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Balancing cells within a pack?
#1
Hello all, new to the battery scene here. My office discarded around 100 laptop batteries this year, so I salvaged all of them and will have around 600 cells to sort through. I've done small electronics most of my life, but I'm not an engineer. Some of this stuff is sparking my brain pretty hard.

I have a small home voiceover studio and currently run everything on battery power. Gets a bit inefficient over time, not to mention not very eco-friendly with all the AA's I go through. Recently set up a bunch of 12v lighting for streaming recording sessions, so overall I'm looking at a 12v environment. A bunch of my equipment is 5v, so I'm using buck converters to step down from 12v->5v.

Based on things I'm seeing here in the forum, seems like I'd be best off to build a 7s 24v pack and use a stepdown to provide 12v to my devices. I'm fine with that.

Let's assume I get a 60% pass rate from my cell hoard. I'll build 4x 7s14p packs, and rotate them as needed (20Ah each will be...laughably overkill for my needs).

My question is this: how do I spec BMS boards for these packs? Somehow I seem to be missing what must be a very obvious answer, in all of my research.

The laptop battery packs balance each cell individually. Everything I'm seeing online shows people balancing entire packs against each other. Assuming that I build packs with like-capacity cells (+/- 50mAh), should I use a BMS that balances each parallel group (as I understand it, this would be an 8-lead BMS)?

Many thanks for any advice you can provide!
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#2
First of all count on 35% Good cells. Thats a more realistic Number.

You need a 7s bms that can cope with the current. Most likely a 20-50A Will be ok. You can Charge it with standard PSU
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#3
For buck conversion I highly recommend using the boards I mentioned here, which are over 95% efficient, and can also handle QC/PD output up to 100W, and are no bigger thnm a 9V batttery (and cheap at around $10).  This is recent tech and blows away most of the ancient converters on eBay etc
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#4
Thanks gents.

@daromer, how should balancing work on a pack with this many cells? Do I balance each series against each other (needing 14 balance wires)?

@gauss163, thanks very much for that. Bookmarked for purchasing.
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#5
^^^ As far as the balancer is concerned, each big nP pack of n cells in parallel is no different than a single "big cell" of much larger
capacity.  It can't get more granular because in general you can't analyze the individual paralleled cells without disconnecting them from the nP pack.

However, this analogy only goes so far if you mix cells in the nP pack, i.e. if your nP pack is a motley crew of cells with different capacity, chemistry, impedance (resistance), health, etc (which is never done by professional pack builders).  Then the parallel pack is a Frankenstein "big cell" whose behavior is difficult if not impossible to predict - since some cells will likely degrade much faster than others,  leading to chaos.  

However, such degradation can likely be delayed to some extent by ensuring that the pack is exposed only to low current and low heat (uniformly, i.e. don't let some cells (or packs) get much hotter than others since then they will degrade much faster than the cooler cells). Such thermal-imbalanced degradation is well-known to anyone who harvested laptop packs back in the Pentium days when laptops ran very hot and the cells closest to hottest components aged much faster (cf. image below) - so you'd get packs with a characteristic aging signature of a couple really bad neighboring cells (closer to heat) and the rest really good.

For nice introductions to cell balancing in battery packs see the links that I posted here.
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#6
Here is an example of hotter groups of cells in a laptop pack (as I mentioned in the prior post).  If you have some idea about the thermal profile of the pack's device then this may be of heuristic use when initially grading harvested cells, i.e. those cells near hotter components will likely be much more degraded than those that ran cooler.

[Image: dgqsQ.jpg]

Image excerpted from Battery Monitoring Basics - TI training, by Texas Instruments.
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#7
(07-10-2020, 10:07 PM)gauss163 Wrote: ^^^ As far as the balancer is concerned, each big nP pack of n cells in parallel is no different than a single "big cell" of much larger
capacity.  It can't get more granular because in general you can't analyze the individual paralleled cells without disconnecting them from the nP pack.

However, this analogy only goes so far if you mix cells in the nP pack, i.e. if your nP pack is a motley crew of cells with different capacity, chemistry, impedance (resistance), health, etc (which is never done by professional pack builders).  Then the parallel pack is a Frankenstein "big cell" whose behavior is difficult if not impossible to predict - since some cells will likely degrade much faster than others,  leading to chaos.  

However, such degradation can likely be delayed to some extent by ensuring that the pack is exposed only to low current and low heat (uniformly, i.e. don't let some cells (or packs) get much hotter than others since then they will degrade much faster than the cooler cells). Such thermal-imbalanced degradation is well-known to anyone who harvested laptop packs back in the Pentium days when laptops ran very hot and the cells closest to hottest components aged much faster (cf. image below) - so you'd get packs with a characteristic aging signature of a couple really bad neighboring cells (closer to heat) and the rest really good.

For nice introductions to cell balancing in battery packs see the links that I posted here.

Im still cooking my Franky...but I wont hit them hard... until I manage to extende the nP so the discharge stay low in daily basis....
Thanks!
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