Are my charged cells within ok range of balanced?

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alpharalpha
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Are my charged cells within ok range of balanced?

Post by alpharalpha » Jan 12, 2018 9:20 am

It's been several days now since I charged them and they read

3.49
3.66
3.54
3.52
3.39
3.44
3.52
3.37
3.60

Thanks.

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Icewrench
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Re: Are my charged cells within ok range of balanced?

Post by Icewrench » Jan 12, 2018 10:21 am

Those volt readings do not look well balanced to me.
I will balance charge my rc lipo packs if they get to be near .1 volt of difference.

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Re: Are my charged cells within ok range of balanced?

Post by fechter » Jan 12, 2018 3:00 pm

alpharalpha wrote:
Jan 12, 2018 9:20 am
It's been several days now since I charged them and they read

3.49
3.66
3.54
3.52
3.39
3.44
3.52
3.37
3.60

Thanks.
Are those LiFePO4?

Either way, not very well balanced. You might want to also measure immediately after charging to see if some have a high rate of self-discharge.
"One test is worth a thousand opinions"

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Re: Are my charged cells within ok range of balanced?

Post by 999zip999 » Jan 12, 2018 5:14 pm

Does it have a bms ?

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Re: Are my charged cells within ok range of balanced?

Post by amberwolf » Jan 13, 2018 2:44 am

alpharalpha wrote:
Jan 12, 2018 9:20 am
It's been several days now since I charged them and they read

3.49
3.66
3.54
3.52
3.39
3.44
3.52
3.37
3.60

Thanks.
What kind of cells?

If those are not LiFePO4, then they're not even charged--they're pretty well close to empty, and very unbalanced.

If they are LiFePO4 then while they are all at least mostly charged, they're definitely not balanced.

Balanced means that at full charge they are within at least 0.1v, and preferably within 0.01v (0.02-0.03v might be ok, too).

But those cells are not even close.

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alpharalpha
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Re: Are my charged cells within ok range of balanced?

Post by alpharalpha » Jan 13, 2018 8:56 am

Yes they are lifepo4 3.2v x 30ah, 9 cell pack (with bms.)

I did a 4 mile ride yesterday running at 20-30amps so here are the voltages taken just now a day later:

3.28
3.30
3.30
3.30
3.30
3.27
3.30
3.30
3.30

(The cells are 3 years old now.)

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Re: Are my charged cells within ok range of balanced?

Post by fechter » Jan 13, 2018 12:29 pm

That looks much better. If they sat around for a long time, they might have gotten out of balance due to uneven self discharge.
If you fully charge the pack, then discharge it slightly and repeat, you can bring it back into balance, but those numbers look pretty good now. I would do a full charge and measure immediately at the end of charge. This will give you a better indication of balance.
"One test is worth a thousand opinions"

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Re: Are my charged cells within ok range of balanced?

Post by dogman dan » Jan 14, 2018 8:03 am

Id say it must be lifepo4.

Re the balance after the ride, with lifepo4 in particular, there really is not much information to be gained by voltage at mid discharge, since the curve is so flat.

If you have a bms, charge back up, wait half an hour for the bms to do any discharging it might do, then try again to charge. To get the charger to restart, you might have to ride around the block, lowering all the cells enough to get the charger to go again.

If you don't have a bms, charge the lower cells one by one to 3.65v max, or at least, to slightly more than 3.5v. 3.5v is full, but lifepo4 tolerates a slight overcharge very well, so you can set a charger to charge the pack to 3.65v average, and force a balance each charge.

If inbalance persists, despite a very low rate of discharge and moderate discharge depth, then you simply have some funky cells. A lifepo4 cell that can't retain 3.3v overnight is pretty worn out. Still usable maybe, but not fresh no more.

