German Powerwall 280xLG 18650MJ1 7s40p+ 450Wp Balcony PV System and a small question

drbacke

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Apr 3, 2019
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Hello,
my first post here and in this one would like to show you to my new (not yet completely finished) small Lithium Powerwall. Maybe you are interested and motivated to rebuild it (or you are afraid of it and run away^^).
The storage is used withmy small balcony pv system (3 Panels@450Wp total output)
Why Powerwall? Before i used2x 80Ah AGM batteries (bad buy), that was much too little capacity. During the day constantly full and in the evening empty.
Now I'm more than happy, I love this system. It's cheap and very powerfull, of course there are some safety issues, but I think it's possible to get it safe.

ButI have aQuestion, maybe someone can help me:
I consider to extend the powerwall with used batteries.
Is there anything against it?
Would this affect the lifespan of the new batteries?





Some data about it:
- Will be extended with used batteries
- No balancers yet, on the way from China (QNBBM Active Balancer)
- Installation is still provisional, wait for a load rack.
- Extra Lithiumfire extinguisher on the way
- Works extremely goodso far, it feels like I have infinite energy :D (approx. 5 days.)

Parts list
280x LG 18650 MJ1 @3.5Ah NCA Batteries (NEW)
1x copper wire tinned, 390-561, 0,2 mm, 35SWG, 32AWG, 717m coil (12,50 on Ebay)
1x 9L Jockel gel fire extinguisher gel extinguisher special extinguisher for lithium ion batteries (119 Ebay)
1x fields soldering grease acid, 50g i.tin 2,89 @ amazon (dont use it)
1x Tabinger 60W soldering iron 200-450C 10,89 @ amazon
3x Velidy 18650 battery 4 x 5 cells spacer 9 @ amazon (cheaper at aliexpress)
1x fields solder wire 1.1.2B / ROM1 Sn60 Pb40, 0.75mm 250g 19,93 @ https://www.rsonline-privat.de
2x single core cable, 1.63mm copper tinned, 22 A, 10,8m for 10.39/piece @ https://www.rsonline-privat.de

I've some more details on my youtube channel (german):https://www.youtube.com/channel/UCaWbn671XbI1GLTgndyI7gQ/
Hope its no problem to post it.



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Geek

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You should avoid using acid flux on any electronics work. On 18650s the acid can eat the nickel plating around the solder joint, exposing steel underneath. Acid flux also never perfectly neutralizes either.
 

Korishan

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And to add to Geeks about the acid flux, after the acid eats the nickle away, the steel is now open to rusting, which will then lead to electrolyte escaping at least.

Other than that, I see you have a good build going happening. Nice solder jobs, btw. Just enough solder on the cells to make fuse wire contact.
 

Geek

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To answer you question about adding second hand cells. I don't believe it will shorten the life expectancy. Just be very careful of cells that show signs of self discharge. Also if you want to further increase the life expectancy do not use the full capacity of the battery. By only charging to 4.1v you will significantly increase the cycle life. As there is is only a small gain in usable capacity going from 4.1-4.2v. Same goes for the cutoff voltage, there is very little capacity left after about 3.5v(say about 3.2-3.3v under load).

By only charging to 4.1v and cutting off at say 3.3v you will loose maybe 20% of capacity.

The same also works for discharge current. By not drawing large currents from your batteries this will also increase their life expectancy.

For more information Battery University has an excellent article detailing how to prolong lithium based batteries.
 

drbacke

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@Geek: Thx for the hint. I tried to wash away the remaining flux. Next time I will use acid free flux.

The used voltage is restricted between 3.4V - 4.1/4.0.
Are you shure about mixing different cells of different capacities?
But are there no cross currents between the different cells?
Shouldn't this lead to an increased "high-frequency" cyclic load?
 

Korishan

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All tanks will drain at the same rate as they are all in parallel (think of a bunch of water tanks connected together). They will generally all fill up at the same rate as well. If the charge/discharge current is low enough per cell, the IR of the cells won't interfere with the flow. So if you end up with <500mA per cell, this is usually well enough below when IR would have any major factor.

