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Why do 2 batteries in series 'tend' to be stable?
#11
(07-29-2020, 04:04 PM)ajw22 Wrote: Every cell self discharges to some degree. Going by my basic understanding of physics, pretty much everything leaks more the higher the pressure(voltage) is.  So a high cell at 4.00V will likely lose charge at a _slightly_ higher rate than a very similar cell at 3.99V.  Thus given enough time, and using very similar cells, they will eventually balance out themselves.

Of course, things will quickly change once cycling starts and uneven temperatures and resistances start affecting the cells. Or a cell develops a fault and starts to SD at much higher rates.
> I understand a 14s BMS is needed here...  that's NOT my topic.
Still doesn't hurt to remind other readers that it's not safe to let LiIons charge without a BMS


Lead acids are similar, yet for different reasons.  Simply put, occasional short term limited overcharging does not hurt a lead acid cell much, so little is done to monitor/avoid it. In fact, chargers may occasionally perform a controlled overcharging ("equalization charge") to ensure all cells are fully charged.
Any excess charge just ends up "boiling" (electrolysis) the water into hydrogen and oxygen, which then is just expelled (non-sealed; needs refilling) or internally recombined (sealed lead acid battery, aka SLA).
Similar to the 2 x 7s2p batteries staying 'in sync', let me expand / try to explain more...
I'm trying to understand the 'science concept' that allows all these 84 packs to stay 'in balance' with no balancing - day after day.   The picture below is 6 x 14s - but still that's 6 groups of 14 in series but yet they stay balanced on their own.  The following have not been balanced at all for months.   


Its just amazing to me that all these cells would have a chemical process that acts so 'uniformly'.   What is the concept behind the uniformity - e.g. is it the chemical process is 'so exact' regardless of manufacturer or what.
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#12
The "science concept" is simply that the self-discharge rate is very low for healthy Li-ion cells. Since your standby pack gets very little cycling it will only be differing self-discharge rates that can lead to voltage/SOC imbalance. This is a low percentage (difference) of an already very low rate, so it amounts to tiny differences.

For a specific example, as a smaller analog of your case, I have a 4S4P powerbank that I rarely use, so I store it around 50% SOC. Further, I trick the BMS into thinking it is in UVP so it is in deep sleep with minimal quiescent BMS power draw.  It's been idle, self-discharging for over 4 years and 1 month, and it only lost about 6% capacity, and the 4P packs remain very closely balanced after those 4+ years, e.g. two packs started out at a 0.74mV difference (3.82228 vs. 3.82154V) and ended up at 0.18mV difference (3.80869 vs. 3.80851V). This might seem amazing when you first meet Li-ions if you were only previously familiar with higher self-discharge chemistries like (non-LSD) NiMH, but it is par for the course for Li-ion.

As to why your other pack is behaving oddly that's hard to say from what little info you provided. It could be due to some unhealthy cells with higher self-discharge (or small internal shorts), or non-uniform-heating (the self-discharge rate depends strongly on temp so even small temp differences can lead to big self-discharge differences, esp. accumulated over long timeframes).
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#13
(07-29-2020, 04:04 PM)ajw22 Wrote: Every cell self discharges to some degree. Going by my basic understanding of physics, pretty much everything leaks more the higher the pressure(voltage) is.  So a high cell at 4.00V will likely lose charge at a _slightly_ higher rate than a very similar cell at 3.99V.  Thus given enough time, and using very similar cells, they will eventually balance out themselves [...]

That's well worth emphasis, but likely it doesn't apply to the OP, since Li-ion self-discharge rate is nearly constant (vs. SOC) except at very high SOC (e.g. see the graphs below, where the self-discharge rate is the slope dQ/dt of the curves). The OP's packs are floating around 60% SOC where the self-discharge rate is essentially constant, so it is unlikely that would cause any nontrivial self-balancing.

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#14
@gauss163 - thank you for more detail.  OK, I get the 'very little SD for healthy packs' and 'in shallow part of charge/discharge curve' and therefore the batteries in series tend to be more stable.

