To charge them fully is a voltage as low as 2.25 vpc to 2.45 vpc. It is a Range. To find the exact number for your batteries takes some experimenting to zero in on it. With PB batteries you just measure the Specific Gravity to find the right voltage. With LFP you have to do it with Discharge/Charge test. It is up to you to find out what you are willing to do and spend time doing it. In the end the difference in voltage is so small, you discover it was just splitting hairs and you end up finding the sweat spot at say 13.64 volts. You spent hours for a .04 volt difference which your controller does not have the resolution to obtain. Been a while since I worked with Midnite Solar, but if I recall correctly you set to 13.6 or 13.7 volts. There is no 13.64. or 4-digit resolution. .

[karrak]

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

[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.

[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

[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

[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.

[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]

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.

[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

[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

[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]

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]

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.