So with your 40p, if you were charging at 500mA, then you would need to an overall amperage of 200A. So I don't believe you'll have any issues there
 

drbacke

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Korishan said:
All tanks will drain at the same rate as they are all in parallel (think of a bunch of water tanks connected together). They will generally all fill up at the same rate as well. If the charge/discharge current is low enough per cell, the IR of the cells won't interfere with the flow. So if you end up with <500mA per cell, this is usually well enough below when IR would have any major factor.

So with your 40p, if you were charging at 500mA, then you would need to an overall amperage of 200A. So I don't believe you'll have any issues there

THX for the explanation, butI think you mean 20A (40*0.5A)? Maximum Amperage in my system is 30A, so this should approximately hold.
But what about the chemicalreaction time of the different cell types? If there will be some differences (and maybe there is a difference with different cell chemistries)you will have very short instationary crossflows i guess.
Ok i'm not shure if this point is really important.But if so this would lead to a very shorthigh frequency cross current I think.
Hope you understandwhat I mean or maybe I'mcompletely wrong :huh:
 

Korishan

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No, I meant 200A, as I was using that as a huge example ;) Altho, now that I do the math (must of fumbled someone earlier on the calc), 200A would be 5A/cell, no good.

If the max amps you will pull is 30A, then 30 / 40 = 750mA/cell


"IF" there are any variances in the chemical reactions, they are probably very negligible, if any at all. Again, we're talking about using very very low amps per cell.

With high frequency cross current, I think it's likened to water hammering. This is caused when large draw on water is happening, then it's stopped suddenly. This can cause pipes that either have trapped air, or loose pipes, to rattle and shutter. However, if you turn the water off slowly, this doesn't happen. This is sorta, but not entirely, similar to what we have here.

As long as the current is low, there shouldn't be any "shuttering" across the cells. If there were, I'm sure someone here on the forum would have found it by now. There are many EE's here, and several have done extensive tests. Also, there are many high system level amp draw setups as well. Pete (HBPowerwall) can pull over 100A continuously, and I think he's even hit the 200A mark at one time. Daromer also has a large setup, as do many others here that draw high currents. No one that I'm aware of have had any failures due to different cell types (other than mixing LiCo with LiFePO4, or other major chemical differences).
 

Generic

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I don't know about crossflows, but I don't believe all cells will be contributing the same amount of current at any given voltage. With used cells from different brands, different IRs, different states of health, different chemistries, etc. every cell will be giving different amounts of current at different voltages. It's not a bad thing and I don't think it affects the life of a pack, it's just something to think about. For me, I've been paying a lot of attention lately to 3.6V cells vs. 3.7V. cells and trying to group them that way so they are all on the same discharge curve, but I don't know if it's that big of a deal with low current applications.
 

Geek

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I wonder if there is a way to protect those cells that have had acid flux used on them. Even after they are cleaned if any remains. If you could stop air and moisture getting onto the surface, they wouldn't corrode. Maybe like brush on some paraffin wax or use some Lanox. Maybe someone has a better suggestion.

Personally, I would test if battery terminal protector effects the wrapper. If it doesn't, I'd use that. Given that they are new cells, and the pack is unlikely to require dismantling for service.
 

Korishan

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Dallski said:
I don't know about crossflows, but I don't believe all cells will be contributing the same amount of current at any given voltage. With used cells from different brands, different IRs, different states of health, different chemistries, etc. every cell will be giving different amounts of current at different voltages. It's not a bad thing and I don't think it affects the life of a pack, it's just something to think about. For me, I've been paying a lot of attention lately to 3.6V cells vs. 3.7V. cells and trying to group them that way so they are all on the same discharge curve, but I don't know if it's that big of a deal with low current applications.

If the cells were being charged/discharged at max C, or even higher, that's when issues "could" arise. However, one thing is absolutely for certain, all cells will have the exact same voltage at any given moment for any cells that are connected in series. Sure some cells may not be able to output same current as another, but that makes no real difference in the whole pack. And if someone was designing a pack to output at 2A/cell or more, they should be using new cells to begin with as the application would require far more stability than what recycled cells can offer.

