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Endless charging against settling voltage in LiFePO4 cell

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sebdehne

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Battery-noob here - I am trying to charge (top balance) my new 280Ah cells (A grade, Docan power), but it seems like the energy I put in disappears somewhere when charging at a too low rate (3-4A). Like when you stop charging and the voltage settles back down after a while... but only now this settling is happening during charging and is "fighting against" the charge I put in.

When I charge the cell the "recommended way" (e.g. set bench top power supply to 3.65V) then the charge current is around 3-4A - but then I never see the voltage at the cell (measuing with Fluke 87) go above 3.370V - it evens drops back down to 3.365V some times and I have been charging like this for a couple of days now.

But when I try to "cheat" and turn up the voltage to 4V (to overcome the voltage drop og the leads) - but still watching the voltage at the cell with my multimeter. That resulted in ~7A and that seems to work: the voltage finally raises to 3.65V at the cell - at which point I quickly turned down the voltage at the power-supply back to 3.65V - and now the cell finally reaches 3.65V and the amps go to near zero.

What is the issue here:
1) I am too impatient, the cell will reach 3.65V eventually, even if I charge at only 1A, 100mA or 1mA
2) the cell is faulty (but I have tried others as well...), seems unlikely
3) such a "large" cell needs a certain minimum current to charge.

If "3)" - where can I read more about this? Where does the enery go if charge rate is too low? And does that mean a certain amount of solar power is required to charge a certain amount of battery capacity?

Thanks for the help.
 
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You are impatient. An empty cell @ 1 amp will take 280 hrs to charge! Don't cheat on the voltage. A sure method to damage your cells. Also, only ever set charger voltage when disconnected from batteries. I think at such a low charge rate for 280Ah cells limit the voltage to 3.45/3.5V. You don't need to push it higher to get full charge.
 
Did you make good quality leads with ring terminals, or are you using the stock skimpy cable and alligator clips? From what I've read it's very important to keep voltage loss on the cables/connection as low as possible.
 
Charleswgibbs said:
Did you make good quality leads with ring terminals, or are you using the stock skimpy cable and alligator clips? From what I've read it's very important to keep voltage loss on the cables/connection as low as possible.
Click to expand...
But a bad connection would only result in a higher voltage drop and a lower charge rate at the cell. Totally agree that is not optimal - but that doesn't explain the fundametal question: is charging at a low rate possible (and will eventually reach 100%), or is a certain minimal charge rate (current) required.

Ellcon123 said:
You are impatient. An empty cell @ 1 amp will take 280 hrs to charge! Don't cheat on the voltage. A sure method to damage your cells. Also, only ever set charger voltage when disconnected from batteries. I think at such a low charge rate for 280Ah cells limit the voltage to 3.45/3.5V. You don't need to push it higher to get full charge.
Click to expand...
Thanks. The cells were not empty at arrival (3.29V). So you are saying that theoretical it should be possible to charge with 100mA and it should reach 100% at some point eventually?

What about this "settling behavior". What if one charges to 100%, disconnects, the voltage selltes back down to maybe 3.45.. and then start charging again. Would you expect that for each time this is repeated, the time it takes to get the voltage back to 3.65V is reduced? And eventually zero seconds?
(just a theoretical question to understand how this works, not planning on putting stress on my cells for real)
 
Right - glad you specified theoretical and not planning to put stress on the cells. :)

Lets take this example to the extreme to make it clear. You have a 280ah cell nearly fully charged. You also have a 100mah little wallwart, but desire to get to 3.65v even if after spending 2 years charging, you are around to witness the last 30 seconds when it finally pushes up into the charge knee. :)

But it doesn't make it. Maybe to 3.4v if you are lucky. Maybe it even looks like it is "settling" even though you still see 100ma of current going into it.

Here's where we go completely off the rails from an industrial standpoint. This lack of push to 3.6v at such low currents is demonstrating what's known as voltage-diffusion. There simply isn't enough energy coming from the wall-wart for the cell to even react, hence the voltage never rises.

But let's not get so propeller-head about it. The recommended minimum for EFFICIENT charging (and plenty of voltage-diffusion to take place) of LFP is about .05C

When it is not efficient, the length of time spent charging to full, and the amount of time doing so is detrimental from an industrial standpoint.

In our case, a one-off top-balance taking very long with low currents can be forgiven. Industrially? No.
 
Same thing was happening to me. Seemed like the voltage would never get the cells into the steep curve and then it happened.. all of a sudden the voltage starting climbing fast and the cell voltage differences also did the same.. I then lowered the charge controller absorb voltage to roughly 3.388v per cell. Now the cells seem full and there’s no amps coming into the cells and the balancer is balancing the cells. Seemed like the balancer wasn’t doing anything when the cells were 3.5+ volts.. I know now that the cells are at the beginning of the steep part of the curve and balancing can happen there..
 
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When top balancing cells, nothing much happens for a very long time....until a lot happens in a very short time.
 
