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LiFePO4 won't charge over 3.3v per cell

KrisThompson

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Aug 14, 2022
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Hello all. New member, thanks for having me. I'm an FPV drone hobbyist and I'm in the process of making a 100aH battery to take out to the field when I spend hours flying. The battery will power the charger that charges my drone batteries so I can stay in the air.

I'm currently using an old car battery.

I purchased 4 Liitokala 3.2v 100aH cells off AliExpress a year ago and only started using them recently. I've wired them up as a 4 cell battery with a balance lead and am charging them off a SkyRC hobby charger at 100w charge. It should take over 12 hours to charge considering I have 1280 watt hours. I am using the LiFePO4 charge profile setting on the charger

Instead, the batteries get to 3.32 - 3.33v per cell after about 5 hours of charging, the charger says it's at "57%" charge and it just sits there apparently charging at 100w but nothing is happening. Even after hours, the cells are not going over 3.33v per cell.

Are my batteries no good? Or do I need a more advanced charger with a customizable LiFePO4 profile.

Regards

Kris
 

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If the cells you bought were used ones being passed off as new, they may have been damaged by previous overcharging to the point where their self discharge is excessive and as the voltage rises, the rate of self discharge can match the charge rate so they never reach the end of the charge.

Observe how quickly and how far the cell voltages drop once taken off the charger. Healthy cells should maintain their voltage quite well.

Buying anything off Aliexpress and then waiting a year before testing it is a dangerous game, as they only give 15 days to open disputes if I recall correctly.

EDIT: looking at your images, your charger is set to a LiPo program, likely with a 4.2V termination. Have you previously fully charged the cells on this program, because this would constitute an overcharge and quite likely is the source of your issue? You should be (must be!) using a LiFe charge program with a 3.65 max (preferably 3.60V) charge termination.
 
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The voltage/SOC profile is very flat, the voltage rises slowly until near full charge.

Limit the fully charged voltage to 3.65 per cell.

Unless you carried out a top balance its possible one cell will reach full before the others.

Charging the 4 cells as a 12v battery without a BMS with cell protection voltage set to 3.65, is not a good plan. There is a good chance of destroying the cells.

LFP charge.jpg
 
If the cells you bought were used ones being passed off as new, they may have been damaged by previous overcharging to the point where their self discharge is excessive and as the voltage rises, the rate of self discharge can match the charge rate so they never reach the end of the charge.

Observe how quickly and how far the cell voltages drop once taken off the charger. Healthy cells should maintain their voltage quite well.

Buying anything off Aliexpress and then waiting a year before testing it is a dangerous game, as they only give 15 days to open disputes if I recall correctly.

EDIT: looking at your images, your charger is set to a LiPo program, likely with a 4.2V termination. Have you previously fully charged the cells on this program, because this would constitute an overcharge and quite likely is the source of your issue? You should be (must be!) using a LiFe charge program with a 3.65 max (preferably 3.60V) charge termination.
Thanks for your message.

That's ok - channel B was on a lipo program with no battery attached, channel A was on LiFe program (I can see how it looks though.)

And I disconnected the charge this afternoon at 3.32v, the voltage fairly quickly dropped to 3.28v per cell, would this be considered a reasonable drop?

I think I understand what you're saying about the self discharge matching the charge rate. Maybe the batteries are not great

And yes 100% agreed I should have tested them a year ago
 
The voltage/SOC profile is very flat, the voltage rises slowly until near full charge.

Limit the fully charged voltage to 3.65 per cell.

Unless you carried out a top balance its possible one cell will reach full before the others.

Charging the 4 cells as a 12v battery without a BMS with cell protection voltage set to 3.65, is not a good plan. There is a good chance of destroying the cells.

View attachment 106944
Hi thanks for your message, the charger has a BMS built in and I'm using the balance charging program. (It monitors individual cell voltage and then tops up each cell individually as each cell approaches full charge. It does this through the little balance lead attached to the battery, the one with 5 wires coming out of it. )

We charge the 4 cell lithium polymer drone batteries the same way

I think the charger is targeting 3.6v per cell, I do have another charger which can charge to 3.65v per cell if that's what you mean
 
The voltage/SOC profile is very flat, the voltage rises slowly until near full charge.

Limit the fully charged voltage to 3.65 per cell.

Unless you carried out a top balance its possible one cell will reach full before the others.

Charging the 4 cells as a 12v battery without a BMS with cell protection voltage set to 3.65, is not a good plan. There is a good chance of destroying the cells.

View attachment 106944
A charger of the type he‘s shown in the picture monitors individual cell voltages and will taper and terminate the charge as any cell reaches the termination point, so your concern about the cells maybe being out of balance is not warranted. Your comment would be relevant if he were charging from a “dumb” current source like a PSU.
 
Thanks for your message.

That's ok - channel B was on a lipo program with no battery attached, channel A was on LiFe program (I can see how it looks though.)

And I disconnected the charge this afternoon at 3.32v, the voltage fairly quickly dropped to 3.28v per cell, would this be considered a reasonable drop?

I think I understand what you're saying about the self discharge matching the charge rate. Maybe the batteries are not great

And yes 100% agreed I should have tested them a year ago
Ok, well if it’s set to LiFe then either you’re not waiting long enough for the charge to complete (taking into account the graph Mike posted above showing how flat the voltage curve is, and that these batteries are massive in comparison to the drone batteries you’ve mentioned), or your batteries came pre-cooked from previous abuse. That‘s possible, but hopefully unlikely.

Leave the charger on for another day and see what happens, it won’t cause harm if the voltages are still below 3.6V.


The drop you mentioned doesn’t seem excessive to me.
 