Re your last reading fully charged, anything over 3.5v is fine. Those are balanced, even if one is 3.5 and another 3.65. This is because 3.5v is full, and any more is just surface charge that often dissipates overnight. Its the ones below 3.5v that you need to get more charge into. I call anything within .05v perfectly balanced myself. so any cell at 3.45v is fine, and typical for any pack. Even .1v off can be tolerated, if you don't need 100% of the potential capacity to do the ride you want to do. Over time, all will take less charge, but if all still charge to 3.4v or better by next morning, its still a good pack.

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Re: Are my charged cells within ok range of balanced?

Post by alpharalpha » Jan 14, 2018 3:34 pm

So I charged them 2x more, first row is the last charge and the 2nd row is the first.
3.60 3.57
3.86 3.74
3.62 3.59
3.65 3.59
3.41 3.40
3.65 3.47
3.72 3.59
3.41 3.37
3.65 3.59


(I have bleeding bms type.)
Also, I took 3 of my original new cells and put 3 from a different older set in, that could be #5,6,8. I will open up pack and see if I can charge those weak cells to 3.65. I have those 3 original cells stored in my 'fridge double ziplocked with silica packs in ea' layer, be interesting to see how they've held up being stored the past few years.

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Re: Are my charged cells within ok range of balanced?

Post by fechter » Jan 14, 2018 7:14 pm

Target voltage for those cells is around 3.65v. The higher ones should bleed down after a day or so.
"One test is worth a thousand opinions"

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Re: Are my charged cells within ok range of balanced?

Post by dogman dan » Jan 15, 2018 8:11 am

Keep repeating the process, till all high ones bleed down to 3.65, and the low ones at least come up to 3.5v and stay there after sitting overnight. Ride around the block to spoof the charger to restart, but wait till the bms bleeds those high cells down to 3.65 to do that.

If they get to 3.5v off the charger, but lose very much more voltage overnight, they are damaged or at least weak cells. Like from 3.5v to 3.35 overnight I mean. The surface charge, that is from 3.5v on up may disappear overnight, and that is normal. If any cells continue to lose voltage after overnight, then you might have a defect in the bms, or some other type of trickle discharging them. This includes an internal short in the cell, a defect if these are new.

Once a pack is used awhile, its not unusual for all the cells to stop holding 3.5v. A pack that still holds 3.4v evenly is still good. Yours may just have some weak cells in it, but even if those weak cells never get past balancing out at 3.35v each ride, its still usable. Just not at the capacity its rated at.

Hopefully your pack will settle down and stay pretty well balanced, once you get it to fully balance, to within .05v of 3.5v on the low side, and not much more than 3.65 on the high cells.

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Re: Are my charged cells within ok range of balanced?

Post by alpharalpha » Jan 16, 2018 4:27 pm

I went for a ride today, and now they are:

3.26
3.29
3.29
3.29
3.29
3.28
3.30
3.29
3.29

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Re: Are my charged cells within ok range of balanced?

Post by fechter » Jan 16, 2018 10:35 pm

Looks better. Best measurement is right after charging then let them sit overnight and measure again. This will pick up ones with high self discharge.
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Re: Are my charged cells within ok range of balanced?

Post by miuan » Jan 17, 2018 8:46 am

A friend passed me his highly unbalanced LiFePO4 BMS equipped 12Ah battery.
I got no more than 2Ah out of it - it was so unbalanced.
Ultimately I got as much as 11Ah out of it.
The way I did it was very easy. Connected a hair dryer to the discharge lead, so the charging process wound never end, and balancing circuit was always running. Just make sure the dryer won't draw more amps than the charger is able to supply.

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Re: Are my charged cells within ok range of balanced?

Post by fechter » Jan 17, 2018 9:07 am

Another way is to use a constant current power supply that puts out less than the balancing shunt current. Even a simple resistor or light bulb in series with a fixed supply could do this. Then just let it go for a few days. Typical balancing shunts are 100mA, but this varies. You can calculate it based on the resistors installed on the shunts.
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Re: Are my charged cells within ok range of balanced?

Post by dogman dan » Jan 18, 2018 8:27 am

Of course lifepo4 looks balanced when not full. very flat discharge curve, so if you were out of balance about 10%, in the middle like that it looks perfect balanced. 20% or more different could only be .05v at that point on the discharge curve.

