How to "revive" a lazy cell from a LiPo pack ?

[Hugues]

Hi guys,

i've got a 7 cells LiPo pack. The last cell is reading 3.9V while the other 6 read 4.2 V.

So i cannot get the 10Ah the pack is rated for, i get more or less 6.5 Ah. The lowest cell reaches the low voltage limit first and the BMS shuts down the pack.

2 ways i can improve on this:
- or i disconnect completely the lazy cell, and get a 6 cell pack with lower voltage
- or i try to "revive" the lazy cell

I've got a LiPo charger that can charge one cell only at 3.9V. My question is:
- how can i charge this single cell only ? My pack connections are shown below.
each cell is connected in serie to the next one with a small tab going into the LiPo,
there is a small balancing cable going from each cell to the BMS,
and of course the main black and red leads at each end.

In order to try to charge this lazy cell only up to 4.2V, i guess i need first to "isolate" it from the rest of the pack and BMS ?
Or can i just plug the 0V of my charger on the minus tab and the 3.9 V on the plus tab, and just monitor it until it reaches 4.2 V ?
Is there any danger to affect the other cells or BMS this way ?
This pack does not have a balancing cable.

 

[Ypedal]

you can tap directly to the + and - tabs of the cell in question, even while it's in series with the others... but limit charge current to no more than 2 amps if using the ballance leads.

 

[neptronix]

I can't believe you're running a RC lipo pack without balance leads. Overcharging or underdischarge is how you get a lipo fire and you're livin' on the edge. The first order of business will be to install balance leads.

 

[Hugues]

I can't believe you're running a RC lipo pack without balance leads. Overcharging or underdischarge is how you get a lipo fire and you're livin' on the edge. The first order of business will be to install balance leads.

Thanks for the warning, we're never too cautious with LiPos.

I'm not sure what is a RC lipo pack and what is not.
Maybe I did not clarify . The pack has an internal BMS. I've got 3 others like this and I bulk charge them since July without a problem . I say this because I've added an extension from the BMS and monitor each cell voltage and they stay very close to each other, < 0.02 V

So I presume you mean by RC lipo pack a pack without any BMS. I've got many like these, I fly gliders, and I'm charging them with my balancing charger. So overall the electronic is the same I guess, on one system the BMS is in the pack, on the other it's in the charger.

But thanks for the warning.

Coming back to my question, do you agree with Ypedal ?

 

[neptronix]

Yes, i agree with ypedal.
However it is a little concerning that you have a BMS on this pack and you still have a lazy cell.

I have a feeling that that particular cell is a goner, IE it has lower capacity and/or a weird RI curve. You can diagnose that problem with a good balancing charger that's capable of logging while it discharges.. IE iCharger, Hyperion, etc.

 

[Ypedal]

Most BMS's canot deal with a large imballance, milliamps per hour take a long time to correct... clip direct to the cell and top that sucker off and be done with it... then monitor it over time, if the cell is really bad it will drift back down quickly ( it is also possible a bad BMS channel itself is causing the imballance... )

 

[Hugues]

thanks guys,

i will "top that sucker off and be done with it." as you say

By the way, Ypedal, do you sell this nice outfit you display on your picture ?
that might be handy for LiPo work.

i'll keep you posted.

 

[Hugues]

Ok, it seems to work.
Charged the lazy cell back up to ~4.2 V
Left the pack overnight and the lazy one did not sink more than others.
Discharge the pack (with 2 x 3 Ohms in series )
And recharged; the cell is now following the others.
Great.
I will put it on the bike today and check the max Ah I can get out of it.
Thanks for the guidance

 

[SamTexas]

Hmm... Sounds like the suggestion I gave you a while ago.

Fully charge the last cell shows 3.8 V only, and when discharged it is closer to 2.7 V !!

1) I'm surprised you were able to get 6.5Ah out of the pack. I would expect a lot less.

- Is there any hope of balancing this individual cell ? I just have a bulk charger, and these packs have no balancing lead.

2) Since you were able to measure the cell voltage, you already have some sort of "balancing lead". If you can find a cell phone charger, you can try charging that individual cell and see if it can hold charge above 3.8V.

- Is it complicated/dangerous to try to order a new identical cell from the same vendor and replace the problematic cell ?

If step 2) fails (meaning that the cell can not be fully charged), I would bypass that cell. So my 7s pack now becomes a 6s pack. Then I would retest to see if I get the full 10Ah. With proper precaution, the bypassing operation should be neither complicated nor dangerous. But to be doubly safe(r), I would discharge the whole pack first.

 

[crusoe]

Is it possible to tap into the balance leads, charge at 2amps (hyperion 1420i), on a cell that reads .38v on a 6s 5ah turnigy lipo?

The cell in question originally was in the same situation as OP, all other cells read 4.2 except the one that read 3.7.

I did a "STORE" charge on the hyperion and it brought back all cells to 3.6V. I charged up my entire 24s2p pack and went for a ride - all seemed fine.

A couple of days later I did a voltage check with cell log, as I do before every ride, and one of the cells now reads .38. It is the same cell, that dorpped to 3.7 originally.

Is there any saving this cell?

 

[John in CR]

I wouldn't even try charging that one while it's packed with the others, since if it went it would take the whole pack with it. Cut it out and replace or bypass and go with 23s.

How deeply do you typically discharge? If you run your pack down, I wouldn't leave it there. At a minimum charge it back up to storage charge. I just immediately charge mine back to full charge 4.15V/cell so my bike is always ready. I did that for 3 years with my Konion packs, and got over triple the manufacturer's cycle life rating and still going, so I believe that with a conservative top of charge voltage damage from storing overnight or over a few nights at full charge is an urban battery myth that someone started with little or no evidence. Leaving them nearly empty seems much more dangerous to me, even if the only risk is that you forget to charge and hop on for a ride at a low SOC.

John

 

[crusoe]

I wouldn't charge it with the others! (unless you meant, on the cell level, within the same 6s pack). I did a store, with just that single battery, inside of a lipo fire bag

So there is no saving that dead cell than? Even if I charged that single cell, on its own, via the balance taps?

I usually discharge to 3.6v.

Hmm, ya that must be a myth then! I heard storing at 4.2 for long periods wasn't good. But your evidence seems to contradict that!

 

[dogman dan]

.38v I would call a gonner. Or call the fire dept. Don't bother with it, it's toast. Buy a replacement pack, and now you have some spare cells.

 

[crusoe]

I've got one, 6s 5ah, lipo that has a cell reading 0.0V. I tried doing a 2A charge via the balance taps of the dead cell (with a Hyperion 1420i). The charge went fine for 5 minutes and then the charger beeped and said "Output cell voltage too low". I tried charging again and got the same message after 5 minutes. The previously dead cell, did go up to .078v after two rounds of those charges. Any insight would be great! Thanks

 

[amberwolf]

As has already been said--IT IS DEAD.

DO NOT CHARGE IT.

If you get any RC LiPo cells down anywhere near that far, and charge them back up (especially at a high current), you're probably asking for nasty things to happen.