Indeed..
ok so If my cells are within .002 of each other at 3.896v per cell.. is there any reason to top balance?
 
Cheap 4-life said:
ok so If my cells are within .002 of each other at 3.896v per cell.. is there any reason to top balance?
Click to expand...
If those are LFP cells they are grossly over-charged.
Suggest get them all down below 3.35 volts per cell ASAP.
 
Cheap 4-life said:
I’m sorry. I meant 3.389v.. still thinking I have nmc cells.. it’s a hard transition :)
Click to expand...
The apex of the high knee starts at ~3.425 volts.
diysolarforum.com

Explanation for Beginners of Top and Bottom Balance

This is a beginners explanation of top and bottom balancing cells that should help you understand what it is all about. To get the document, click on the orange button at the top of the page. NOTE: This is not intended to be a tutorial of how...
diysolarforum.com diysolarforum.com
diysolarforum.com

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...
diysolarforum.com diysolarforum.com
 
John Frum said:
The apex of the high knee starts at ~3.425 volts.
diysolarforum.com

Explanation for Beginners of Top and Bottom Balance

This is a beginners explanation of top and bottom balancing cells that should help you understand what it is all about. To get the document, click on the orange button at the top of the page. NOTE: This is not intended to be a tutorial of how...
diysolarforum.com diysolarforum.com
diysolarforum.com

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...
diysolarforum.com diysolarforum.com
Click to expand...
Yeah I read all that. My question is if there is any reason to do top balance if the cells are already so close in voltage. I’ll never charge above 3.389 anyways unless I need to balance them
 
Cheap 4-life said:
Yeah I read all that. My question is if there is any reason to do top balance if the cells are already so close in voltage. I’ll never charge above 3.389 anyways unless I need to balance them
Click to expand...
LFP needs to be charged into the high knee regularly so that the bms can maintain the top balance.
Even automotive grade cells drift.
The non automotive grade cells drift more.
Why 3.389 volts btw?
 
Cheap 4-life said:
Yeah I read all that. My question is if there is any reason to do top balance if the cells are already so close in voltage. I’ll never charge above 3.389 anyways unless I need to balance them
Click to expand...
I've seen people do this. But not back when LFP wasn't so cheap.

After proving that the system is relatively stable at the top, they drop back down to under 3.4v cv. That assures that the battery will NEVER be fully charged. Oh yes, you will see the normal CV response and being under 3.4v means you'll never get fully charged. And for some, that's ok. The LFP capacity they chose was big enough to do this.

What a lot of people miss is that when under 3.4v, the potential for a full charge is not there. 3.4v is a transitionary LFP chemical window between assuring that you'll never achive a full charge, OR by surpassing 3.4v just a little bit, the cells are willing to do so - whether you actually reach full charge or not.

This 3.4v transitionary window can depend upon actual cell chemistry, so give it some wiggle room. Like 3.375 max if you want to ensure you'll never achieve a full-capacity charge, or 3.425 to tell the cells it's ok to do so.

This is also a consideration for those who choose to do float for whatever reason. Without getting too lost in the float-war noise, floating at 13.6v (3.4v cell) puts the cells right on top of that transitionary point, where depending on manufacture, some cells might be "willing" and some won't. Those that are willing to do so, if kept floated like this, but not really going anywhere, could lead to degradation by sitting on that transitionary knee, and not really deciding which way to go. :)

This is why you see some recommendation to float at 13.5 instead of 13.6v. But that can be inconvenient for more common chargers, that use 13.6v float like they did in the lead-acid past.
 
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John Frum said:
LFP needs to be charged into the high knee regularly so that the bms can maintain the top balance.
Even automotive grade cells drift.
The non automotive grade cells drift more.
Why 3.389 volts btw?
Click to expand...
I understand that. But do I need to do it now? The cell voltage difference is only .002 volts all the way up to the max voltage I will be using regularly.
3.389v seemed to be where the cells rested at so I’m thinking that's where I will float them that way they aren’t being constantly charged (like they would be at a higher voltage) when they are already full enough..
 
Cheap 4-life said:
I understand that. But do I need to do it now? The cell voltage difference is only .002 volts all the way up to the max voltage I will be using regularly.
3.389v seemed to be where the cells rested at so I’m thinking that's where I will float them that way they aren’t being constantly charged (like they would be at a higher voltage) when they are already full enough..
Click to expand...
See @Substrate 's post ^

I charge to ~3.46875 volts per and float for a few hours at ~3.34375 volts.
In those few hours my pack only discharges ~1.6 amp hours.
This is a 280ah pack with a fairly constant 10 amp load.
 
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Substrate said:
I've seen people do this. But not back when LFP wasn't so cheap.

After proving that the system is relatively stable at the top, they drop back down to under 3.4v cv. That assures that the battery will NEVER be fully charged. Oh yes, you will see the normal CV response and being under 3.4v means you'll never get fully charged. And for some, that's ok. The LFP capacity they chose was big enough to do this.