In reference to the 84 packs in the powerwall - they are all matched in terms of 260ah @ 3.7v noninal - e.g. they all have very similar ah (+/- 5%) and IR even though they are a mix of cells.  They are charging up to 75% SOC and down to 25% SOC (roughly) each day ...  but yet its pretty amazing to me that they stay almost exactly within minor voltage difference (0.05v) for many days...  months even.  Of course, that's what makes all this work - but part of me expects them to vary much more  than they do...   Batteries can be frustrating but Lithium-ion 18650 seem like an amazing piece of engineering Smile
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#15
It's all about the quality of the cells and the manufacturing process. I believe you purchased cells that are all similar. If you did what hbpowerwall did and purchased laptop packs at all stages of their lives you will find that the quality of his powerwall isn't as optimal as yours. When you have a sanyo along a panasonic and mixed with samsungs and lgs, you will find that their manufacturing processes will be slightly different and causing some kinds of imbalance. Then you also have them at different capacity ratings vs the expected ratings. All those would lead to an imbalance that over time will differ from pack to pack. My packs for example are only made out of two different batteries, a panasonic 2400mah and samsung 2400mah. I've never had to balance my packs much and I stay within the 80% DoD and never drain or overcharge them.

Think about tesla batteries in general, their packs are built to last hundreds of thousands of miles, and even Elon is claiming a million mile now. Their balance leads are tiny yet they're able to stay balanced. It's all about the quality of the manufacturing process to make them as identical as possible.
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#16
So if the cells are well matched pack to pack in type, chemistry & capacity, agree that would naturally reduce any tendency to drift out of balance.

Perhaps guass163 would know this one: is there a difference, eg in charge efficiency at the top end of charge, say between 4.0 & 4.2V, that would help this "stay-in-balance" effect.
Say there's two identical packs in series, one pack was 4.05V & one 4.10V. Would the one at 4.10V tend to be "wasting" a bit more of the input charge, eg say ~95% efficient & the 4.05V pack absorbing the same charge current "better" at say ~98% efficiency.
Running off solar, DIY & electronics fan :-)
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#17
(07-29-2020, 08:23 PM)OffGridInTheCity Wrote: They are charging up to 75% SOC and down to 25% SOC (roughly) each day ...  but yet its pretty amazing to me that they stay almost exactly within minor voltage difference (0.05v) for many days...  months even [...]

That voltage difference 40mV(3.63-3.59) in post #11 is not "minor" - it corresponds to about a 7.3% capacity difference (cf. graph below). This is 10.8 times larger than the 6mV difference (= 0.67% at 3.92V) in post #1. But a 7.3% capacity deviation is not so amazing. This deviation will likely increase given the 50% DOD daily cycling. Note: I used the rightmost green curve (60 min rest) below to estimate SOC from resting voltage. I'm assuming these are not (mostly) LiCo cells, since then 3.63V would be closer to 3%, so far from 25-75%, cf. Sanyo here.

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#18
>That voltage difference 40mV(3.63-3.59) in post #11 is not "minor" - it corresponds to about a 7.3% capacity difference (cf. graph below)
To clarify - the 84 packs were mid-balanced to 0.04v max difference to start with.  They didn't start out with 0.0v max difference and then drift to 0.04v.    And note that from previous discussion, the Batrium longmons are mis-measuring the 'actual' voltages a bit and are not calibrated with each other - so there's a bit if fuzziness in the system. 

During the daily cycle they charge up to 3.9v (and drift to 0.05v max difference) and then discharge down to 3.54v with a 0.03v max difference.  This pattern repeats day after day, several months with no balance.    There are variations by 0.01v sometimes but if they occur it's usually due to some unusual circumstance such as a 103F day where the average consumption goes up to 5500w/hour for the day instead of the typical 85F day and 3000w/hour - e.g. higher/more-demanding current draw on the battery bank.  Once the circumstance passes - things return to the normal pattern.   
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#19
^^^ Then that is a bit more surprising. Are you 100% sure no balancing is being done? I'm not familiar with Batrium. What does the "bypass" stuff mean in the image in post #11 (and the white squares and triangles).
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#20
(07-30-2020, 01:18 AM)gauss163 Wrote: ^^^ Then that is a bit more surprising. Are you 100% sure no balancing is being done? I'm not familiar with Batrium. What does the "bypass" stuff mean in the image in post #11 (and the white squares and triangles).
Yes - absolutely sure. I have it affirmatively disabled.   

The white triangles in the blue bars are the temperature measurement - C on the right hand side.  They show 25C +/-.   

The top bar will show red vertical bars of bypass amps (left hand metric) and the white triangles are the longmon / bypass temp (right hand metric).  Notice no red bars and right side shows 25C +/- a bit.  

Here's a snap from a youtuber - showing the red bars when bypass is occurring...
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