LifePO4 batteries for use with Midnite Classic 150?

[jflorey2]

What is happening in the absorb phase in this case is that the charging voltage is staying constant at ~3.45V/cell and the current is dropping from ~0.16C to ~0.0C. From the graph the SOC where the 0.1C charge curve crosses the 3.45V line is ~94%. If the battery is out of balance where the lowest cell has a 1% lower SOC than the average and the highest cell has a 1% higher SOC we can work out what voltage difference this will result in. At a charge rate of 0.1C and an SOC is 94% following the 0.1C charge curve to 93% gives a voltage of ~3.44V and 95% gives a voltage of ~3.46V. The difference being ~0.02V.

I agree. Yet you are seeing differences of .15V (I think it's higher than that, but let's go with that.) That would seem to indicate the best and worst cells are out of balance by about 5% when close to full charge, using the .05C line. Is that your interpretation as well?

[Sunking]

I am not sure what you mean, following the 0.2C curve from 92-95% gives ~0.1V, the 0.1C curve from 92-96% gives ~0.75V, the 0.05C curve gives ~0.01V?

What is happening in the absorb phase in this case is that the charging voltage is staying constant at ~3.45V/cell and the current is dropping from ~0.16C to ~0.0C. From the graph the SOC where the 0.1C charge curve crosses the 3.45V line is ~94%. If the battery is out of balance where the lowest cell has a 1% lower SOC than the average and the highest cell has a 1% higher SOC we can work out what voltage difference this will result in. At a charge rate of 0.1C and an SOC is 94% following the 0.1C charge curve to 93% gives a voltage of ~3.44V and 95% gives a voltage of ~3.46V. The difference being ~0.02V. Doing the same where the 0.05C curve crosses the 3.45V line gives a voltage difference of ~0.08V. It is the slope of the curve at whatever SOC and charge rate that will determine the voltage difference caused by any battery imbalance at that SOC

Simon

Karrak you just proved you do not know WTF you are talking about again or have any clue how Lithium Ion cells charge and discharge. You cannot even read your own fake charge graph which I know is completely FAKE and one you copied off the Internet. I know this for fact because you do not have the lab test equipment to do CC charge/discharge rates. Second dead give away is current does not begin to taper until you reach the CV phase at 3.6 volts. FYI that is the FLAT TOP that occurs on your graph on the right top side of the graph.

MODs what more do you need to know Karrak is a Fraud and a liar. jflorey2 and I have caught Karrak with his hand in the cookie jar. Ban this pretender permanently. Hell he cannot even read a graph.Karrak is like Dan, no one will ever notice them gone or care if they do notice.Pull the trigger.

[karrak]

OK. But per your other charging graph, charging from 92-96% resulted in only a .1V change - so in your case, a lesser change in charge state resulted in significantly larger change in voltage. (And that's assuming the maximal difference between cells within your battery.) Is it that your pack is fairly old, giving a wider difference in capacities?

I am not sure what you mean, following the 0.2C curve from 92-95% gives ~0.1V, the 0.1C curve from 92-96% gives ~0.75V, the 0.05C curve gives ~0.01V?

What is happening in the absorb phase in this case is that the charging voltage is staying constant at ~3.45V/cell and the current is dropping from ~0.16C to ~0.0C. From the graph the SOC where the 0.1C charge curve crosses the 3.45V line is ~94%. If the battery is out of balance where the lowest cell has a 1% lower SOC than the average and the highest cell has a 1% higher SOC we can work out what voltage difference this will result in. At a charge rate of 0.1C and an SOC is 94% following the 0.1C charge curve to 93% gives a voltage of ~3.44V and 95% gives a voltage of ~3.46V. The difference being ~0.02V. Doing the same where the 0.05C curve crosses the 3.45V line gives a voltage difference of ~0.08V. It is the slope of the curve at whatever SOC and charge rate that will determine the voltage difference caused by any battery imbalance at that SOC

Simon

[jflorey2]

I think the main reason for this is that during the 130 minutes of the absorb phase an extra ~3.2%SOC was added to the battery which in this case took the battery from around ~97%SOC to ~100%SOC. As the battery SOC increased any imbalance between the cells resulted in an increase in the voltage difference between the cells.

OK. But per your other charging graph, charging from 92-96% resulted in only a .1V change - so in your case, a lesser change in charge state resulted in significantly larger change in voltage. (And that's assuming the maximal difference between cells within your battery.) Is it that your pack is fairly old, giving a wider difference in capacities?

[Sunking]

Jeff, as you have discovered, Karrak is a Fraud and a Liar. He just got caught with his hand in the cookie jar. Good enough to have him banned permanently.

[karrak]

That's for a single cell, and it shows ~.025V difference between 30% and 70% charge. Yet your curves show a difference of .25 volts between cells. What accounts for the DIFFERENCE in your cell voltages, and why does the difference increase as the charge current drops?

I think the main reason for this is that during the 130 minutes of the absorb phase an extra ~3.2%SOC was added to the battery which in this case took the battery from around ~97%SOC to ~100%SOC. As the battery SOC increased any imbalance between the cells resulted in an increase in the voltage difference between the cells.

A slight correction, the difference between the cells was ~0.15V (3.51-3.36) not 0.25V as you stated.

Simon

[vast]

You need a CC that allows you to set Bulk = Absorb = Float = 54.4 volts. What you DO NOT WANT to do is fully charge LFP batteries. No reason to go above 90%. You will double to cycle life staying under 90%. Under no circumstances over discharge them as that is instant death. You want to limit charge voltage to about 3.4 to 3.45 volts, Never let them go below 3 volts. That gives you an operating range of 48 to 54.4 volts. With Sinopoly old Thenderturd cells you can go up to 55.2 volts, but I advise against it.

