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LiFePO4 : CC / CV / absorption / bulk / float / confused again!

Frank in Thailand

making mistakes so you don't have to...
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It keeps being mixed up in my brain..
Hard to let go of lead acid thinking I guess.

The average solar charge controler doesn't have only LiFePO4 settings.

They normally have lead acid (Flooded, Gel, AGM) and "User"
Some might have "lithium" as setting.
User probably will have the options for:
- boost
- absorption
- float

For 12 and 24v it's for LiFePO4 "easy" as the lithium setting will be 4 or 8 cells.

Trouble start at 48v and higher.
48v is S15 or S16.
Depending on the builder it's one of them (probably S15) or you can choose.

In my situation, having S16.. my lithium settings are for S15.

So..
"User" settings.

And that's where the confusion starts.

Please take me by the hand and tell if this is correct.
(Like I'm a 6 year old) :)

CC phase is the first phase.
Depending on your setup (quality of LiFePO4 cells) it can be 60 to 90% of the charge
(Normal charging is to 60%, fast is to 90% ultra fast is up to 97% SOC CC)
I guess most use fast charging.

CC is called "boost" charge.
Absorption I don't know, but overall advice is not to use for LiFePO4.
CV then should be float.

For lead acid you always set the boost on higher voltage then floating....

For LiFePO4 this should be the other way around, yes??

As boost is CC and float CV, one wants CC to get to +/-90% SOC and CV to the rest.

90% SOC is lower voltage then 95%

Where ever I look, I read peope setting their boost and float the same, or boost higher than float.

Is my understanding correct that this is a wrong setting?

That boost should be lower then floating?
 
"Boost should be lower then floating" - No.

16S is the correct 48V config for LFP.

Phase 1: Bulk/Boost is CC up to the absorption voltage. Depending on target voltage, you can get to very high states of charge.
Phase 2: Absorption is CV with tapering current to top off the cells. Not necessary on LFP especially if you're using a lower charge current (less than 0.5C).
Phase 3: Float - maintaining the battery at full state of charge. LFP will continue to charge to about 95% SoC at 13.6V float (X4 for 48V)

"Boost" sometimes refers to a forced repeat of a bulk/absorption cycle from float periodically. "Boost" isn't a term typically used on the higher end equipment.

Bulk/Boost/Absorption all refer to the same target voltage - 14.6V or less for 12V.
Float should be 13.6V or less for 12V.

LFP SoC results can vary wildly depending on charge current. The best way to determine target voltages for SoC management is to use a coulomb counting battery meter for your particular charge parameters.
 
I need to correct myself for the previous post.

I wrote "Boost" where I should have written "Bulk"

S15 or S16 LiFePO4 is not something to discuss what is best, rather information about pre set settings in solar charge controlers.

If they made LiFePO4 setting for 48v and choose S15...
We can discuss all you want but it isn't going to change the options for the solar charge controller :)

So actually 2 settings.
Bulk and float.
That's all the choice available for "user" settings.

I'm having a hard time getting from CC to CV.

If bulk is CV..
Then the starting cycle starts with CV and not CC?

I have S16, 51.2 nominal voltage.

Sure, during charge or discharge the (real) voltage is different from rest (= real voltage)
Depending on the amount of current and capacity (Ah) of the battery, as well as it's SOC will give "unreal" readings.

That being said, @ 0.2C will give on average daily the same increase or decrease, between 30% and 80% SOC.

Voltage measurement still will give +/- 20% inaccurate SOC, bit it's also all the "information" the solar charge controller has.

And based on this information, voltage, it should switch from CC to CV at set voltage level.

After reaching this level, the amount of current should decrease slowly where the voltage should remain the same.
(= CV)

Please correct me if I'm wrong.

For lead acid this happens "automatically" as the chemistry changes and it can't absorb current fast anymore.

For LiFePO4 you can charge till full with CC, with the risk of "plating"
Only high end battery can endure this.

