Revive or replace dead 18650 cells ?

[jeromebg]

Hi, just bought a used Segway MiniPro for my son with a defective battery.
I opened the battery and alls cells are around 3v except 2 paris that are at 0.3v.
If I want to replace dead cells it looks I have to disassemble all the cells from the BMS or I couldn't solder the new ones, right ?
So I was wondering if I could try to "wake up" the dead cells with a RC charger (Bmax 6) without disassembling anything , or it's really not a good idea ?
Thank you for your suggestions !

[BeBuLamar]

I would replace. Reviving a dead cell is never a good thing in my opinion. If it works it won't be for a long time.

[CaptDon]

Holy crap that is a lot of cells!!! If you really want that unit as a keeper I would replace the entire pack. Chances are all of the cells are reaching end of life and you would be in danger of weak cells going into reverse charge or developing the dreaded 'copper short' which could be explosive if a bad cell is in parallel with more cells. Reviving weak and severely over discharged 18650 cells is a waste of time since all you end up with is a weak low capacity cell with a high self discharge rate which could also self destruct during a charge cycle.

[wraper]

Never charge overdischarged Li-ion cells. Even if you succeed, they'll be a fire hazard.

[Bud]

Cells are not soldered, they are spot welded to conductive metal strips, you'd need a basic spot welder for that. But the biggest possible problem could be with the BMS chip locked, if it detected the fault. You may need to research on iNet about battery rebuild for your Segway and if it is possible to unlock the chip. This can be a legthy and difficult task.
The other option is to get the pack to a battery rebuild shop and let them do it.

[timeandfrequency]

Hello jeromebg,

Replace all of the dead LGEBMG11865 cells(*) with equivalent quality parts from a trustable supplier.
Buy a few more spares : it's likely than other cells might fail at medium term.

Once the dead cells replaced, you may also want to locate the weakest ones.
At first use the scooter normally a few times (let's say 5 charge and dischage cycles).
Then charge the pack again and use the Segway to empty the battery quite firmly to about 20-30 % residual autonomy. Disconnect the battery pack.
Measure the voltage of each cell.
Those with the lowest voltage are usually the ones having sub par residual capacity and are therefore good candidates for a preventive replacement.

As Bud underscored, avoid soldering the cells with a soldering iron and Tin solder.
Use a spotwelder and battery stripes instead.


(*) the low charging current of max 0.2 C mentionned in the datasheet is somehow surprising. This means that charging a cell from 0  to 100 % cannot be done in less than 5 hours. A bit annoying for an electric mobility gear

[tunk]

You can revive some "dead" 18650s, 0.3V may be a bit too low.
And there's reason for dead cells, and in a battery pack,
even if you can revive them, they may soon turn bad again.
If you replace some cells they should be identical, i.e. same
make and model, and similar capacity, internal resistance
and rate of self discharge.

[jeromebg]

Ok, thank you for your advices.
I have a spot welder but I'am a bit afraid of disassembling the battery if it can take fire...
I think I will buy a new battery, but they seems hard to find, original ones are way to expensive and chinese one are 4400mah instead of 5700mah like the one I have...

[timeandfrequency]

I think I will buy a new battery, but they seems hard to find, original ones are way to expensive and chinese one are 4400mah instead of 5700mah like the one I have...

Seems not that cheap. Unfortunately.

[tunk]

You got nothing to lose trying to revive them.
And if you're careful, it shouldn't be a safety problem.
Usually when I try to revive cells that's below 3V, I put
a resistor in series with the cell and use a bench PSU.
E.g. you could use a 1k resistor and set your PSU to 1.3V, 
this will give you 1mA charge current. When/if the cell
reaches 0.8V you could increase the PSU voltage to 1.8V.
And so on until you reach 3.0V when you can start normal
charge.

[wraper]

You got nothing to lose trying to revive them.

Except your home and your life. They will be a permanent fire risk that can happen at any time.

And if you're careful, it shouldn't be a safety problem.

Simply  

[CaptDon]

Any time two or more lithium ion 18650 cells are in parallel there is a fire risk if you fool around mixing and matching and 'reviving' dying cells. Tunk's advice is bad when you are playing with parallel cells!!! Even the lowly 18650 cell all by itself can be a hazard and there is a much more serious problem when paralleled!!! If one of the parallel cells shorts it must dissipate all of its own energy as heat but must also dissipate all of the energy from its parallel neighbors!!! This usually happens during the charging phase which is worse yet as the good cells may be at nearly full energy capacity. Look at how many cells you have in close proximity. If that baby goes into thermal runaway initiated by just one cell failing you are going to have a nightmare on your hands!!! I have fooled around with reviving weak cells used in series ONLY when they WERE NOT paralleled by other cells and it is a waste of time!!! The revived cell always discharges first, has high internal resistance, has a high self discharge rate and may even reverse polarity depending on the quality of the BMS to disable the battery pack before really bad things happen. It isn't worth your house, your life or your family to listen to really poor advice!! Even if you replace a few weak cells now, you will be forever married to the battery pack as the rest of the 'high milage' cells also fade away and die.

[Grandchuck]

So many fires started by lithium cells and batteries!  Err on the side of caution!

[thm_w]

(*) the low charging current of max 0.2 C mentionned in the datasheet is somehow surprising. This means that charging a cell from 0  to 100 % cannot be done in less than 5 hours. A bit annoying for an electric mobility gear

Some sort of typo, it says 0.5C standard charge above that, maximum 1C.

[amyk]

The cells with almost no voltage have developed internal shorts, so you're unlikely to be able to recharge them.

There are some people who have had success with "flashing" the shorts away, but that might be a temporary solution at best.

[Gyro]

That's NiCds, hopefully nobody would ever try to flash a Li-ion.

[Psi]

Holy crap that is a lot of cells!!!

hehe,  126V, that's got some serious energy.

[CaptDon]

Gyro, I thought the same thing reading that post. Usually when a lithium ion develops a short the result isn't pretty!!

[Peabody]

So is the Segway pack 2P15S?  Looks like it.  If parallel cells are so dangerous, I wonder why Segway would use that configuration.

[BeBuLamar]

So is the Segway pack 2P15S?  Looks like it.  If parallel cells are so dangerous, I wonder why Segway would use that configuration.

Tesla use parallel series for their batteries using only 18650 cells. I don't think parallel cells are dangerous just that you wouldn't want to flash a cell that is still in parallel with others because you would put power in the other cells as well. If the cells are only in series then you do not need to disconnect them and yet still supply power to only 1 cell.

[CaptDon]

Everyone uses the parallel configuration. The laptop you are using probably has 4S X 2P. We simply accept the risk!! My friend had a 3P camera battery which was untouched for a year sitting in her office. It burst into flames setting the wooden shelving on fire!! Lucky she was home!! My family doctor had a laptop plugged in to the charger go up in flames around 23:00 when no one was around. How did they know?? The fire was on camera and DVR. I have a Heathkit IM4180 which used 10 AA nicads, converted to 4S Lithium. I have a home made 6 input audio mixer, also converted to 4S Lithium. I have a DMX512 lamp controller, converted to 4S Lithium. I personally will not risk a parallel fire hazard. The hospital where I worked a few years back had a devastating fire that started in an I.T. closet from a piece of test equipment that was lithium ion powered and left on the charger over the weekend. The fire was fed by all that PVC CAT5 wiring!! These are three lithium ion fires that I can bare witness to. Not a friend of a friend of a friend here say bullshit story. There is safer lithium technology out there. I do remember about 15 years or so back Segways were in fact known to burst into flames.