Most important thing you do is how you intend to do the Bulk or Initial Balance. Do not use a BMS to regulate charge. They require you to Over Charge the cells, and the number 1 cause of premature failures and fires.

In menu, under CHARGE MENU -> VOLTS, I can setup only Absorb voltage and Float voltage. There is no setting for Bulk. Maximum voltage for Float is Absorb voltage minus 0.1V (example: Absorb = 54V, Float = 53.9V).

LVD is 48V from inverter.

[jflorey2]

Sorry, I should have stated the charge rates with that graph that I posted. At the start of the absorb phase at 0 minutes the charge rate was ~0.16C dropping down to ~0C at the end of the absorb phase at ~130 minutes. I agree with you that if you were charging at rates greater than ~0.5C that the 3.45V charge voltage would be in the flat portion of the curve.

That seems backwards.

When a bank is first commissioned, the cells are equalized; their voltages are matched at some SOC. This is not perfect of course but you can come very close, within millivolts. They tend to keep that matched voltage at least at first, which is why it's possible to run without a BMS under some conditions.

However, their internal resistances may not be the same. Thus during HEAVY discharges and charges, their apparent voltage may differ more, since V=IR (and R in this case is the somewhat variable internal resistance of the cell.) But when the charge/discharge rate is decreased, they should return to close to their original balanced state.

Your diagram shows the opposite - a widening divergence in cell voltage as charge rates (and thus currents) drop.

All the manufacturers charge curves that I have seen are at charge and discharge rates of 0.5C or higher. The graph below shows the charge curves at charge rates that are more applicable to off grid use. As you can see at charge rates of 0.05C (C/20) which is a common absorb termination current the SOC of the cell is ~99% and in the very steep section of the curve.

That's for a single cell, and it shows ~.025V difference between 30% and 70% charge. Yet your curves show a difference of .25 volts between cells. What accounts for the DIFFERENCE in your cell voltages, and why does the difference increase as the charge current drops?

[Sunking]

Would some mod please make Karraks ban permanent. Get rid of this fraud and liar.

[karrak]

The cell voltages diverge until they are about .25V different. That's a lot for a chemistry with such a flat voltage profile during charge/discharge. How do you account for that? Are you sure it's not instrumentation error?

Sorry, I should have stated the charge rates with that graph that I posted. At the start of the absorb phase at 0 minutes the charge rate was ~0.16C dropping down to ~0C at the end of the absorb phase at ~130 minutes. I agree with you that if you were charging at rates greater than ~0.5C that the 3.45V charge voltage would be in the flat portion of the curve. All the manufacturers charge curves that I have seen are at charge and discharge rates of 0.5C or higher. The graph below shows the charge curves at charge rates that are more applicable to off grid use. As you can see at charge rates of 0.05C (C/20) which is a common absorb termination current the SOC of the cell is ~99% and in the very steep section of the curve.

ChargeDischargeCurves.jpg
Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[Sunking]

Before I write my first post here I read many threads in this forum. This is my rule: not write a new post in thread before I read all posts. Also, I respect all opinions. Maybe someone say something "wrong" - for me it is not wrong, it is a thing which must be avoid, like in brainstorming.

So, Sunking I respect your opinions about LFP. Can you help me with settings? There is something which I can improve in my system?

You need a CC that allows you to set Bulk = Absorb = Float = 54.4 volts. What you DO NOT WANT to do is fully charge LFP batteries. No reason to go above 90%. You will double to cycle life staying under 90%. Under no circumstances over discharge them as that is instant death. You want to limit charge voltage to about 3.4 to 3.45 volts, Never let them go below 3 volts. That gives you an operating range of 48 to 54.4 volts. With Sinopoly old Thenderturd cells you can go up to 55.2 volts, but I advise against it.

Most important thing you do is how you intend to do the Bulk or Initial Balance. Do not use a BMS to regulate charge. They require you to Over Charge the cells, and the number 1 cause of premature failures and fires.

[vast]

Before I write my first post here I read many threads in this forum. This is my rule: not write a new post in thread before I read all posts. Also, I respect all opinions. Maybe someone say something "wrong" - for me it is not wrong, it is a thing which must be avoid, like in brainstorming.

So, Sunking I respect your opinions about LFP. Can you help me with settings? There is something which I can improve in my system?

[Sunking]

The cell voltages diverge until they are about .25V different. That's a lot for a chemistry with such a flat voltage profile during charge/discharge. How do you account for that? Are you sure it's not instrumentation error?

Jeff, Karrrak cannot answer the question, he is a fraud, dangerous, and a spammer. That is why he has been banned so many times. 0.25 vpc on a LFP battery is roughly 75% difference in SOC. Karrak does not know WTF he is talking about and giving very dangerous advice. The kind that makes battery fires.

OP you have been warned by two engineers to ignore Karrak.

[jflorey2]

When you measure the cell voltages also makes a big difference as to how spread out they are. As you can see at the start of the absorb phase the voltages are close together. They are at the most out of balance when the solar controller goes from the absorb phase to float at around 130 minutes. During the absorb phase some of the cell voltages go down while some go up!

The cell voltages diverge until they are about .25V different. That's a lot for a chemistry with such a flat voltage profile during charge/discharge. How do you account for that? Are you sure it's not instrumentation error?

[Sunking]

Karrak is a fraud and should be banned permanently. Extremely dangerous with very little knowledge. Now is a SPAMMER.