Our average Alibaba LiFePO4 cells (mine are) won't endure that abuse long.

What goes first?
CC or CV?

To my understanding that is CC.

Also that the bulk phase goes before the float phase.

What would make logical reasoning the Bulk CC is lower voltage then floating (CV)

I'm confused, and see behaviour from my solar charge controller that doesn't work as I would expect from setting the bulk higher then the float.

It does reach the set bulk voltage, sure..
But at maximum current

Then floating..
And the current draw drops slowly.

What makes my visual information to be bulk CC and float CV.

Chargers that have nothing to do with solar, nothing with lead acid, just AC to LiFePO4 DC, those charging curves are I think most "real" , not a trade-off from what Is and what we want.
Build for LiFePO4.

Those show CC to -/+ 90% SOC, en the last 10% at CV, where the maximum voltage during CC is lower then the end on CV.

I think you start to get my confusion.

I'm known for not just flat out accepting the "known truths"...
Seeing is believing.
What I see that is happening isn't consistent with the "known truth"
 
Per my first message, bulk is CC, phase 1. Once absorption voltage is hit during bulk, then CV by tapering current to maintain voltage. Once the charge termination current is hit, it drops to float, which is a different CV.

Bulk and Absorption as a voltage value mean the same thing. Bulk voltage of 14.6 or Absorption voltage of 14.6 just means the voltage at which is transitions from bulk to absorption.

Chargers just designed for charging know that LFP don't need float to maintain a charge, so they terminate; however, in a solar power system, you need float to maintain the battery at the nearly fully charged point while the panels power the loads.

Any pre-programmed 12, 24 and 48V LFP programs assume 4S, 8S and 16S, respectively.

Here's a 0.3C charge to 3.65V:

1610777836509.png
Blue voltage
Red current (10A start)
Brown capacity.

As you can see, the cell is almost fully charged at 0.3C (CC) and 3.65V. The absorption (CV) phase is arguably not needed unless you truly want 100%.

Here's a 0.3C charge to 3.4V:

1610777950015.png

As you can see, 3.4V is hit very quickly - 30 minutes in and at a very low SoC. The current then tapers, and it takes almost 6.5 hours to charge to 95%.

The contrast between the two charts is why I mentioned that it's important to use a battery monitor to establish limits if you're trying to target an SoC based on voltage. Termination current is also important.

Bulk/absorp not higher than 14.6V, float at not higher than 13.6. Note that at 13.6 (3.4V/cell, chart #2), you will still get charging during float and they may push as high as 95% regardless of any intention at using a lower peak SoC. If you don't want any charging during float, set to 13.2. For 16S LFP (48V) , multiply those numbers by 4.

The above, and my observations are completely consistent with "known truths." Perhaps you have misinterpreted "known truths." Is there any way you can break down your "confusion" into a succinct statement?
 
I will give it my best shot ?
Not native English, so my choice of words may be *not the usual way to say things".

My hybrid MPPT inverter (off grid usage) have 4 different settings.

- Lead acid (with no distinction between Flooded, AGM or Gel)
- Lithium (S15 !)
- no batt (battery less operation)
- user.

According to the factory if you go from lead acid to user, it uses "lead acid profile"
If you first set and safe lithium and then choose user, it's lithium profile.

I tried to get the distinction between the 2 profiles... No information yet.

The user settings have just 2 options: bulk and float.

I used set the bulk to adviced setting of 56 as the float , 54.4v

(I take the measurement from the Inverters and at the battery, where the battery voltage is leading for my settings. During charge with +/- 30A it's 0.4v lower at the inverter then at the battery)
(I'm working on better backbone to the battery for the 3 parallel hybrid units, right now temporary just thick wires, 50mm2)

I don't want to overcharge the cells right now, not even 100%, I'm happy at 85%

With the permanent setup I can get closer to the 95%.
Only 100% when and. If Balance is needed.