Cool ideas for forcing balancing. I just suggest the ride around the block thing because it works for customers and bike shop owners that know nothing about electronics. I learned a few things about CS working at E-Bikekit for 4 summers. You need solutions that anybody can handle.

Lots of short rides, then charge full, and eventually the bms will get closer to fully balanced. The trick is letting the bms trickle down the high cell before you take that short ride. So charge, wait an hour, ride, charge again. If you have nothing much else to do, you can force a balance in a day, 4-8 cycles.

This does assume your cells are fine, and I bet yours are, or it would look worse even at mid discharge.

What tends to happen is the battery arrives slightly out of balance, like any battery that has been stored awhile. Riding it will fix it.

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Re: Are my charged cells within ok range of balanced?

Post by 999zip999 » Jan 18, 2018 3:48 pm

You can use a light bulb for a tail light solder a couple wires to it and use it to bleed the cell that's above 3.65 volts. I have 3 voltpheaks single cell charger made for lifepo4 and stop at 3.5v perfect. I cant find them online.
Lifepo4 drain fast from 3.8v to 3.45volt as very little capacity there. Fluff.

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Re: Are my charged cells within ok range of balanced?

Post by dogman dan » Jan 19, 2018 7:53 am

That's how I do my batteries that don't have a bms. Just discharge the one high cell with a car light bulb, unless a low cell is really low.

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Re: Are my charged cells within ok range of balanced?

Post by alpharalpha » Jan 21, 2018 9:57 am

I built this pack 3 years ago and shortly after thought I had a problem with #1 cell discharging, turns out the cell was fine, don't recall what the issue was; I found the last few 30ah cell like I had; they had been sitting in a warehouse, weren't perfect then, don't know why I replaced them with my new numbered set, I think I wanted to see how they performed and get some use out of them, so there are 3 cells in my pack that don't match the others (the 3 new cells have been stored in my fridge for a few years.) I can easily get to the cells in my pack so I just put a single cell charger on those low cells and now after a few days they are:

3.54
3.59
3.59
3.56
3.54
3.55
3.59
3.53
3.59

I think it was cell 1 or 2, 5 & 8 that I switched out but not sure. I will ride it now until bms discharge cutoff and see what's what. the numbers from the original set should still be on the bottom, like I said I can access the cells.
IMG_0190a.JPG
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I just took the 3 original cells out of the fridge
IMG_0195a.JPG
IMG_0195a.JPG (63.26 KiB) Viewed 296 times
IMG_0197a.JPG
IMG_0197a.JPG (30.86 KiB) Viewed 296 times
fortunately I copied the numbers from the stickers. #111 that says bad cell I think was from when I thought cell #1 had a problem. Also when I first got the cells and didn't know what I was doing I charged without a bms and cell #8 puffed out a little, but I put it in the middle of the pack and it compressed and didn't seem affected; that could be why it's lower than the rest. I'm kinda interested now to see what's going on as it's been 3 years. The cells were sold in numbered sets of 6 to a box so since I bought 9 the last 3 while still a numbered set and close to the first set weren't sequential to the first. I might have written bad cell for the one that puffed up, I'll know when I loosen the tie down and look underneath the cells.
I had absolutely no idea what I was doing when I started but figured instead of buying a pack, I'd build my own so if something went wrong I'd be familiar with everything to fix it.

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Re: Are my charged cells within ok range of balanced?

Post by 999zip999 » Jan 21, 2018 12:51 pm

Just do some light rides I don't like to run a battery to lvc. Not the first run. Find out it's Behavior first. Your numbers looks great.

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Re: Are my charged cells within ok range of balanced?

Post by alpharalpha » Jan 22, 2018 7:04 pm

So I did a light ride, 2 miles at high amps. and now they're:

3.29
3.29
3.29
3.29
3.29
3.29
3.30
3.29
3.29

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