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Battery Management System IC

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moottii

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Hi!
I want to use an IC for Battery Management System like bq76PL536(Texas Instrument product). I have to control 4s8p (4 series*8 parallel) cells.. It's something like this:
1.jpg
I'm aware that these kinds of ICs are designed for series cells. But my question: is it practical and possible to control x*8 cells with one bq76PL536 or not? I mean is paralleling cells practical and if no, what should be done?
 

Vbase

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Yes, you can use bq76PL536 for controlling groups of paralleled cells. It is a common practice.
Don't forget one of the rules of reliability is the less components the more reliability.
 

moottii

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Since resistance and capacity of each cell is different, I thought parallel cells will overcharge/undercharge during charge/discharge of battery. Could you explain how paralleling doesn't damage cells?
Thanks
 

Vbase

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If one cell is damaged then the other cells in parallel voltage differ to the cells in series and the charge controller detects that.
If you use well interconnected cells that come from the same production batch then they match well enough to last. If you use cells that are very different from each other your battery of cells may never work.
In a laptop or electric car the cells are almost identical, if you change one of the cells it might bring down the whole battery.
 

SunnySkyguy

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When combining cells, the degree of mismatched ESR and SoC affects reliability. For serial, voltage sensing on charger per cell is needed, which is why they have 3 pin on multi cell batt's. Serial, doubles ESR and ESR rises rapidly when any cell approaches 10% SoC so reverse protection is often used or better, cell equalization FETs.

For parallel same thing. If not matched by factory, parallel cells can age faster by current hogging and if not matched Voc, results in self discharge until equal, which can result in excessive current as ESR is almost a short circuit. In this case if cells are of unknown age and capacity, then series equalization Rs is added to match ESR of all batteries, which may be in range of 100 mOhm and now adds more loss but more reliable.

In series the weakest cell drains fastest. In parallel, strongest battery drains faster until equal, but mismatch weakens initially faster and Rs if added is more lossy. So factories may make arrays in Series parallel like LEDs 2S2P for more capacity and resilient voltage to match load.

Rule of thumb is cell ESR must be 10% of motor coil ESR for fast start and low dip.

But in cars it is often 33% so starter current in coldest winter allows 4V drop from 12V when battery ESR is highest.(cold) thus the infamous CCA lead acid test at 0deg C and7.2V

- - - Updated - - -

Typically factory matched cells have ESR and capacity of 1% worst case, only when new. Aging increases the degree of mismatch as ESR rises with aging and not all at the same rate.

Something like 10% mismatch if in series will fail quickly, but in parallel will not, but then Ah capacity is reduced a lot.
 

moottii

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Thank you guys
So if I'm not wrong, all the cells in one pack must be similar (max 1% mismatch) for desired functionality of the pack. Series cells are controlled continuously with analog to digital converters (inside the BMS IC).
Since BMS IC sees all the paralleled cells as one single cell, we can not control each parallel cell separately. We can only choose them to be similar.
Am I right? :)
 

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I would agree except a fuse may be needed per parallel cell inside in case of short such as LiPo batteries in case of impact.

the fuse also contributes to ESR and narrow mismatch tolerance limits. This also widens with aging and shows how industry tends towards Serial Parallel arrays of tiny standard cell which can be made in tolerances in <<1%

... exactly same reason with LED arrays in one substrate.
 

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