Look, it is a potential issue. I think the biggest issue here is that the two batteries on the left share a cable, while the one on the right doesn’t. The left side also has double the capacity. How this is going to even out in practice is a good question, but you should expect them to charge and discharge unevenly.
That is a problem. Especially if you discharge deeply.
It is not one that is going to kill batteries though. Not like with lead acid.
Edit: Also, one has to distinguish what imbalance we’re talking about.
It is not going to cause imbalances between cells in the same module.
It is going to cause imbalances in the relative SOCs of modules. Which is not the biggest issue in the world, as the increase in impedance at the top and bottom tends to help distribute that somewhat. But it is something to be aware of.
I thought so as well, especially with freedom won, I will guarantee you within 6-7 months he’ll have a pretty serious imbalance… Trust me… on one site with only two batteries the imbalance was serious in less that 2 months after installation.
Always remember, there is a huge difference between theoretical and practical experience. I get called to at least 2 installations a month with battery issues, BSL, Pylon, Sunsynk, Dyness, CFE, E-Towers, to name a few, all of them showing imbalance anything between 2 to 7 months after installation if you don’t follow one simple rule… Negative of Fuse holder to first battery and positive to last or vice versa.
The supplier than blame the firmware and ask us to update. Without rectifying the wires, no update will fix the issue.
When that happens, the theory is wrong and needs to be updated
OK, that is way more severe than I would have thought. Could also have something to do with some BMSes not having the capability to sense low down, so this might have been a combination of unequal recharge, plus SOC drift.
At least it confirms what I thought: It will cause SOC imbalance between modules. It will not cause voltage imbalance between cells in a module.
In general, if you put two batteries in parallel, at different states of charge, and you charge them together, then one of them fills up and stops accepting charge. At this point, the other one simply accepts all the charge until it is also full.
This was also practically determined in telecoms, where they frequently put banks of different ages in parallel, and where they can live with it because the batteries are generally kept full: The higher-impedance path has ample time to also recharge in other words.
Anyway: I’ve moved slightly on my opinion. I started at “naaah, no need to worry” and I am now at “you’ll probably be fine for a few weeks, but you need to know about the issue”.
Again, we know that nature lends itself pretty well to being described mathematically. If the maths doesn’t agree with what we observe, we made a mistake somewhere. I would like to know what that mistake is. Sorry, that is just how I am. I am not content with “oh well, I suppose practice don’t align with theory”.
Thought experiment. Put two LFP batteries in parallel. One is at 100%, the other is at 40%. Charge them. Prove to me that the one that is at 40% will NOT, in time, reach 100%. That is the challenge.
If it ends up that far apart, there wasn’t enough solar to recharge, and because of the poor division one of them took the brunt of it. That is wrong, that is a problem, that needs to be fixed, I have conceded that.
But please show me how my thinking is wrong otherwise.
Different wire sizes, bad connections, so many things we see in the field, 35mm from fuses to ist battery and then 16mm from there to second battery.
We got so many support cases that we made the battery connection part of each and every training session whe have with our installers. 90% of them don’t follow these rules and 100% of them immediately relates to the imbalance issue once we mention it. “ahh is that why we are having the issue”
The 100-40% issue were on a 4 battery stack after 7 months. It took us more than a month getting them back in sync. Even changed masters a few times and ended up charging them up individually to 100% and then reconnected them. A year later they are still performing well. (ET’s)
Its a lot more common than you think, people just don’t like advertising their failures and installation mistakes too much.
I made a few myself in the beginning, thinking it won’t matter, had to go back to fix it.
No, I think you misunderstand him and me. He FOUND the battery in that state. I am not contesting that an SOC imbalance can develop, I am contesting that it will PERSIST upon recharge.
Show me the mechanism. You have a lot of experience with these batteries. Tell me… why would the one at 40% SOC stop charging. Out of compassion with its mate?
Even with lead acid, the low string will come up, it just takes a lot longer with them.
I hear what you guys are saying, and am also not hearing.
Do you believe my batteries are wired so that 2 is in parallel and the 3rd one is standing alone?
For what it’s worth, they reach full charge just about every day, and I never ever allow more than 50amp draw from the inverter but I will allow them to discharge to anything between 20-30% at times, though this is seldom.
That is true and i was just starting to type the same reply, I found them like that, but have to add, the only thing in common on 95% of similar installations were the fact that this principle were not followed. What else added to the huge imbalance, I dont know, but what i do know is that firmware updates did not fix it, when we changed the wiring to what I believe work bests, we could almost immediately see an improvement in the majority of the installations with smaller SOC discrepancies. The bigger issues took longer to resolve.
Module A is 100% full, and the highest cell is at 3.5V. Based on this, the BMS (the master, which acts for the whole bank) tells the inverter “please don’t charge me too hard, I have a high cell!”.
Model B, however, is only 40% full, and now doesn’t recharge properly.
In other words, what I am discovering here, or reasoning out for myself (embarrassingly and publicly ), is that an SOC imbalance combined with a BMS that doesn’t expect it, can be a recipe for a bad day.
If the BMS doesn’t intervene (as with the Victron SmartLithium batteries, which I have far more experience with), the low string simply accepts the rest of the charge and all of them pull up to 100% in one cycle.
Also, I have somewhat lowered my opinion of the eTower. So I revise my earlier statement, I probably would not wire them like that.
Add the 4th batt, it is designed for that. Good foresight.
If not happening, then have them rewired as 3 banks in parallel … one set of wires … in my simplistic brain, it is physics. Or, more parts, more wires, all contribute to things in the end.
I’m a lazy bugger … don’t like to redo things later … and O boy I have done my fair share of “WTF not to do”. This is not one of those … you have a good plan …
As Jaco said, takes months to get “there”. Small differences every single day all add up over time. Way I see it.
I am running 2 heavily mismatched banks in parallel. One is a 16s3p 120Ah (360Ah total) pack with 2nd life cells, a decent BMS and beefy wiring.
The other is a bank of 4S 200Ah 12V Champion1 batteries (cheap junk with a ridiculously high internal resistance).
It works, and everything stays in balance. BUT it results in continuous small charge-discharge cycles, which will certainly degrade overall battery life.
Basically, each time you turn on a load, the 360Ah battery delivers all the current for the first few seconds, until pack voltage starts sagging, at which point the cheapy joins in. When the load is removed the cheapy then discharges into the 360Ah bank to balance the charge again.
So on every large charge/discharge event, the battery with the lowest Ri takes the initial punch, and then balances afterwards. SOC can differ by 10-20% after long heavy loads, but they always balance again within and hour or two.
When not under load, then as long as the cell chemistry is the same (and the temperature is the same), then the SOC is tied closely to the voltage. Since the voltages are the same, the batteries will charge/discharge to the same SOC.
The only time this does not happen, is when one (or more) cell is out of balance, in which case the battery voltage is determined by the out of balance cell, instead of SOC. This is how I discovered that my Heltec balancer had died… SOC deviated rapidly and overall capacity dropped. Replace the balancer and all is hunky-dory again.