Ok, with those settings, I don't see the current dropped...
It stays just about max as it can do and I have regular cells getting close or at 3.65.

When I temper the capacity (I can set the charge ampere from 10 to 90A in steps of 10A) this drops to 3.4x and the charge continued as planned.

In my mind I would say that this is something that should be done via CC, CV. Not me manually change the capacity of charging, but the charging profile should do this for me.

The source of my confusion.
It doesn't limit the charge.

When I change the settings, and set the bulk at 54v, float at 56, I see it stops at 54v, keeps it there, and the current slowly drops.

That would make sense.
Also would make it CV?
As before it have reached that Voltage it feeds all possible capacity it can obtain from the solar panels.
(For me now about 100A)
54v is 99% charge.
Stopping the charge (and discharge) when it is at 54v and insee the current reducing from 100 down to 50A the voltage after 30 minutes rest is about 53.4, 85%

That's my observation.

Right now I have the float higher then the bulk, and it does what I understood what lithium charging should do.

Setting the bulk @56 or the suggested 58.4 will provide all possible current to the battery untill it reaches that Voltage.

Now if I read the charging curves correct, it should go fast (high current) up to 90% SOC and then go slower to 100%.

With bulk on 58.4v it never slows down.

I do have the DT24P AMMeter, with 300A shunt. Not yet installed.

I'm slow with those things due to medical limitations.

I need to prepare all individual components to be "start ready" before I can build the whole thing together.

That takes me several days (and I'm happy to have battery less operation)
The preparation already takes months.

First step now is to understand and get those settings correct.

Then rebuild my LiFePO4 battery

Then connect the inverters with backbone.

BMS and all the rest.
Getting there, it just takes ages :)

I just finished silencing my 3 hybrid inverters.
This was needed due their location, close/in to our bedroom
That took its time.
Virtually silent now!
IMG_20210116_141358_088.jpgIMG_20210116_141403_587.jpg
I used Silent wings 3 fans, lifespan of+35 years.
And phantek I had, who probably will die within 5.(then replace with SW3)
 
48v is S15 or S16.
Depending on the builder it's one of them (probably S15) or you can choose.
In my situation, having S16.. my lithium settings are for S15.
I may just be ignorant, I mostly ignore 48V, but I don't recall ever seeing a major manufacturer that treats anything other than 16S as the default for LFP 48V
Please take me by the hand and tell if this is correct.
(Like I'm a 6 year old) :)
Coincidentally, I have a Meanwell explainer on 2, 3, and 8 stage charging open in another tab. I'll share it here, not necessarily because its the best article out there, but because its one click away. Read if you like.
CC phase is the first phase.
...
CC is called "boost" charge.
Bulk is the term I'm more familiar with but I think these terms are partial or full synonyms.
Absorption I don't know, but overall advice is not to use for LiFePO4.
CV then should be float.
My first impulse was to disagree, but now you are making me think a little harder about this (uh oh :rolleyes:)
Most or all LFP charge profiles I recall seeing (both 2 stage and 3 stage) have an absorption phase (which I understand to mean a CV phase and current taper. As I understand it, its not necessary unless (1) you want to charge to 100%, or (2) you need additional time at an elevated voltage to allow the BMS to balance. @snoobler's graph shows the first and second stage, the portion I highlighted in pink is the absorption phase:
1610777836509.png

Float is a second lower voltage CV phase. I think sometimes max current is limited in this stage (@snoobler can you confirm or correct this)
For lead acid you always set the boost on higher voltage then floating....
For LiFePO4 this should be the other way around, yes??
No, float should be <3.4V or disabled
And Bulk/Absorb should be whatever you want your max charge voltage to be.
Possibly you are thinking of "re-bulk"?

