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Does BMS board used with series string of 18650 cells accelerate self-discharge?

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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?


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?

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.

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.

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.. :)


--- Quote from: iMo on April 07, 2024, 07:46:06 am ---Any BMS discharges the cells, that is based on the fundamental laws of physics.. :)

--- End quote ---

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.


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