Author Topic: Does BMS board used with series string of 18650 cells accelerate self-discharge?  (Read 2123 times)

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Offline wb0gazTopic starter

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Question - what should be expected in terms of self-discharge when a series string of LiIon cells is wired to a balancing BMS but not otherwise in use?

Details:

I built a 6S1P battery out of 18650 cells (all new) and a balancing BMS board labeled HXYP-6S-CW15. The battery (starting out fully charged) was left in indoor conditions since last summer (so maybe 8-9 months) with exactly zero use.

I removed the cells and installed them in a Miboxer C8 charger (handles each cell individually), intending to inspect the state of charge and allow the charger to bring the six cells to full charge before replacing in the holders attached to the balancing BMS.

I expected at the outset that the cells would be somewhat discharged due to self-discharge, and expected the state of charge to be similar among the six cells.

What I encountered was that  the six cells were widely differing in state of charge, several below 2V, only one above 3V.

The Miboxer C8 charger went into a recovery mode where a small current is supplied to the cell as long as the cell appears to be making some progress in charging, then switches to a normal charge regime when the cell exceeds 2V.

The cells were purchased as new (bulk packed); I do not believe they had prior use or abuse.

Is this normal/expected behavior when a series string battery is "wrapped" in a BMS?


 

Offline Siwastaja

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Excessive off-state/quiescent current is a very usual design failure mode in crap BMS designs.

You get your answer by reading the BMS datasheet / manual. Quiescent current should be listed there. If the information does not exist, then you can be pretty sure the product is total crap.

Often, the self-discharge of cell is so small that quiescent current of the BMS dominates, unless the BMS is specifically well designed for low leakage.
 

Offline jbb

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I haven’t used or measured them, but there seems to be a class of BMS with all sorts of bells and whistles like blinky lights, Bluetooth or WiFi. That’s a recipe for high quiescent current and it’s not clear if the manufacturers are carefully hoarding their microamps or saying ‘yeah whatever, we gotta spin the boards by Friday.’

Of course, I also see the value of the configuration-by-phone and nice diagnostics. Ultimately, I wish the manufacturers would publish the quiescent draw.
 

Offline iMo

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I have several various 18650 packs I built for a flexible powering of my experiments (usually small currents like 100mA max) and I have a pack for my radio (XX Amps). I do not use balancing, I do not use special chargers, I've been charging them off a lab voltage power supply via a limiting resistor (max 1A at the beginning, to 3.9V per cell). No issues so far..
Any BMS discharges the cells, that is based on the fundamental laws of physics.. :)
« Last Edit: April 07, 2024, 07:48:12 am by iMo »
 

Offline Siwastaja

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Any BMS discharges the cells, that is based on the fundamental laws of physics.. :)

No "fundamental" law of physic dictates the existence of leakage current - realities about available materials do. While zero leakage is practically impossible, it is entirely possible, without going to space grade costs, to design the BMS functionality (cell-level voltage measurement and possibly balancing) such that it consumes tens of nanoamperes from the cell taps, and this will be much less than the leakage with the cell itself, and as such, can be ignored.

The real question just is, which BMS's actually are engineered to very low leakages and which are not? - if there isn't even specification available, it doesn't start well.
 

Online bdunham7

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Unless you are willing to let your newly charged cells sit for another 8-9 months disconnected from the BMS, then you won't know whether the discharge was due to the BMS, the cells or both.  One thing to note, the cells were not likely in "widely differing states of charge".  It is more aptly described as their state of discharge being from 99% to 101% of capacity.  What make/model of cells were they?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Siwastaja

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If balancing is not used at all, then it is quite important to initially balance the cells. Large scale manufacturers can possibly trust whoever is supplying them cells that they are initially balanced, but at least checking is a great idea. If you are a hobbyist or small producer of products, just go through initial balancing. Simple way to do that is to connect all cells parallel first (before this, make sure voltage differences are within tens of millivolts to avoid large currents between the cells), then fully charge to the final voltage.

The job of in-application balancing is to keep available pack capacity maximized as cells age. The safety functions are provided by cell-level low-voltage and high-voltage checks which are orthogonal to balancing; balancing is optional, cell monitoring "mandatory" part of a BMS.

If even cell-level voltage monitoring is not used at all (i.e., no BMS), then it is hugely important to
1) initially balance the cells,
2) make sure cells have similar capacity (i.e. tight manufacturing tolerances)
3) make sure cells are thermally coupled i.e. stressed similarly during the years of use
4) make sure the overall manufacturing quality of the cells is top-notch, i.e., small and similar self-discharge, similar aging (capacity fade, ESR increase) characteristics.
5) quite obviously, connect nothing external to the middle taps

Such no-bms approach in series stacks, even 6s, has been successfully used by companies like Bosch, even though this is hard to believe to many armchair internet experts. That being said, I don't think anyone does this anymore: design or try to find a proper BMS. I have mixed feelings between recommending a no-bms or crap-bms solution - neither is really a good idea, but indeed, a well thought out no-bms solution is certainly better than worst case crap BMS.
« Last Edit: April 07, 2024, 06:18:34 pm by Siwastaja »
 

Offline wb0gazTopic starter

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Thanks to everyone that took time to respond to my question - - very helpful indeed!

I plan to discard the BMS module (and it's spares) as I could not obtain any useful spec information, and restart the project (with fresh cells) with a BMS module (without bells/whistles) subject to getting clear spec data.

In the meantime, two of the six cells involved in the problem ended up taking a full (?) charge, then immediately ended up showing 0V at terminals, so I will pursue how to safely dispose of all six cells involved.
 

Online ConKbot

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Ive had bare 18650s I pulled from packs, and never did anything with sit around for 5 plus years and still be at 3.4V. With a cell like that, yes, self discharge will be increased significantly by including a BMS. But if your application doesn't include sitting for 5 years doing nothing, then you're probably fine with a BMS that will discharge the pack entirely over the period of a year.
And if your application doesn't involve sitting for 5 years doing nothing, you should probably be looking at primary cells, and designing your circuit yourself for ultra low standby draw.
 


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