My understanding is there are 3 stages but two settings in a normal 3 stage charge.
  1. Absorption Voltage: sets the target voltage for stage 1 (CC/Bulk) and stage 2 (CV/Absorption)
  2. Float Voltage: stage 3 (Lower CV/Float)
Where ever I look, I read peope setting their boost and float the same, or boost higher than float.
Can you point to any of these places I don't recall ever seeing this, are you sure you aren't confusing float and absorb?
Is my understanding correct that this is a wrong setting?
Yes
That boost should be lower then floating?
No

Maybe it would be useful to see some examples to anchor the discussion:

VictronMastervoltBattlebornWill P.SBMS0Avg
Bulk/Absorb3.553.553.63.6253.53.57
Float3.3753.3753.353.4No3.375
 
Last edited:
I've read it :)
(Meanwell article)

My hybrid inverter just have 2 settings.
Bulk (accedently used the word boost in the first post) and float.

Like I tried to explain in the second, when I place the bulk on "high" voltage (higher then 54v) it keeps bulking, pushing all current available to the LiFePO4 cells, untill it reaches the set voltage

When set at 54v my requested end voltage (About 85-90% SOC) is +/- 53.4, without any cells spiking on/over 3.65.
As it reduces the current slowly and keeps that voltage at 54v.

I'm not 100% sure if it's 85-90%, I take the measured voltage around the time the solar panels produce little less then our (fluctuating) power consumption.
About 15-30 later.
Then that voltage under slight load (discharge) is 53.4v.

At the moment I have the float at 56v.
"Useless setting" as it's not being used this way.

Setting it this way does give me the desired results.
Tomorrow I can test again with higher settings for bulk... But already know what's going to happen... It doesn't reduce amperage till it hits the set voltage.
No way anything that Balance can keep up with 100A charge...

To my understanding that's why it's getting reduced..(the current)
So that the total current at the higher voltage is lower then at lower voltage (= SOC)

And that's a bit my point..
It doesn't reduce.

Unless..
Bulk at 54, float (now) 56volt.

For sure it can be just my "special" China manufacturer that have this implementation.. :)
Would not be that surprising ?

To answer the question about S15 for 48v.

I'm not sure about NOW.
In November 2019 to February 2020 during my research to get LiFePO4 as replacement for lead acid, I came across several companies that made pre-build 48v (exact) battery array.
And companies that made both.
48 or 51.2v

Where begin last year the balance was about 50/50 for S15/S16 it's clear that S16 "has won"

S15 is available:

I do agree and have urged the manufacturer to make atleast 2 settings possible for their "Lithium" setting, for now it's just S15.

Why?
Easy
The battery pack they sell is S15 :) (50ah)
(And during their development S15 was just as common as S16)
 
While 3.65 is a nice target voltage, it's not a voltage that is used (??)

If you charge a cell to 3.65 and stop charge, next day they are 3.4 or 3.35v.

If you charge up to 100% and start discharge, it drops "directly" to 3.4-3.35v

For me in use 24/7 the AMMeter when in place will provide (in time) the most accurate SOC.

Till then, it's a bit guessing. :)

I can be mistaken, but isn't this curve the most desirable?
Least stressful on the cells?
= Most possible amount of cycles out of that installation?
(The one that goes fast to 3.4v and slow the rest)

1610789685394.png

As started, I'm confused.

For battery there are always a few things standard:
Trade-off between charging speed and cycle life.
Trade-off between available capacity a d speed of discharge
EV drains a lot faster then solar and won't get the same (less) Ah out of the cell.

For EV speed if charge is important.
For solar there is atleast 8 hours to charge.
It doesn't need fast.

I can be mixing up things again, I'm good at this.:)
 
1610777836509.png

With this cuve the cell gets full current till 3.60v

How the heck is that one going to be balanced?
100A is a lot to burn off...

My BMS can to max 1.5A passive and I have active Balancer that does max 2A.
Together not enough to keep that cell from rocketing beyond 3.6v...

Unless I reduce the current..
Then things go OK.

Something somewhere clearly goes different than expected.

That can be my expectation, or the outcome of the normally adviced settings.
 
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