How to detect bad cell in parallel?

[libqw]

Hey All: My first post here, absolutely newbie. I watched some video on youtube showing how to detect bad cells connected in series. but what about in parallel? If I connect 20 18650 together in parallel and one of them gone bad.
1 What would be the syndrome? I figured if the battery is open, then it should be no harm I just lost one cell? But if it is shorted then the entire pack would be shorted too? I would get no voltage at all?
2 In above case is there a way to detect which cell is bad? Without fully dissembling the pack?

[PNPmacnab]

Just as in solar panels, if you have more than two there is a risk of a lot of current going to one if it shorts. That is a lot of cells in parallel and likely they are not new. That is a disaster waiting to happen. Tesla fuses cells with a small fusable wire as protection. Even a very small wire like #26 can carry tens of amps. A sensitive voltmeter could measure voltage drop under load and determine a weak cell. Eventually you will have problems and you better have ways to maintain these banks. The best thing I have seen are two cells wired in parallel with one cell stuck in a battery holder. Not a lot of thought goes into those power wall videos on youtube.

[Mike90250]

How about a IR thermometer to read the temps of each cell, shorted one will be warmer, or have flame shooting out. If it lets loose, the fumes are EXTREMELY TOXIC

If the shorted one pulled the whole pack below 2V it's likely other cells could be damaged too, and ready to turn into a roman candle

[PNW_Steve]

I have seen FLIR used to find bad cellls in Telecom UPS batteries.

[libqw]

Thanks for the reply, good idea to look at the temperature, I was so focused on using multimeter only.

[Sunking]

Lithium batteries rarely fail open. Most all the failures and fires are from shorted cells. They do not have to be in parallel to short and fail. The two major causes are over discharge and the real threat is from metallic particles flaking off between the ultra thin separators, and or contamination during manufacture process. Matters not how it happens, it is what happens that is real reason for concern. It sets up thermal runaway that cannot be stopped. Parallel cells only mitigate and compound the problem. When a cell shorts, all parallel cells feed the runaway and accelerate rapidly. You might be able to detect it with thermal sensors, but once detected is too late and time to bail out. It is almost impossible to extinguish and burns like thermite were throwing water on it only makes it angry and hotter.

[Sunking]

I have seen FLIR used to find bad cellls in Telecom UPS batteries.

That works for AGM batteries as they are known to go into Thermal Runaway. However it is very easy to stop Thermal Runaway in AGM batteries, simply turn off the charger and problem solved. That does not work with Lithium. Once started, you cannot stop it.

Fortunately in Telecom Plants using AGM Batteries, you stick a therm o-couple on a pilot cell deep in center mass. Temperature compensation is inverse, as temp rise, voltage goes down, which lowers the temp. If that does not work, the controls on the charger disconnects the battery from the charger to stop runaway. When Lithium goes into thermal runaway is like Thermite and once lit, there is no putting it out.

[nebster]

You can use a thermal camera to identify a cell that is on its way to having a problem, before it has the big problem (open, closed, or thermal runaway). This is what the engineered LFP pack providers do as part of QC/validation, and it is also what some of us DIY builders do. Many would consider it a best practice when building up a pack, especially now that the cameras are becoming less expensive.

Thermal analysis also lets you see if you have any weak interconnects, which is a more likely commissioning issue in a DIY build with a large number of cells. Under load, such a tool lets you literally see if a nut is undertorqued or if a string is imbalanced relative to another, because the wiring temperature will vary.

The risk of a cell failing closed-circuit is precisely why it is not appropriate to dictate "parallel-first, then series" as the One True Design. Such a topology only offers equivalent overcurrent protection if the builder is willing to fuse in between every paralleled cell, like Tesla does on their modules. But doing with large format prismatics is not only expensive, it also takes up valuable space and may greatly complicate the wiring geometry. Moreover, the fuses themselves contribute substantially to circuit resistance, driving up ohmic losses, decreasing pack efficiency, and increasing local heating in an area where that may be undesirable.

[Ampster]

............
Thermal analysis also lets you see if you have any weak interconnects, which is a more likely commissioning issue in a DIY build with a large number of cells. Under load, such a tool lets you literally see if a nut is undertorqued or if a string is imbalanced relative another, because the wiring temperature will vary.
............

Great point. I remember years ago on my VW conversion I was pulling into my driveway which has a slope and I notice a popping sound behind me where the pack was located. When parked the car I notice one of the bolts connecting the series string of cells was loose. I felt it and it was hot. It had been arcing and that was the sound I heard. I had to file the top of the cell, the bus bar and throw the bolt away because the arcing had deposited bits of molten metal and the contact points were not smooth. I just found some heat vision programs for my Android.

[Guest]

I think, when you have cells in parallel and one of them fail short, in this case you will see that whole connection being at 0v. In this case the battery management will shut the whole pack and prevent charging. Also li iron seldom fail short and never explodes.

Moderator Note:
Li Ion (not iron) batteries nearly always fail shorted, and if there is much charge in them, they self-destruct in a hot thermal reaction, that many people describe as Fire or Explosion.
Do not attempt to mis-inform naive uses of this site again or you will be banned.