proper charging parameters for LiFePO4

[bkfamily1]

I have a 280Ah LiFePO4 battery. The voltage vs charge level data from the manufacturer is as follows.

Battery Voltage & Actual Capacity
13.6V 100%
13.4V 99%
13.3V 90%
13.2V 70%
13.1V 40%
13.0V 30%
12.9V 20%
12.8V 17%
12.5V 14%
12.0V 9%
10.0V 0%

Question: does this seem typical for a 12.8V LiFePO4 battery?

I have a 40A MPPT charge controller.

The available settings with the default Li settings are as follows.

Overvoltage Shutdown 16.0V* Boost +2.0 if Boost is manually set
Equalization Voltage —
Boost Voltage 14.4V* if manually set, Overvoltage will be Boost +2.0
Float Voltage —
Boost Return Voltage 13.2V
Undervoltage Warning 12.0V
Undervoltage Return 12.2V
Low Voltage Disconnect 11.0V
Low Voltage Reconnect 12.6V
Equalization Duration —
Boost Duration —

Question: Do the default Li settings look OK for my battery? If not, what would you suggest?

Thanks

 

[sunshine_eggo]

That chart is for resting voltages. You can get to 100% @ 13.6V, but it may take a very long while.

Change:
Float to 13.5V
Boost duration to 30 minutes.

If you have the option of a long, slow charge, set boost to 13.8V and duration to 2 hours.

 

[bkfamily1]

Thanks, Sunshine. Reading the controller manual, the controller starts out charging with the maximum current it can deliver (within its charge capacity) based on the current the panels can deliver given the current sunlight conditions. Once it reaches the boost voltage, it switches to constant voltage charging and charges at the boost voltage. By default, it will charge at boost voltage for 2 hours. Then it drops down to the float voltage and stays there. So, per the default settings, it will charge with as much current as it can deliver until the battery gets to 14.4V (while charging, not resting). Then it will hold that 14.4V boost voltage for 2 hours. Then it would drop down to the float voltage, but no float voltage is set for Li batteries oddly?? I do not know what it will do then. Do I understand how normal charge cycles correctly?

Having read that, I understand your thoughts as follows. Ideally, I would have enough charge time available each day to lower the boost voltage to 13.8 and continue to hold that for 2 hours. Then drop to 13.5 float which would be the target full charge voltage. Correct?

I have read that to preserve maximum battery life, one should not charge to 100% or discharge to 0%. For LiFePO4 batteries, I understand that generally I should limit the discharge to 15-20% and the maximum charge to 90-95%. Do you agree?

If I am trying to charge to only 90-95%, based on the table for my battery, then I would think the float voltage should be 13.3V (90%). The boost voltage can be higher than that, I guess, but I do not want to come out of the boost cycle above that 13.3V. If I am coming out over 13.3V, then I should either lower the boost voltage or shorten the boost cycle. Is that correct?

Thanks again or the reply and help!

 

[bkfamily1]

I just saw this thread and post. Great info. I guess if I took the time to read, I would not have to post Qs that have already been answered (probably 100s of time)!

MPPT User Settings for LiFePO4 Battery.

New member here, please forgive me if this has been covered before. I have replaced my AGM battery with a 200AHr LiFePO4 from Renogy. My MPPT Charge Controller is a 40A Renogy Commander which has no LiFePO4 Charge Settings, but does have "User Settings" for charging parameters. Renogy Tech...

diysolarforum.com

Thanks again for answering (again).

 

[sunshine_eggo]

I just saw this thread and post. Great info. I guess if I took the time to read, I would not have to post Qs that have already been answered (probably 100s of time)!

MPPT User Settings for LiFePO4 Battery.

New member here, please forgive me if this has been covered before. I have replaced my AGM battery with a 200AHr LiFePO4 from Renogy. My MPPT Charge Controller is a 40A Renogy Commander which has no LiFePO4 Charge Settings, but does have "User Settings" for charging parameters. Renogy Tech...

diysolarforum.com

Thanks again for answering (again).

