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Why do 2 batteries in series 'tend' to be stable?
#1
Hi Folks

I have 2 separate healthy (no SD, balanced) 7s7p 18650 batteries in series in my 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 these 7s7p batteries mostly just sit all day at the 27.xV shown are 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 7s2p or 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, rechargeable batteries in series tend to be stable Smile

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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.
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#2
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).
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#3
>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, RED UR18650E(s) - https://secondlifestorage.com/showthread.php?tid=1718
A
ll 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).

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

They are in an APC and 'floated' to the voltages shown above.  The APC protects against the twice daily 10-20ms power switchovers between grid and inverter with a 1000w load and APC self-tests every 2 weeks - e.g. minor discharge/charge cycles. 

>What is the ambient temperature?  Is temp distribution fairly uniform throughout the pack?
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.
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#4
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).

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.
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#5
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...
Running off solar, DIY & electronics fan :-)
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#6
(07-29-2020, 09:55 AM)Redpacket Wrote: 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...
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 theory that 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. 
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#7
^^^ 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?
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#8
(07-29-2020, 03:22 PM)gauss163 Wrote: ^^^ 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?
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?
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#9
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).
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#10
(07-29-2020, 03:49 PM)OffGridInTheCity Wrote: All these batteries are the same as described above.

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

(07-29-2020, 03:49 PM)OffGridInTheCity Wrote: I didn't find an explanation in your comments above...   

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