Why do 2 batteries in series 'tend' to be stable?

Hi Folks

I have 2 separate healthy (no SD, balanced) 7s7p 18650 batteries in series inmy APC 3000. This mimics the original 2 seperate 1s2p 12v lead acids.
Here's a pic so you can see what I'm talking about - each one minics the standard 24v APC RBC7 lead acid battery that would normally go in an APC with about same ah.




This setup has been running for over 2 years and yet the separate batteries (which are healthy)do not 'get out of sync' with each other. Note that these7s7p batteries mostly just sit all day at the 27.xV shownare only used a few secs/day when the APC smooths the ATS switch to inverter and back to grid each day. In other words, these batteries are not discharged/charged significantly - mostly just sitting at a steady SOC.

I understand a 14s BMS is needed here... that's NOT my topic.

My question: What is it about healthy batteries (18650 7s2por 12v lead acid) in series that lets them stay at a reasonable voltage day after day with respect to each other. Is it just luck or is there something about battery chemistry where 'healthy' batteries (that are not charged/discharged much) just tend to reach an equalibriam?
We see this all the time with lead acid - e.g. 4 x 12v in series = 48v but they are never balanced or worried about until 1 starts to fail.

Again, this is not about BMS, but just wondering why healthy, rechargeablebatteries in series tend to be stable

------------------
P.S. This is the APC where I recently blew up 2 x Charger 16Ts... so BMS efforts have cost me $300 for nothing so far. I'm considering my next move, but meanwhile I've gone back the original pre-Chargery situation with BattGo's as cheap/inadequate BMS.

It would help to know a bit more:

1) How were the packs constructed? Do they use the same model cells at same SOH (e.g. all new or all harvested from the same packs?) Was there any matching done by AC/DC IR and capacity during construction?

2) How are the packs maintained? Are they being float-charged at some voltage, or only recharged when they self-discharge some amount?

3) What is the ambient temperature? Is temp distribution fairly uniform throughout the pack?

Generally, packs constructed with the same model cells and in the same SOH will remain well balanced if there is not much cycling (as in your case), and temps are uniform. Then any unbalancing will be due mainly to differing self-discharge rates (and - to a smaller extent - differing degradation processes) and any such differences should be small for same model cells (esp. if from the same batch, e.g. if they were harvested from the same packs).

>How were the packs constructed? Do they use the same model cells at same SOH (e.g. all new or all harvested from the same packs?) Was there any matching done by AC/DC IR and capacity during construction?
They are used Sanyo, REDUR18650E(s) -https://secondlifestorage.com/showthread.php?tid=1718
All 90% original spec during OPUS tests / similar capacity within 100mah- IR is 'normal' (don't remember as it was over 2 yrs ago these were harvested).

[size=small]>How are the packs maintained? Are they being float-charged at some voltage, or only recharged when they self-discharge some amount?[/size]
[size=small]They are in an A[/size]PC and 'floated' to the voltages shown above. TheAPC protects against the twice daily10-20ms power switchovers between grid and inverter with a 1000w load and APC self-tests every 2 weeks - e.g. minor discharge/charge cycles.

[size=small][size=small]>What is the ambient temperature? Is temp distribution fairly uniform throughout the pack?[/size][/size]
[size=small][size=small]About 15F above ambient (inside the APC unit) e.g.70F (winter) to 90F (summer) thru the year. Yes, the temp is uniform thru the pack.[/size][/size]

In that case they'll likely stay well-balanced due to the points I mentioned above. Further, they will likely last quite a long time since you're doing very shallow cycles centered around 60% SOC, which is close to optimal for health (similar to what NASA does for space missions). For another example see the graph below from this 2018paper.

You can figure out how many equivalent full cycles you've used by adding up the total capacity lost during all switchovers and self-tests, then dividing that by the pack capacity. Likely it is a very small number of full cycles since each discharge is very small.

Is it me or is the way you have described the 2x packs & UPS, they are actually in parallel?
They're both 7s7p in a "24V" UPS right?
When in parallel, them staying "balanced wrt each other is a no-brainer of course...

