DIY Electric Car Forums banner
1 - 6 of 6 Posts

Super Moderator
1,556 Posts
Tangent: Not sure why you created 2 threads 2 minutes apart for the same issue..

when a number of cells become 'unbalanced' with others in the pack, and do not remain balanced after repeat single cell charging/discharging to a set voltage... is there any way to recover them, or are they dead?
You are using the word "unbalanced" but that is not really what "balance" means. Yeah, technically they are also out of balance, but that's not the problem.

"Balance" refers to have equal states of charge from, presumably identical cells.

The problem you have is that your cells are no longer capacity-matched, due to damage, and thus will always become quickly out of balance when charged or discharged in series.

It's like lining up an adult with a toddler and telling them to each take 3 steps forward. You notice the toddler is not keeping up. So you line them back up again, and again after 3 steps the toddler is not keeping up. The problem isn't that you're not lining them up good enough, the problem is that your baby is ugly. Err, that your toddler has shorter legs.

here are a number of cells that just don't seem to hold the charge nearly as well, and even with repeat charges to the set voltage, they 'settle' to a lower voltage than other cells... and when I try a pack charge the cells are obviously out of balance with a number of cells heading up to 4v while a few don't seem to keep up.
Hard to parse your grammar here, but, I think this could be a couple different things.

1 - Some cells have lower capacity (due to damage). That means they fill quicker and empty quicker because there's less capacity in them.

2 - Some cells have higher internal resistance (due to damage). This means that they fill slower and discharge slower, and get hotter when they do. When charging, you may be giving it 3.65 volts, and it may be showing 3.65 volts, but it's not adding all that as energy. Chargers usually "terminate" when the charging current drops to a certain amount, so if your resistance is high the charging current will be low especially at the top end, and it will stop before it was done filling.

3 - Some cells may have dendrite damage internally, small shorts that constantly drain the battery. So they self-discharge.

In the case of the 3rd, they're probably garbage, in the case of the 2nd your charger isn't likely to use them effectively and you'll have to rig up some halfassed solution. In the case of the 1st you could add some capacity to the cell, maybe purchase some 18650 LiFes of approximately the missing capacity, hook them up in parallel, and hope you're closer to matched capacity that way.

Super Moderator
1,556 Posts
What I am looking for now is a way to identify each cell's IR
The bad news is that, generally, high internal resistance goes hand in hand with lowered capacity.

I think you just have lowered capacity, and nothing can help that other than boosting the low cells with some smaller add-on cells like 18650s.

I'm not sure you grasp what's going on, you seem to keep using the same terms for a variety of things. Isolate what you're measuring.

Charge all your cells up to their max voltage independently. Then let them sit, then measure them. If you have self-discharge then that's a separate issue.

If they maintain their voltage, but some cells drop faster under use, then those cells are lower capacity, "balancing" is not going to fix them.

Super Moderator
1,556 Posts
I am not as concerned with reduced capacity as I am with inability to maintain a decent top-balance with repeat charge and use cycles. Unbalanced end of charge voltages obviously will damage cells further.
Okay, so, I think you still don't get it.

"Unbalanced" is not the issue. Reduced capacity of some cells is the issue. You keep calling it a balancing issue as if it can be resolved by better balancing, but I'm trying to tell you that, from your symptoms, it can't.

I think I grasp things quite well thank you. Sorry my language is too imprecise for you.
I don't matter. I'm trying to articulate that you are mixing terms and problems and the advice you're given based on what you're asking might not be useful to you.

obviously.... but I am not seeing self discharge, nor is that what I asked about.
Well, I'm glad I clarified, because it certainly did seem that that was what you were asking about.

Ex) "HOWEVER, this time around, there are a number of cells that just don't seem to hold the charge nearly as well, and even with repeat charges to the set voltage, they 'settle' to a lower voltage than other cells."

When you say they "settle to a lower voltage" it makes me think you're charging them all to 3.7v but then you come back later and some have fallen to 3.5v or whatnot.

That's exactly why I'm trying to point out that it's important to accurately describe your issues, so that when people try to help diagnose them for you they can give you useful answers.

pretty hard to tell if some cells are dropping faster under use since the middle of the curve is so flat. i.e. at 10%-70% DOD there is hardly any difference in voltage.
Okay, here is a procedure:

1 - Charge all cells in series until one of them reaches your max voltage (3.65 or 3.7v or whatever). This is for convenience.

2 - Charge all the remaining cells up to their max voltage independently.

3 - When all cells have the same max voltage (not just the sum of voltages being what you guess to be correct), discharge them in series. Set a timer if you feel like it.

4 - When one cell falls below it's minimum voltage, stop the test. That cell is the weakest cell. You can yank it out of the series chain, then continue the discharge.

5 - When the next cell falls below it's minimum voltage, stop the test. That cell is the next weakest cell. You can remove it from the chain too.

6 - Do this as long as you like, recording times as you go. This will give you a ballpark for how much life the cells have. If your first cells hits your minimum voltage and the others are all significantly higher, you know it's lost a lot of capacity. If a few others are close behind, you know those are all weak. If they all arrive fairly close to each other, your pack is decently capacity-matched.

