Good evening Powerites,

Thanks Jason for the timely powerwall topic.

It was funny to see a load shedding thread on another forum descend to a bragging contest of "my solar system is bigger than yours... 😉

I cannot compete with Leszek or Delphinus's workmanship but while I can get 18650 Lithium-Ion cells for (almost) free I will keep on using them for our houses on the farm. KISS is my motto. Herewith photos of one of my 14S powerwalls, this one currently 100P . Don't laugh, it works fine, although I have to occasionally don my BMS hat and use that thing with the 2x24V bulbs to check strings under load to diagnose and replace faulty cells.

My reason for showing this is to inquire if a forumite perhaps has an idea for a better architecture for a 18650 cell powerwall?

Ja, ons kan self!

(Yes, we can DIY)

@Abe53

 

Please share the love and let us in on your secret of getting 18650's for almost free?

That's awesome

Apart from what seems to be an issue with balancing & fusing of individual cells for each string, I really like the simplicity of this design! namely having a rack/cabinet with slats/shelves where you just slide in trays of the 18650's lying in strips of conduit.. I'm looking for something cheap and easy to maintain and this seems like a strong contender, as the usual 18650 cell spacers cost a fortune to import at the moment and make me question if the DIY route is even still worth it versus the price of just buying a complete lifepo battery...

Have you made any changes to this, particularly for making it easier to balance the cells? The way you have it set up at the moment, there should be a conductor between each series connection of where the cells touch, if the cells were supposed to be properly balanced with a BMS... which would be an ideal place for fusing as well. Perhaps interrupting each section with some kind of spacer with spring contacts on each side which connect with a fuse wire to that conductor where the balance lead connects

Maybe you could use another strip of trunking, but flipped 90 degrees (so vertically on the tray rather than horizontally) between each block of parallel cells... the "spring tension" of each wall of the trunking could help secure/snap the batteries in place although it would need to be pretty thick trunking to work properly.  I saw someone use paper fasteners (eg https://www.croxley.co.za/product/croxley-25mm-paper-fasteners-pack-of-100s/) (which you can also buy in bulk for cheap) as battery contacts, which seems like a brilliant idea. you could then just drill some holes in the sides of the trunking for the fasteners which then connect to the fuses/buss bars. Pop rivets could also work? It seems like adding more cost & complexity to the system and thus more points of potential failure as well, but technically it would be more vital from a safety point of view, to ensure automatic BMS functionality in place regardless, IMO

See: https://www.youtube.com/watch?v=wwD5b7Kb4xs

And: https://www.youtube.com/watch?v=bYcRAXrCt3I , https://www.youtube.com/watch?v=zawS3wwSgvM

So yeah, if we can just find a better way to fuse & balance on a cell-level then this design can be a winner

By the way, what are you using at the ends of each trunking to press the batteries together? Would love some more details, close-ups, how you do your wiring etc..

Thanks for sharing

Edited January 20, 20232 yr by FragtioN

Hi Frag, apologies for not replying sooner, busy building to take another three houses on the farm off-grid...

As to architecture, was hoping this thread could come up with suggestions of improvements, but no, thus soldiering on with what I have.

Actually, each 14s string already has a 4A fuse. (It is in the perspex spacer at positive end) 

As to string compression tension for reliable electrical contact, a metal manufacturer told me spring steel strip is unobtanium. So I keep using 13mm Terry clips halved.

String balancing turns out to be a non-issue. A cell failing closed is compensated by the other 13 taking up higher voltage, still within specs. Once two cells fail, one cell downline is sure to pop its fuse, which is easy to reset with my probing tool. (I keep a clear line to outside to eject a shorted suddenly heating cell 😉

A cell failing open just takes one string out, in this instance 1%, and is diagnosed and replaced with the next BMS service. A retiree's time is free. (On the golf course we wave through the players pushing behind, telling them we do not want the game to end ... 😉

Safe journey, Abe.

 

I'm too stupid to give you battery advice

My dumb advice is that your battery bank appears to be sagging a little bit, maybe put another support in the center.

And a fire extinguisher near the door because this scares me

On 2022/12/21 at 9:14 PM, wingnut said:

@Abe53

 

Please share the love and let us in on your secret of getting 18650's for almost free?

Yes please share. 

Just an update on the health of my first 18650 powerwall:

Have not checked the 53s 34p for the last year but coming home at sunset in load shedding (blackout) I switched to the Eaton PX9 because son wanted a pizza and I wanted to spare the Solis's 14s 100p for in case EKSDOM did not come back . With oven, microwave, kettle and more, the wall only dropped from 206V to 199V after dinner. So I switched off all strings and checked them today. All 1802 x 18650 cells at healthy Volts and pulling their weight, nice to see! No BMS, no pumping up or drawing down, every cell finds its happy place.

13 hours ago, Abe53 said:

No BMS

Wow, no BMS with incendiary cells, that is either very brave or something else...

Good evening from our home powered by 18650's in Ph6 loadshedding.

P1000, do you perhaps drive a car with a tank full of incendiary fluid? At speed in close encounters with taxis driven by reckless maniacs? (They say heaven welcomes taxi drivers, they convert more heathens than the priests... 😉

Yes, 18650's have a small risk of overheating and burning (not exploding) if mismanaged. However, I minimize the risk by:

1. Testing all cells, checking temperatures by hand and with a laser meter while charging at 1A, weeding out the heaters. (The red Sanyo's are the usual suspects)

2. Using them very conservatively, low cutout at 3.1V, charge to max 3.9V (4.25V allowed) Draw max 1A (<0.5C) Charge max 0.5A

3. Fitting every string with a 4A fuse. (Yes, once popped 36 fuses when switching on big loads with only three trays of 14s 10p batteries on the wall.)

4. DC rated CB and fuse on + and  -

5. The 18650's are not soldered, just butted + to - (after mangling the + to protrude) contained only by a 25x16mm PVC trunking, if disturbed, the battery just falls apart cutting the circuit.

I have survived 69 years with some close encounters, not at all worried about the thousands of 18650's on the wall in the passage.

On 2022/12/21 at 9:14 PM, wingnut said:

@Abe53

 

Please share the love and let us in on your secret of getting 18650's for almost free?

Bump