Revive or replace dead 18650 cells ? - Page 1
Forum section: electronics / beginners

author: jeromebg
author_rank: Newbie
timestamp: April 11, 2025, 11:43:04 am
content: 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 !

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 11, 2025, 12:00:13 pm
content: 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.

author: CaptDon
author_rank: Super Contributor
timestamp: April 11, 2025, 12:21:07 pm
content: 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.

author: wraper
author_rank: Unknown
timestamp: April 11, 2025, 12:26:19 pm
content: Never charge overdischarged Li-ion cells. Even if you succeed, they'll be a fire hazard.

author: Bud
author_rank: Super Contributor
timestamp: April 11, 2025, 12:44:45 pm
content: 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.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 11, 2025, 01:23:31 pm
content: Hello jeromebg,Replace all of the deadLGEBMG11865 cells(*) with equivalentquality partsfrom 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

author: tunk
author_rank: Super Contributor
timestamp: April 11, 2025, 01:40:02 pm
content: 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. samemake and model, and similar capacity, internal resistanceand rate of self discharge.

author: jeromebg
author_rank: Newbie
timestamp: April 11, 2025, 01:45:24 pm
content: 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...

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 11, 2025, 01:51:11 pm
content: Quote from: jeromebg on April 11, 2025, 01:45:24 pmSeems not thatcheap. Unfortunately.

author: tunk
author_rank: Super Contributor
timestamp: April 11, 2025, 02:14:53 pm
content: 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 puta 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 cellreaches 0.8V you could increase the PSU voltage to 1.8V.And so on until you reach 3.0V when you can start normalcharge.

author: wraper
author_rank: Unknown
timestamp: April 11, 2025, 02:23:14 pm
content: Quote from: tunk on April 11, 2025, 02:14:53 pmExcept your home and your life. They will be a permanent fire risk that can happen at any time.QuoteSimply

author: CaptDon
author_rank: Super Contributor
timestamp: April 11, 2025, 05:27:07 pm
content: 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.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 11, 2025, 06:23:26 pm
content: So many fires started by lithium cells and batteries! Err on the side of caution!

author: thm_w
author_rank: Super Contributor
timestamp: April 11, 2025, 10:14:45 pm
content: Quote from: timeandfrequency on April 11, 2025, 01:23:31 pmSome sort of typo, it says 0.5C standard charge above that, maximum 1C.

author: amyk
author_rank: Super Contributor
timestamp: April 11, 2025, 10:53:12 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2025, 08:31:17 am
content: That's NiCds, hopefully nobody would ever try to flash a Li-ion.

author: Psi
author_rank: Super Contributor
timestamp: April 12, 2025, 08:37:11 am
content: Quote from: CaptDon on April 11, 2025, 12:21:07 pmhehe, 126V, that's got some serious energy.

author: CaptDon
author_rank: Super Contributor
timestamp: April 12, 2025, 01:05:56 pm
content: Gyro, I thought the same thing reading that post. Usually when a lithium ion develops a short the result isn't pretty!!

author: Peabody
author_rank: Super Contributor
timestamp: April 12, 2025, 01:49:44 pm
content: So is the Segway pack 2P15S? Looks like it. If parallel cells are so dangerous, I wonder why Segway would use that configuration.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2025, 02:46:32 pm
content: Quote from: Peabody on April 12, 2025, 01:49:44 pmTesla 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.

author: CaptDon
author_rank: Super Contributor
timestamp: April 12, 2025, 09:52:19 pm
content: 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.