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German Powerwall 280xLG 18650MJ1 7s40p+ 450Wp Balcony PV System and a small question
#1
Wink 
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 with my small balcony pv system (3 Panels@450Wp total output)
Why Powerwall? Before i used 2x 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.

But I have a Question, 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 Lithium fire extinguisher on the way
- Works extremely good so far, it feels like I have infinite energy  Big Grin  (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°-450°C 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, Ø 0.75mm 250g 19,93€ @ https://www.rsonline-privat.de
2x single core cable, Ø 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/UCaWbn67...TgndyI7gQ/
Hope its no problem to post it.


 
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#2
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.
drbacke and Korishan like this post
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#3
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.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#4
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.
Ibiza likes this post
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#5
@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?
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#6
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
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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#7
(04-04-2019, 11:00 AM)Korishan Wrote: 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, but I think you mean 20A (40*0.5A)? Maximum Amperage in my system is 30A, so this should approximately hold.
But what about the chemical reaction 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 short high frequency cross current I think. 
Hope you understand what I mean or maybe I'm completely wrong  Huh
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#8
No, I meant 200A, as I was using that as a huge example Wink 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).
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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#9
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.
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#10
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.
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