Super Moderator
1,556 Posts
1. time. I have 48x100ah CALB LiFePO4 cells; draining each to 80% DOD and recording the time would take hours per cell unless I come up with a super heavy resistance. And then take another 4 or 5 hours per cell to charge back up with my Revolectrix at 20amps.
No. You're not following the instructions. If you read the procedure I gave you, you will notice that I said to discharge them IN SERIES so you will be discharging them all at the same time.

I specifically wrote a foolproof method for you that saves you as much time as possible.

This is why I keep coming back to end-of-charge top balancing... to see what ideas anybody has on how to make the best of this pack without the expense of replacing the whole thing. I'm looking for some way to quantify which cells will not be 'top-balancable' anymore,

You keep calling a capacity issue a "balancing" issue.

Balancing will never increase the capacity of a cell. That is not how it works.

You can keep asking but the answer will be the same every time. Balancing does not fix capacity issues. No one will have any "ideas" on how top balancing will fix your capacity issues, you might as well be asking for ideas on which seat position changes the color of your paint job.

The only way to figure out your capacity is to check your capacity.

This will involve charging them and then discharging them.

If you do not have time to charge and then discharge your batteries, and to make occasional measurements as to the voltage along the way, you will never know their capacity or identify bad cells.

You can rephrase the question over and over about how balancing them will tell you this info, but you will never get an answer.

Balancing just ensures that your capacity measuring is accurate. So, yeah, top balance them first, cell-by-cell. Then discharge them and occasionally measure each cell until you have an idea which ones to keep an eye on and roughly when the pack will be bottoming out. That will identify which cells have less capacity.

Super Moderator
1,556 Posts
The only load heavy enough to discharge the series is.... driving

1 - Bulk charge your battery pack.

2 - Top balance each cell.

3 - Go for a drive based on what you already know about your likely range, try to time it to be near the end.

4 - When you get back, check your voltages and see which cells are significantly lower than the rest of the pack. Those are the shitty cells.

5 - Remove/replace/supplement the shitty cells.

I totally understand that balancing will not increase capacity. That's not what I am worried about. What I am worried about is that it appears *some* cells are not STAYING balanced and returning to end-of-charge at the same time after a couple charge/drive cycles

I'm trying to narrow in on why you're not getting this.

If you have cells that are not capacity-matched, they will immediately become unbalanced when used.

If you have 5 LiFePO4 cells charged to 3.65v each.. but one has lost 50% of it's capacity, its voltage is going to plummet twice as fast as the cells of double capacity.

At the same time, for the capacity it does have, from the (reduced) surface area of the plates, it's drawing twice the current. So, for example if they are 100AH cells and you're drawing 100 amps... that's 1C. If one cell is reduced to 50AH max, you're still drawing 100 amps from it, that's 2C. So that chemistry is going through double the abuse, double the waste energy, etc.

Ditto on charging, you'll be charging it at double the C-rate of its neighbors, double the waste.

If you waste energy as it's coming out or into the battery, it will have less capacity compared to the other cells.

So that could be one explanation.

Another explanation can be that, if you're draining the pack until it's total voltage is low, then the weak cells are going to be way beyond their safe bottom limit.

For example, 10 cells:

If spec is each cell to a max of 3.65 volts...

3.65 + 3.65 + 3.65 + 3.65 + 3.65 + 3.65 + 3.65 + 3.65 + 3.65 + 3.65 = 36.5 volts.

If you want to cut off at 2.5 volts per cell....

2.5 + 2.5 + 2.5 + 2.5 + 2.5 + 2.5 + 2.5 + 2.5 + 2.5 + 2.5 = 25v.

But if you have no knowledge of the cell-by-cell voltage, only the pack total, what will actually be happening is probably:

2.77 + 2.77 + 2.77 + 2.77 + 0.00v + 2.77 + 2.77 + 2.77 + 2.77 + 2.77 = 25v.

Which means 9 cells not empty, and 1 cell completely blasted to zero, further damaging it every cycle.

So when you go to recharge it, there's less and less and less capacity in that cell every time. Again, not a balance issue, a capacity issue.

In any case, you cannot know these things until you observe and measure them. There is no further shortcut. You have to do what it takes to figure this out. You can't make good decisions with bad information.

Super Moderator
1,556 Posts
John61 said:
I was under the impression that, at least with LFP, even one "draw down to dead flat 0.0V" event will result in instant death , time to scrap that cell.Is that not the case?
Yes, sometimes, reverse charging is worse, leaving it that way for a while is worst. But you got distracted by a tangent and are missing the forest through the trees.

I was just throwing numbers at it as simply as possible. My point wasn't "It reaches exactly zero". My point was "One will be low enough to incur damage while the total voltage is still at a level that would fool you to thinking it was okay."

Substitute 0.00v for any number below 2.0v or whatever you want. Maybe it reaches 1.00v, or 1.5v, or whatever. The actual number doesn't matter, I picked something extreme to try to simplify it and illustrate the point.


Duncan said:
DTbaker seems to have NOT a cell imbalance problem but a cell self discharge problem. If he starts with a balanced pack some of the cells slowly diverge from the others. If that simply happens with time then those cells are self discharging
That's what he was saying earlier, so that's what I was explaining at first, but he later clarified:

"obviously.... but I am not seeing self discharge, nor is that what I asked about"

He is not talking about self-discharge in any way.

He has capacity issues he is calling "balancing" issues, and doesn't want to take the time to put them through even a single charge/discharge cycle to discover any information that would help fix his problem or even find out what it is.
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.