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Flat Wall Design -- 13s80p on a sheet of ply
#1
Lightbulb 


Hello! I'm looking into a 18650 powerwall and every example/build/etc I've seen often does all the cells in compact, soldered setups (unless it's Jehu's pcb variant). 

What I imagine would be a nice big patch of wall (in the image above, the wood is a 4x8 sized ply) that would hold 13s (48v) with anywhere from 60 to about 90 parallel's worth of cells. In the diagram, those black boxes are actually modeled to be cell holders.

Yes, totally would not be an efficient use of space, but slap a piece of plexi on top and it morphs into art (imo). I also really like the idea of being able to very easily swap out cells, should ones go bad, without having to desolder, or even disconnect and disassemble the entire thing.



As seen from a more simplistic wireframe view.

If the bus bars connecting here were of sufficient copper mass, this would work, right? What I can't think of is a graceful solution to how to fuse these. Or...would you even need to?

Alternatively, if this is egregiously stupid (given I can't find any examples of it), where's my dumb? Or maybe it has been done, and you can help me find it?

Thanks!
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#2
"48V" Li-Ion systems are usually 14s as voltage ranges matches gear better...
You should add fuses somehow.
You'll need BMS attachment points too.
Assuming all cells in each row are in parallel (better if so)?
Running off solar, DIY & electronics fan :-)
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#3
Yes it would be Art but rather non functional or at least minimal functionality.
Having a string of parallel cells that long will give you a considerable voltage drop between first and last cell.
Mechanically it looks nice but electrically not so much.
Electrons are not impressed by looks  Tongue

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#4
To fix Wolf's point, you would connect at diagonally opposite corners....
Running off solar, DIY & electronics fan :-)
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#5
(08-12-2020, 10:08 AM)Redpacket Wrote: "48V" Li-Ion systems are usually 14s as voltage ranges matches gear better...

Huh, interesting. Looking up most inverters in the "48v" rating seems to say they operate at 40-60v. So, yeah, 14s has the least lost high/low. Thanks.

(08-12-2020, 10:08 AM)Redpacket Wrote: You should add fuses somehow.

Yeah, just not sure how to set that up, but fair.

(08-12-2020, 10:08 AM)Redpacket Wrote: You'll need BMS attachment points too.

Indeed.

(08-12-2020, 10:08 AM)Redpacket Wrote: Assuming all cells in each row are in parallel (better if so)?

Yup, you can imagine that all of the cells are all facing the same direction in each horizontal row. IE: positive to the top.

(08-12-2020, 01:40 PM)Redpacket Wrote: To fix Wolf's point, you would connect at diagonally opposite corners....

Excellent point, thanks. Is it worthwhile making the connected wires match length, or just have a short and long ones?
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#6
(08-13-2020, 02:32 AM)chuyskywalker Wrote: Is it worthwhile making the connected wires match length, or just have a short and long ones?
I'd suggest that depends on the number of panels & how you have the BMS wired, etc.
Basically, anything in parallel within one panel should have equal cables so current flow paths see equal resistance.
This is most important for the "main" current flows, eg charging & inverter load currents. For the BMS balance leads, not so much (maybe center of row).
Say you had several of your panels in parallel, you could have eg all the -ve cables longer & +ve cables shorter (but not a mix unless your BMS strings were isolated, one string per panel).
Hope that makes sense?
Re the fuses, if you had eg a 3mm gap beside or "above" each holder maybe you could squeeze a fuse in there?
You might be able to alternate sides (beside version) or similar to get better use of space?

Also be careful of holder quality, you don't want contact problems or dents when inserting/removing cells (or broken fuses!).
A holder where the contact to external tab is one piece would be better than two metal bits joined somehow...
Wanting good contacts is why most folk solder or spot weld.
Running off solar, DIY & electronics fan :-)
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#7
I asked an eletrical engineer friend about wiring to one end of a long pack vs forcing current to go thru by having + and - on opposite ends.   The answer seemed to be that the 'level of amps' matters.  For example - if you're charging/discharging 100ma/cell (100p = 10a per pack) then its no big deal.  If you're charging/discharging 2a / cell (100p = 200a per pack) then its a bigger deal.    

So part of all this is the design of the battery bank - what kind of load...  charge/discharge is planned.   If its a standard DIY powerwall - perhaps in the 500ma/cell max range - then for power, its not a huge deal.  

 For voltage sense leads... it has been observed that Batrium longmons vary, regardless of wire length, by as much as 20mv (0.02v) / cell kind of thing.  The observation is that the packs will appear balanced - even though they might be technically off a bit.   So its a matter of scale.     Having equipment/measurement fuzziness of 0.02v means that you should not charge higher than 4.18v (to be safe)...  e.g. fuzziness...  BUT other than that, it has no practical affect on day to day operations (charge/discharge).    

So perhaps just wire things up and check the packs directly with a volt-meter and if you find it small (e.g. <0.03v) max difference - then maybe its not worth overly worrying about it and just limit max charge to 4.17v/cell Smile
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