Its not possible to initially balance Lifepo4 cells so all have the same SOC

[HeywoodJ]

Running a massively parallel system just requires a slightly different mind set than the "run them between 20% SoC and 80% SoC and they will last forever" that seems to be the mantra. Treat them a bit more like LA and get them "full" every once in a while. As Jedi said above, balancing in the knee (top or bottom) is relatively easy. A decent pack, with decent hardware and all you have to do is get the cells up into the knee and hold them there for a while to get the balancing happening.

We have literally thousands of modules that are Lishen 40A cells in 8P12S or 8P16S configuration running a 5%-95% cycle daily, and keeping them balanced is trivial - run them up above 95% about once a month or when the delta gets above whatever limit and run a CV charge with the maximum cell voltage as the control input. If your BMS and charging equipment can't do that then you're using the wrong BMS.

[hjohnson]

Originally Posted by s/v Jedi

You don’t know that because you don’t have data from individual cells. What your BMS thinks is a cell, are actually two cells in parallel, interacting with each other all the time. They may be equal voltage or 60mV apart or more, you simply don’t know.

I do know. By definition, their voltage has to match because they’re interconnected by good quality copper bus bars. It would be essentially impossible for them to have differing voltage. This is basic electrical theory.

When I did the initial charge, the system brought each pair of cells up to 3.65Vpc.

[s/v Jedi]

Originally Posted by hjohnson

I do know. By definition, their voltage has to match because they’re interconnected by good quality copper bus bars. It would be essentially impossible for them to have differing voltage. This is basic electrical theory.

When I did the initial charge, the system brought each pair of cells up to 3.65Vpc.

Ouch, that is not how it works. By connecting cells in parallel, you do not make them equal resistance nor equal soc%. What your connection does is that during load or charge, one cell is doing more than the other and at idle, one cell is discharging into the other.
If you would remove the connection, the truth will appear as voltages will change.

When somewhere in the linear part of the soc% graph, you can see very similar voltage but still largely different soc%. So the two cells may appear to be at say 80% soc while in reality one is at 85% and the other at 50%.

[UFO]

Originally Posted by s/v Jedi

Ouch, that is not how it works. By connecting cells in parallel, you do not make them equal resistance nor equal soc%. What your connection does is that during load or charge, one cell is doing more than the other and at idle, one cell is discharging into the other.
If you would remove the connection, the truth will appear as voltages will change.

When somewhere in the linear part of the soc% graph, you can see very similar voltage but still largely different soc%. So the two cells may appear to be at say 80% soc while in reality one is at 85% and the other at 50%.

Surely a 30% difference in SOC would be with some really crap quality cells?


When I got my 16 cells (from memory) they were all sitting between 3.02V and 3.05V with the bulk of them at 3.04V - After top end balancing to 3.6V (took about 5 days) and resting for a few days they all remained within less than 10mv of each other. My BMS balances (up to 1 amp) and monitors each set (yes not individual cells, so those cells could have the variance you talk about) and they are always less than 20mv apart even under a 280Amp load -


Now the question.....



Surely if I had a bad cell in any set I would see a greater variance than 10mv per set or am I wrong there?

[Emmalina]

I just use these and everything is spot on ! https://www.lazada.co.th/products/ac...msnoOO&stock=1

[s/v Jedi]

Originally Posted by UFO

Surely a 30% difference in SOC would be with some really crap quality cells?

When I got my 16 cells (from memory) they were all sitting between 3.02V and 3.05V with the bulk of them at 3.04V - After top end balancing to 3.6V (took about 5 days) and resting for a few days they all remained within less than 10mv of each other. My BMS balances (up to 1 amp) and monitors each set (yes not individual cells, so those cells could have the variance you talk about) and they are always less than 20mv apart even under a 280Amp load -

Now the question.....

Surely if I had a bad cell in any set I would see a greater variance than 10mv per set or am I wrong there?

For bad cells, yes, but cells with different internal resistance aren’t bad per se… they’re just not matched.

Assume you have two cells with the same voltage and you connect them in parallel. Now you start charging this pair with 10A current. The only way that each cell gets 5A is when they are precisely matched. If they aren’t, one gets 6A and the other 4A, leading to substantial imbalance and one with much higher soc% than the other.

Then, when you stop charging and thus the higher charge voltage is gone, the fuller cell will have a higher rest voltage and start discharging into the other.

Compare to series-only: all cells get the exact same charge current, even if they have differing internal resistance, they all get the same Ah charge.

[hjohnson]

Originally Posted by s/v Jedi

Ouch, that is not how it works. By connecting cells in parallel, you do not make them equal resistance nor equal soc%. What your connection does is that during load or charge, one cell is doing more than the other and at idle, one cell is discharging into the other.
If you would remove the connection, the truth will appear as voltages will change.

When somewhere in the linear part of the soc% graph, you can see very similar voltage but still largely different soc%. So the two cells may appear to be at say 80% soc while in reality one is at 85% and the other at 50%.

All top (or bottom) balancing does is bring all the cells to the same voltage at the time of the balancing. It doesn’t change the fundamental properties of the cell. It just means that all cells have the same voltage at the moment they were disconnected from the charging bus that was used to balance them. Letting the active balancer do its job of balancing out all the cells has exactly the same effect as top balancing the cells; it brings all the cells up to 3.65v and holds them there until the charging current tails off. That results in exactly the same thing as if I had connected all the cells in parallel and hooked them to a dc supply set to 3.65v. That initial balancing took about 8 hours or so.