The issue is the idea that "lithium batteries don't need to be floated" like lead acid. As a result, there are some dumb implementations of "lithium" charging. If one is going to fully charge a LFP battery and place it in storage, it is true that it doesn't need a float. If it's going to be used in a solar power system, a float voltage is used to ensure the solar will power loads AFTER the battery is fully charged. The last thing you want is to charge up to full and then watch the battery drain while the sun is shining...

 

[bkfamily1]

Good additional info Sunshine. Reading your comments and others, I am surprised that the default Li settings for my SCC do not include a float voltage. I am indeed going to set the float as you suggested. Your latest reply clarifies why that is important.

 

[sunshine_eggo]

Good additional info Sunshine. Reading your comments and others, I am surprised that the default Li settings for my SCC do not include a float voltage. I am indeed going to set the float as you suggested. Your latest reply clarifies why that is important.

Don't be surprised. MANY charging devices do not implement LFP charging optimally. RV AC-DC converters are another market where they've gotten it completely wrong.

 

[bkfamily1]

I see many recommended charging parameters that do not talk about the duration of the boost/absorption cycle. Is that parameter not included in most SCCs? How does one figure out what is right for their situation? Start short to avoid overcharging and watch to see where your voltage is after the cycle, then increase or decrease as needed?

I understand your comments on using float for a LFP to ensure that the charger does not "shut down" for the day while load is draining the battery. Makes sense. My controller has a Boost Return Voltage. Iam verifying with Renogy, but I believe that can be set for the SCC to start the boost/absorption cycle again if the battery voltage drops to a certain level. What are your thoughts on the advantages/disadvantages of using float vs setting it up to start charging again if it sees the battery dropping from load using that parameter?

 

[sunshine_eggo]

Boost return is simply the voltage at which the charger will initiate a boost charge. This is typically because the loads have exceeded the PV and drained the battery. It is very common to see a controller re-boost as the PV tapers off for the day.

Most LFP is fully charged at 3.65V when the current has dropped to 0.05C, e.g., a 12.8V 100Ah LFP is fully charged at 14.6V when the charge current has dropped to 5A. If one continues charging past that, it's over-charging.

At 3.45V/cell, current has to taper pretty close to zero.

There are other variables, but charging to 3.65V requires a very short absorption period - maybe 15 minutes. 3.45V requires about two hours.

Your initial charge rate also factors into it. Charging at 0.5C may require longer absorption times. 0.1C may require very little absorption time.

BTW, boost = bulk = absorption = all mean the peak charge voltage where current begins to taper off.

 

[bkfamily1]

Thanks for hanging in there for all my questions!

My battery is 280Ah, and my charge controller is limited to 40A charging. I have 400W of panels, so realistically, I will never reach 40A of charge rate. I think my panel-limited max charge rate is around 30A. So at that max rate, I am charging at about 0.1C, maybe slightly more.

So you suggested shortening the absorption time to 30 minutes because I am charging at 3.65V/cell, and my charge rate is only 0.1C or so, right? I also assume that if I drop the boost voltage a bit to avoid overcharging, I will need more time?

Would the best way to evaluate the absorption time be to monitor the charging amps during absorption to see how long it takes the amps to drop to 0.05C, 14A for my case? It would also seem that sunlight and panel output variation may change the time needed? Or it does not since the system should be able to maintain the voltage during absorption once it gets to that voltage?

Given that my system is alone with minimal load when I am not at the camp, I am guessing I should monitor it with that same minimal load to evaluate since that is what the system will see the vast majority of the time. I would rather undercharge when the load is larger when I am there than overcharge when the load is low when I am not there most of the time.

 

[sunshine_eggo]

Would the best way to evaluate the absorption time be to monitor the charging amps during absorption to see how long it takes the amps to drop to 0.05C, 14A for my case?

Yes.

It would also seem that sunlight and panel output variation may change the time needed?

It can.

Or it does not since the system should be able to maintain the voltage during absorption once it gets to that voltage?

Ultimately, it's hard to plan for every possibility. You establish what is typical and work for that.

Given that my system is alone with minimal load when I am not at the camp, I am guessing I should monitor it with that same minimal load to evaluate since that is what the system will see the vast majority of the time. I would rather undercharge when the load is larger when I am there than overcharge when the load is low when I am not there most of the time.

It's also perfectly reasonable to change programs when you are there and away.