Redpacket said:

Is it me or is the way you have described the 2x packs & UPS, they are actually in parallel?
They're both 7s7p in a "24V" UPS right?
When in parallel, them staying "balanced wrt each other is a no-brainer of course...

Click to expand...

No - they are in series. An APC 3000 runs at about 55v.

The reason I ask this - is that on the 'other APC' a while back I had this sequence occur after all being stable for quite a while....

Week1:Battery1 (7s7p)= 27.1v + Battery2 (7s7p) = 27.3v = 54.4v GOOD

Week2: Battery1 (7s7p) = 25.7v +Battery2 (7s7p) = 28.7v = 54.4v Getting out of whack - 4.10v/cell

Week3: Battery1 (7s7p) =24.5v +Battery2 (7s7p) = 29.9v = 54.4v DANGER - 4.27v/cell

It seemed as if Battery1 started to self-discharge and Battery2 increased it's voltage to compensate - to maintain the 54.4v for the APC. This continued until I caught it at 4.27v/cell according to the BattGo on Battery 2.

My question -is it just luck that 2 Batteries in series will remain stable if healthy (no self discharge)or is there something in the chemistry of batteries that 'tend' to keep them stable when in series. Obviously - no one balances lead acid... and an APC like this will have 4 x 12v batteries in series and they stay stable.

I just don't know of any theorythat would let batteries in series stay stable more than a few weeks or months - but yet all APCs use lead acid that depend on the seperate 12v batteries to stay stable.

^^^ I already explained why they stayed balanced in the first unit, viz. the major factor causing any imbalance in your case is going to be self-discharge,because any divergence due to cycling degradation will be minimal due to the very low amount of cycling in standby use. It may be helpful to review the major causes of degradation.

As for the other unit, please provide the same details requested above. In particular, were the cells not the same brand or same SOH, or was temp distribution less uniform?

gauss163 said:

^^^ I already explained why they stayed balanced in the first unit, viz. the major factor causing any imbalance in your case is going to be self-discharge,because any divergence due to cycling degradation will be minimal due to the very low amount of cycling in standby use. It may be helpful to review the major causes of degradation.

As for the other unit, please provide the same details requested above. In particular, were the cells not the same brand or same SOH, or was temp distribution less uniform?

Click to expand...

All these batteries are the same as described above.

I didn't find an explanation in your comments above...

It seems like *any* battery (healthy or not) would have some drift and should get out of whack over just a few months. Maybe a healthy battery just doesn't have 'drift'? or the 'drift' is so small that its not a matter of months but years type of scale?

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).

OffGridInTheCity said:

All these batteries are the same as described above.

Click to expand...

The exact same packs, or similar packs using same cells from same lot?

OffGridInTheCity said:

I didn't find an explanation in your comments above...

Click to expand...

I can elaborate if you can be more precise as to what is not clear to you.

ajw22 said:

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).

Click to expand...

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 manufactureror what.

The "science concept" is simply that the self-discharge rate is very low for healthy Li-ion cells. Since your standby packgets very littlecycling it will only be differing self-discharge rates that can lead to voltage/SOC imbalance. This is alow percentage (difference) of an already very low rate, so it amounts totiny 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 around50% 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 chemistrieslike (non-LSD) NiMH, but it is par for the course for Li-ion.

As towhy 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).

ajw22 said:

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 [...]

Click to expand...

That's well worth emphasis, but likely it doesn't apply to the OP, since Li-ion self-discharge rate is nearlyconstant (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.

@gauss163 - thank you for more detail. OK, I get the 'very little SD for healthy packs' and 'in shallow part of charge/discharge curve' andtherefore 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 anamazing piece ofengineering

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.

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.

OffGridInTheCity said:

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 [...]

Click to expand...

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.

>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 to0.05v 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 daywhere the average consumption goes up to 5500w/hour for the day instead of the typical 85F day and3000w/hour - e.g. higher/more-demanding current draw on the battery bank. Once the circumstance passes - things return to the normal pattern.

^^^ 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).

gauss163 said:

^^^ 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).

Click to expand...

Yes - absolutely sure. I have it affirmatively disabled.

The white triangles in the blue barsare 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...