Ok, well if it’s set to LiFe then either you’re not waiting long enough for the charge to complete (taking into account the graph Mike posted above showing how flat the voltage curve is, and that these batteries are massive in comparison to the drone batteries you’ve mentioned), or your batteries came pre-cooked from previous abuse. That‘s possible, but hopefully unlikely.

Leave the charger on for another day and see what happens, it won’t cause harm if the voltages are still below 3.6V.


The drop you mentioned doesn’t seem excessive to me.
Awesome thank you for your help. I'll give it a good long charge and see if the voltage goes up any higher! Thank you
 
How many amps does your charger feed the battery, is it only 1.5A?

If 100Ah and 43% remaining (57% charged), it should take you around 43 / 1.5 = 28 hours more to fully charge it.
 
How many amps does your charger feed the battery, is it only 1.5A?

If 100Ah and 43% remaining (57% charged), it should take you around 43 / 1.5 = 28 hours more to fully charge it.

It's 100w so at the moment as I type this, it's charging at 7.4A and 13.3v. I will attach a picture
 

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The voltage/SOC profile is very flat, the voltage rises slowly until near full charge.

Limit the fully charged voltage to 3.65 per cell.

Unless you carried out a top balance its possible one cell will reach full before the others.

Charging the 4 cells as a 12v battery without a BMS with cell protection voltage set to 3.65, is not a good plan. There is a good chance of destroying the cells.

View attachment 106944
This graph is really helpful. Maybe I'm just not waiting long enough! I have it charging over night tonight
 
I purchased 4 Liitokala 3.2v 100aH cells off AliExpress a year ago and only started using them recently. I've wired them up as a 4 cell battery with a balance lead and am charging them off a SkyRC hobby charger at 100w charge. It should take over 12 hours to charge considering I have 1280 watt hours. I am using the LiFePO4 charge profile setting on the charger
These lithium iron phosphate cells are usually shipped at about 30% state of charge. Their maximum rate of loss of charge is usually stated on the spec sheet as somewhere around 3% per month, so in shipping they may get down to 20% if they sit somewhere for a long while.

If your cells started at 25%, they would require 75ah to fully charge. At 100w with the battery at 13.2v, you are charging with a current of about 7.5amps, so it should take around 10 hours to completely charge. As was stated by others, the middle of the charging curve is nearly flat in terms of voltage. After 5 hours, I would expect that you are about half done.

Can you charger display the actual current being delivered?
(Looks like someone else beat me to the same comment, but you get the math, right?)
 
These lithium iron phosphate cells are usually shipped at about 30% state of charge. Their maximum rate of loss of charge is usually stated on the spec sheet as somewhere around 3% per month, so in shipping they may get down to 20% if they sit somewhere for a long while.

If your cells started at 25%, they would require 75ah to fully charge. At 100w with the battery at 13.2v, you are charging with a current of about 7.5amps, so it should take around 10 hours to completely charge. As was stated by others, the middle of the charging curve is nearly flat in terms of voltage. After 5 hours, I would expect that you are about half done.

Can you charger display the actual current being delivered?
(Looks like someone else beat me to the same comment, but you get the math, right?)
Thanks yes the charger is displaying 7.4 amps as the charge rate at the moment. I'm actually cautiously optimistic at the moment because the charger is reading 59% charge and I don't think it has ever read that before.

Before I decided to make this post, I thought it wasn't charging because the voltage was hardly moving but after reading some of the replies and seeing the graph I think I may have been switching the charger off, thinking that the battery wasn't taking charge, where really it was possibly simply charging through that really flat spot around 3.3v! I hope so

I'm excited to see what the charger reads tomorrow morning. I will let you guys know. Thanks again
 
And note that LiPo is not the same as LiFePO4.

The chargery BMS makes you choose lipo or life so the abbreviations are not consistent amongst different chargers or BMS units.
 
And note that LiPo is not the same as LiFePO4.

The chargery BMS makes you choose lipo or life so the abbreviations are not consistent amongst different chargers or BMS units.
No that's ok I understand. It's a two channel charger and I had channel B set for charging my drone lipo batteries and channel A in the photo is set to LiFe profile ✔️ it confused me at first
There is also a profile for Li ion to add to the confusion.

I really think I just was just giving up too early but the battery was still taking charge. I'm use to the seeing the steady voltage increase that you get from lithium polymer
 
Yeah just wait it out. LiFePO4 will set flat for most of the charge, and at the very end it will rise very quickly.
 
So did it get fully charged @KrisThompson ?
Hey sorry for the late reply I've been chipping away at this. Everyone was correct and I learned something! I just needed to be patient. It FINALLY made it to 3.6v for one cell but the others are around 3.4 and it's taking forever forever forever to balance because the charger only balances at 300mA ?
I'm curious to know if a BMS would balance charge this at a higher current!


Thanks again for your help and for every one else input! I honestly thought I had completely dudd batteries at first. The charging voltage curve on these LiFePO4 is incredibly flat
 
Hey sorry for the late reply I've been chipping away at this. Everyone was correct and I learned something! I just needed to be patient. It FINALLY made it to 3.6v for one cell but the others are around 3.4 and it's taking forever forever forever to balance because the charger only balances at 300mA ?
I'm curious to know if a BMS would balance charge this at a higher current!


Thanks again for your help and for every one else input! I honestly thought I had completely dudd batteries at first. The charging voltage curve on these LiFePO4 is incredibly flat
If the cells have never been top balanced, you could be waiting almost forever for your charger to balance them out at a measly 0.3A. You are probably better off disassembling the battery and reconfiguring the bus bars so all the cells are in parallel, then putting your charger back on and charging them as a 1S single cell up to 3.6V.

After that, with them all at the same state of charge, they can go back into their 4S configuration and future charges will only need a very brief balancing stage at the end.
 
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