Now, because drbacke is using NEW cells, there's nothing to worry about any kind of oddities. They are new, so he could pull 2A/cell for a period of time and the packs should be able to handle it just fine. The LG MJ1 cells can do a 10A max discharge and 3.4A charge rates. But running at 40p with an estimated 30A max usage, he'll never hit the limits of the cells capabilities.
 

drbacke

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Geek said:
I wonder if there is a way to protect those cells that have had acid flux used on them. Even after they are cleaned if any remains. If you could stop air and moisture getting onto the surface, they wouldn't corrode. Maybe like brush on some paraffin wax or use some Lanox. Maybe someone has a better suggestion.

Personally, I would test if battery terminal protector effects the wrapper. If it doesn't, I'd use that. Given that they are new cells, and the pack is unlikely to require dismantling for service.

You are right. The acid flux containsF-SW21 which is "weak/limited corrosive", maybe this is actually a problem.
They sold it to me as electrical flux, so i didn't checked it further. My fault.
Which material is the cell wall build of?
I'm not sure if this is a real problem or can be dangerous, maybe someone else has more information?

Maybe I can use flux cleaner and the a protective resin afterwards. What about the cleaner, can it damage the plastics of the cell?
 

Generic

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I only mentioned it because drbacke was considering adding used cells to the pack that contained the new cells and was worried about the used ones negatively affecting the new ones.

And yes, the only thing all cells share in a parallel pack is voltage.
 

drbacke

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Ok I got some more Information about the "flux problem":
It seems to be Ammonium chloride which is not problematic at 20C with most stainless steels (the cell walls are of stainless steel, but I dont know which steel exactly). It also contains little amount ofZinc chloride, which is lightlycorrosive to stainless steel :( . I will clean it up with a flux cleaning spray and afterwards put a protective resin on top.

THX for the hint, these are the little things that can get dangerous with time.
 

Korishan

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Wow, even the flux is "weak", just happens to be strong against exactly what you're working with. that sux. At least you caught it before too much damage was done. You may have even caught it before any thing happened. Just check the metal really closely and see if has any etching. But putting a protective layer over it after cleaning it off definitely won't hurt, if anything it'll help protect against humidity.
 

drbacke

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Korishan said:
Wow, even the flux is "weak", just happens to be strong against exactly what you're working with. that sux. At least you caught it before too much damage was done. You may have even caught it before any thing happened. Just check the metal really closely and see if has any etching. But putting a protective layer over it after cleaning it off definitely won't hurt, if anything it'll help protect against humidity.

I will disassemble the system and try to get the packs clean andhope its possible without complete disassembling each battery :s
If I'm finished, I will post you the result. THX a lot
 

Geek

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drbacke said:
Geek said:
I wonder if there is a way to protect those cells that have had acid flux used on them. Even after they are cleaned if any remains. If you could stop air and moisture getting onto the surface, they wouldn't corrode. Maybe like brush on some paraffin wax or use some Lanox. Maybe someone has a better suggestion.

Personally, I would test if battery terminal protector effects the wrapper. If it doesn't, I'd use that. Given that they are new cells, and the pack is unlikely to require dismantling for service.

You are right. The acid flux containsF-SW21 which is "weak/limited corrosive", maybe this is actually a problem.
They sold it to me as electrical flux, so i didn't checked it further. My fault.
Which material is the cell wall build of?
I'm not sure if this is a real problem or can be dangerous, maybe someone else has more information?

Maybe I can use flux cleaner and the a protective resin afterwards. What about the cleaner, can it damage the plastics of the cell?

F-SW21 based flux can be washed off with alcohol. I would opt for isopropyl, as metholated spirits can leave a residue.
 
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What do you power with your Powerwall? How do you distribute power? Could you give me a small summary?
 

drbacke

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Maniac_Powerwall said:
What do you power with your Powerwall? How do you distribute power? Could you give me a small summary?

Air Conditioner (onlny in summer, not necessary here in april ;) ), TV, Light, fridge, depends on the battery state... But in the moment I dont't have to think about it, there is enough energy.
It's not connected to the grid. I build upan own electrical wire network with some extension cables, simple and effective.
 
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