Leaky 18650 pack - Page 1
Forum section: electronics / general

author: najrao
author_rank: Regular Contributor
timestamp: June 20, 2019, 07:06:17 am
content: I would be grateful if someone helps me solve this problem:Bank is 3S 12P, in a straight rectangular 3×12 formation. Nickel strip welded connections, BMS without equalizer for charging only (<10A), to be loaded to 20A directly. Problem is ONE of three 12P groups LEAKS slowly, completely discharging the >20Ah in some 2000h. The other two groups hold charge (dropping <50mV) over the same period. I want to identify the one culprit cell and use the rest, possibly cutting out two good cells from the other groups for Ah balance.How do I identify the bad cell? It is very difficult to undo the welded connections just to check randomly. There are no visible clues.

author: sleemanj
author_rank: Super Contributor
timestamp: June 20, 2019, 07:12:37 am
content: Split the 12 in to two 6's, charge both, see which one leaks, split the 6 that leaks into two 3's, charge both, see which one leaks, split the 3 into 3 singles, charge and see which one leaks, job done.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 20, 2019, 08:33:00 am
content: Can you make a fault like this show up with thermal imaging?

author: Gyro
author_rank: Super Contributor
timestamp: June 20, 2019, 08:57:20 am
content: Youmightbe able to do it by measuring the voltage drop across the Nickel strips with a sensitive millivoltmeter. Nickel strip is fairly resistive in the grand scheme of things to you ought to be able to sense the voltage drop across it between the leaky cell and the adjacent good ones.

author: najrao
author_rank: Regular Contributor
timestamp: June 20, 2019, 11:11:53 am
content: Jeroen, I am not sure how you expect to do that, the leakagepoweris only at mW level and will not show up as heat.Gyro, thank you. Just what I had in mind too. A 20mm long nickel strip 0.15 thick and 6 wide would have a resistance of may be 2mR, and 10mA through it would drop 20uV. Not very far from measurement contact potentials. I have an Advantest picoammeter, so let me see. Please wait a couple of days.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 20, 2019, 04:48:33 pm
content: You can probably get it to show up on thermal imaging if the leakage contains even a trace amount of moisture, which it should. Evaporation will cool the wet areas enough for them to show up vividly.

author: Gyro
author_rank: Super Contributor
timestamp: June 20, 2019, 06:01:36 pm
content: He's talking about electrical leakage, not physical leakage.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 20, 2019, 06:14:01 pm
content: I’m not sure how. The cell is defective. Maybe there is a way to stress it. Either by high charge or discharge. Or maybe by applying AC current on them.

author: najrao
author_rank: Regular Contributor
timestamp: June 21, 2019, 03:44:52 am
content: I am sorry I did not consider that 'leakage' could be construed as of the electrolyte.I have now recharged the leaky cell group to 4.15V (on charge), and will monitor the self leakage for a week or so. I am not too optimistic of the leakage current showing up a drop across the intercell connection, but will still give it a go. I will report findings. Thanks all.I am back sooner. One of the cells in the leaking group showed some evidence of corrosion at the positive terminal, so suspect venting. I checked the intercell mV drop, which was surprisingly high at just this cell -- over 300uV! In the same group, this mV drops off progressively, indicating that the cells far removed from the leaky one share the burden to a lesser extent. The intercell drop is much higher than my prediction of 20uV, perhaps because the current leak is highest after a full charge, and would reduce progressively.All other intercell connections show < 25uV drop on my 80000 count DMM.I have eliminated this cell, and balanced out by adding a spare cell from another group (bodged).I have learnt a good way of checking bad cells in a parallel group, thanks to eevblog team. Thanks all.

author: Gyro
author_rank: Super Contributor
timestamp: June 21, 2019, 06:15:48 pm
content: It looks like you may have suffered both kinds then.It's a bit worrying (for the safety of the pack) that one of the cells may have vented though.

author: amyk
author_rank: Super Contributor
timestamp: June 23, 2019, 02:16:05 am
content: Quote from: najrao on June 21, 2019, 03:44:52 amFYI that is exactly what almost everyone will think when they see the term "leak" applied to a battery. The term you're looking for here is "high self-discharge".

author: james_s
author_rank: Super Contributor
timestamp: June 23, 2019, 03:50:11 am
content: My experience with battery packs is that most of the time when one cell is bad, the rest are not too far behind. Trying to replace just one cell is usually a fool's errand.

author: najrao
author_rank: Regular Contributor
timestamp: June 23, 2019, 06:56:15 am
content: Quote from: amyk on June 23, 2019, 02:16:05 amYou are absolutely right sir. My mother is said to have dropped me repeatedly on the head as a baby.Quote from: james_s on June 23, 2019, 03:50:11 amI agree, but just cannot get a replacement right away. The bodged cell group still self-discharges more than the other two, though far better than before. I am working on a simple "equalize on float" circuit to supply the self-leakage current (<20mA for 12 cells in parallel) and clamp the cell voltages at 3.95-4.05 V. That should make the dying battery live a little longer.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 23, 2019, 03:54:01 pm
content: Quote from: najrao on June 23, 2019, 06:56:15 amAssuming the cells are of good quality and you just saw a freak incident, then if the rest of the cells have not been taken severely below the low-voltage cutoff voltage, then there is no reason why others would have suffered from one cell self-discharging quickly.If they are some random Alibaba specials you have no idea about, then assuming they are all bad is a good assumption, even without such incident.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 23, 2019, 08:22:36 pm
content: Yes, thank you amyk! Leakage and self-discharge are exclusive terms, though not always mutually exclusive as we have seen here.

author: najrao
author_rank: Regular Contributor
timestamp: June 24, 2019, 05:12:04 am
content: Quote from: Siwastaja on June 23, 2019, 03:54:01 pmI can hardly be sure of the battery's history. The 'leaking' cell group HAD discharged down to 1.95V, but I perked it up by charging at < 500mA for 12 cells till the voltage rose to 3.3 in over 3h. Indeed, I have done this before many a time: really dead cells simply fail to go beyong 2.5V or so.I want to learn how dropping below 2.5V kills a cell. After all, during manufacture it started at zero volt, andsomeprocess set it up to get to the initial 2.0+V. Charging very slowly first seems to be that process. Any cell which is not dead can be revived, and slow charging eliminates the kaput ones very well. The reviving process itself does not seem to pose a serious fire risk. I may be entirely wrong, and welcome correction.Would 'revived' cells still take on the full Ah charge and hold it? In my experience, yes and yes. It would thus seem that throwing awayallcells which are below 2.5V is doing that the proverbial baby with the bath water.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 24, 2019, 07:09:06 pm
content: The problem has to do with anode depletion. From what I've read, at very low voltages the anode will start to dissolve into the electrolyte. This can cause a sort of bridge to start to form between + and - which will short the cell eventually if it's allowed to grow too far. The same can occur in the other direction if the cell is over charged. The problem isn't being over-dischsrged or over-charged, but being so for a significant length of time. The longer the cell is at one or the other extreme, the more damage is done. When the cells are manufactured, they are immediately charged, so they spend very little time at low voltage.Next problem is even if there is no short, the depletion of the anode will eventually start to cause reduced capacity of the cell. If you have one group of cells with high self-discharge that you took out of a group, chances are the other cells will have been damaged somewhat as well. Whether this is something you want to worry about is an individual matter. Personally I would test each cell individually by discharging and charging and checking capacity, internal resistance and self-discharge. If they are all fairly close then you're good to go as far as I'm concerned.