What a lot of people miss is that when under 3.4v, the potential for a full charge is not there. 3.4v is a transitionary LFP chemical window between assuring that you'll never achive a full charge, OR by surpassing 3.4v just a little bit, the cells are willing to do so - whether you actually reach full charge or not.

This 3.4v transitionary window can depend upon actual cell chemistry, so give it some wiggle room. Like 3.375 max if you want to ensure you'll never achieve a full-capacity charge, or 3.425 to tell the cells it's ok to do so.

This is also a consideration for those who choose to do float for whatever reason. Without getting too lost in the float-war noise, floating at 13.6v (3.4v cell) puts the cells right on top of that transitionary point, where depending on manufacture, some cells might be "willing" and some won't. Those that are willing to do so, if kept floated like this, but not really going anywhere, could lead to degradation by sitting on that transitionary knee, and not really deciding which way to go. :)

This is why you see some recommendation to float at 13.5 instead of 13.6v. But that can be inconvenient for more common chargers, that use 13.6v float like they did in the lead-acid past.
Click to expand...

I was charging the cells for the first time and trying to get them above 3.45 to top balance. Under 3.45 the voltage difference was only .002v. When they reached 3.5ish the voltages started to rise very quickly and the cell voltage difference went from .007v to as high as .075v when the cells reached 3.53ish. At the same time my charge controller was pumping 50amps into the pack. I should have waited for the amps to start lowering to keep the pack at absorb voltage and not go over and then once amps lowered the bms could actually started to do some balancing. But I guess I got scared ??‍♂️ from seeing the cell voltage differences go up so much, although I knew they were going to. I lowered the absorb voltage to 3.42v per cell so the charge controller wasn’t putting as many amps into the battery. The voltage difference basically instantly went back down to .03 from .075. Still far from .002 that the cells started with. But now with the amps lowered coming into the batteries from lowering absorb voltage the bms was actually starting to balance and got the cells down to .018. I’m sure some of that cell difference lowering was simply because the cells were no longer at such a steep part of the curve..
This led me to my question. Is there any reason for me to top balance my cells now? Or can I simply wait to top balance till I start seeing some drifting at 3.38 volts. Next time i try to top balance I won’t be scared :) and will actually wait till the amps lower on their own at the higher absorb voltage. But the other problem was that the amps were just not lowering. The voltage was up there at 3.5 and absorb voltage was reached and it seemed like the amps were not coming down although I’m now thinking they would have.. anything under 30amps coming in from the controller want raising the voltage to get to top balance voltage.. the voltage kept going back down as the amps coming in would lower.. it was the worst waiting game ever.. I now understand what so many meant about voltage not being a good indicator of SOC with LFP.. With NMC voltage was a great indicator
 
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Cheap 4-life said:
I was charging the cells for the first time and trying to get them above 3.45 to top balance. Under 3.45 the voltage difference was only .002v. When they reached 3.5ish the voltages started to rise very quickly and the cell voltage difference went from .007v to as high as .075v when the cells reached 3.53ish. At the same time my charge controller was pumping 50amps into the pack. I should have waited for the amps to start lowering to keep the pack at absorb voltage and not go over and then once amps lowered the bms could actually started to do some balancing. But I guess I got scared ??‍♂️ from seeing the cell voltage differences go up so much, although I knew they were going to. I lowered the absorb voltage to 3.42v per cell so the charge controller wasn’t putting as many amps into the battery. The voltage difference basically instantly went back down to .03 from .075. Still far from .002 that the cells started with. But now with the amps lowered coming into the batteries from lowering absorb voltage the bms was actually starting to balance and got the cells down to .018. I’m sure some of that cell difference lowering was simply because the cells were no longer at such a steep part of the curve..
This led me to my question. Is there any reason for me to top balance my cells now? Or can I simply wait to top balance till I start seeing some drifting at 3.8ish volts. Next time i try to top balance I won’t be scared :) and will actually wait till the amps lower on their own at the higher absorb voltage. But the other problem was that the amps were just not lowering. The voltage was up there at 3.53 and absorb voltage was reached and it seemed like the amps were not coming down although I’m now thinking they would have.. anything under 30amps coming in from the controller want raising the voltage to get to top balance voltage.. the voltage kept going back down as the amps coming in would lower.. it was the worst waiting game ever.. I now understand what so many meant about voltage not being a good indicator of SOC with LFP.. With NMC voltage was a great indicator
Click to expand...

Probably the biggest reason to top balance is to ensure the capacity of the pack is the same as its weakest cell.
When/if you discharge into the low knee the weakest cells will nosedive first.
 
Cheap 4-life said:
Under 3.45 the voltage difference was only .002v. When they reached 3.5ish the voltages started to rise very quickly and the cell voltage difference went from .007v to as high as .075v when the cells reached 3.53ish.
Click to expand...
This demontrates the pack is un-balanced.
 
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