As far as the SoC of an individual cell, I also don’t really care about that, I care about the SoC of the pair, and that’s all that really matters. One cell cannot “discharge into the other” as you say, because there is no voltage difference between them. What happens is that the equivalent series resistance of one of the cells will go up a little more than the other, and the other cell will carry more of the load. Conversely, when charging, the cell at lower SoC will accept more of the current than the one that is at higher SoC. But both will be discharging, or charging depending on the situation. The only time that one cell might have pushed some current into its pair is when I initially connected them and they evened out the few 10s of mV difference between them.

[hjohnson]

Originally Posted by s/v Jedi

For bad cells, yes, but cells with different internal resistance aren’t bad per se… they’re just not matched.

Assume you have two cells with the same voltage and you connect them in parallel. Now you start charging this pair with 10A current. The only way that each cell gets 5A is when they are precisely matched. If they aren’t, one gets 6A and the other 4A, leading to substantial imbalance and one with much higher soc% than the other.

Then, when you stop charging and thus the higher charge voltage is gone, the fuller cell will have a higher rest voltage and start discharging into the other.

Compare to series-only: all cells get the exact same charge current, even if they have differing internal resistance, they all get the same Ah charge.

The other thing that you’re missing is that my BMS is controlling my charging sources. As the pairs of cells approach their Max voltage, the BMS commands my charging source (alternator, inverter/charger, mppts) to back off and limit the current into the batteries. It fades down the current limit, until that limit gets down to (eventually) 0A into the battery. The system is never pushing huge current in when you’re close to 100%.

The other thing that you’re missing is that the “cells” we buy aren’t a single cell like you’d find in a lead acid battery. If you look at an x-ray of an LFP “cell” it’s actually made up of multiple pouches inside the housing, all wired together in parallel and bathed in the electrolyte. Doesn’t matter if it’s Winstron, CALB, or EVE, they’re all built in roughly the same way.

[s/v Jedi]

Originally Posted by hjohnson

The other thing that you’re missing is that my BMS is controlling my charging sources. As the pairs of cells approach their Max voltage, the BMS commands my charging source (alternator, inverter/charger, mppts) to back off and limit the current into the batteries. It fades down the current limit, until that limit gets down to (eventually) 0A into the battery. The system is never pushing huge current in when you’re close to 100%.

The other thing that you’re missing is that the “cells” we buy aren’t a single cell like you’d find in a lead acid battery. If you look at an x-ray of an LFP “cell” it’s actually made up of multiple pouches inside the housing, all wired together in parallel and bathed in the electrolyte. Doesn’t matter if it’s Winstron, CALB, or EVE, they’re all built in roughly the same way.

All wrong, but that is fine, be happy

[hjohnson]

Originally Posted by s/v Jedi

All wrong, but that is fine, be happy

25 years of electrical engineering experience here, my friend. I know what I’m talking about. But you do you.

[s/v Jedi]

Originally Posted by hjohnson

25 years of electrical engineering experience here, my friend. I know what I’m talking about. But you do you.

Then you should check facts before posting. If you are an EE like me, then you know a prismatic cell is very different from a pouch cell. It’s just silly so discussion doesn’t work at that level.

I became an EE in ‘86 and actually keep up to date with all modern tech.

[hjohnson]

Originally Posted by s/v Jedi

Then you should check facts before posting. If you are an EE like me, then you know a prismatic cell is very different from a pouch cell. It’s just silly so discussion doesn’t work at that level.

I became an EE in ‘86 and actually keep up to date with all modern tech.

I never said they were pouch cells, those are a different beast. However, when I was doing my initial research into this topic before building my system, one of the sites included an x-ray image of a CALB 280Ah cell. The internal structure was clearly not a single monolithic cell like what you’d see on a very large lead acid, for example. Instead it was a whole stack of thin elements, wired in parallel to the terminals. All the elements are bathed in the same electrolyte so they’re not fully independent of each other, but at the same time it’s not just a 3D comb of plates intermeshed like what you’d find in your average lead acid starting battery.

[UFO]

Originally Posted by s/v Jedi

For bad cells, yes, but cells with different internal resistance aren’t bad per se… they’re just not matched.

Assume you have two cells with the same voltage and you connect them in parallel. Now you start charging this pair with 10A current. The only way that each cell gets 5A is when they are precisely matched. If they aren’t, one gets 6A and the other 4A, leading to substantial imbalance and one with much higher soc% than the other.

Then, when you stop charging and thus the higher charge voltage is gone, the fuller cell will have a higher rest voltage and start discharging into the other.

Compare to series-only: all cells get the exact same charge current, even if they have differing internal resistance, they all get the same Ah charge.

Back to the original question and in regards to my system, would you expect to see a bigger variance between the sets than 20mv under large loads such as 280amps out or 150 amps in?

[s/v Jedi]

Originally Posted by UFO

Back to the original question and in regards to my system, would you expect to see a bigger variance between the sets than 20mv under large loads such as 280amps out or 150 amps in?

Yes, I would expect to see 130-150mV variance.

[Wandering1]

Originally Posted by hjohnson

I never said they were pouch cells, those are a different beast. However, when I was doing my initial research into this topic before building my system, one of the sites included an x-ray image of a CALB 280Ah cell. The internal structure was clearly not a single monolithic cell like what you’d see on a very large lead acid, for example. Instead it was a whole stack of thin elements, wired in parallel to the terminals. All the elements are bathed in the same electrolyte so they’re not fully independent of each other, but at the same time it’s not just a 3D comb of plates intermeshed like what you’d find in your average lead acid starting battery.

We have taken some cells apart to have a look at them. That not how they are constructed. They are more like a Swiss roll cake. Single cell folded into a case. There is only a single cell with large surface area. Just like a lead battery