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Solderless Powerwall Project - Simplest & Cheapest Design?
As you mention, there are several approaches and many details to building a battery from 1,554 cells - but it is fun!

Let me tackle a few items, and I'm sure you'll get additional posts.

>I'm going for cheap, simple, quick and easy
Solder vs Solderless
There is a 3rd option - which is spot-weld.  I'll assume by solderless you mean things like PCBs with holders where you push/pop in/out the cells but don't solder or spot-weld.  This idea has it's attraction but the economies of scale kill you.  When it's a matter of 100 cells then an extra xx cents/cell is no big deal but when it get's to be 1,000(s) of cells...  then the extra cents per cell turn in to hundreds of dollars.    You should always do your own math/thinking - but you probably find its cheaper to solder/spot-weld than buy printed circuit solutions or solderless solutions.

When I build a 1s100p pack I solder and use....
- 16 x 4x5 cell holders =  $0.60c * 16 = $9.50    ( I do 100-120p but will hold up to 160p )
- 4 x 28" 6awg twisted copper bus bars @ 0.59c/foot = $5.50
- 4 x Lugs = $3.00
- Solder - maybe $3.71
- Fuse wire / tinned copper wire (cell -> busbar) - maybe $3.00?
- 2 x Hot Glue sticks + 6 14" twist ties ~= $1.00??

That adds up $25.71/pack in raw material * 7 = $180 for a 7s  battery.  I have 84 packs - so you can see that 84 *25.71 = $2159.64 in 'just pack build materials'.  It really adds up as you scale Smile.    That's why I say, just do the math and see what's good for your situation.
If you use spot-weld you'll need a spot-welder + nickle strips in addition to the above but you'll use much less solder and won't need the fuse/tinned wire - so I think it'll turn out to be 'in the ball park' of the solder route if you do enough cells to pay for the spot-welder.    

> if a BMS is required. 
Yes, you need a BMS.   A BMS as a minimum protects against over-voltage and under-voltage / pack(cell).  Typically it will also protect based on temperature.  Typically it will offer balancing as well.   The key thing is to shut off power going in (charge) or power coming out (discharge) from the battery if any one of the packs(cells) in series get out of the proper voltage range and/or temperature range to protect the cells.  In particular, if you allow an 18650 cells to overcharge for any significant amount or time it will result in a fire and each cell is likely to ignite the cell next to it - very big deal.   If you let the battery go too low a packs(cell) in series can go negative causing the pack to take current in reverse which does severe damage - and again we're back to fire risk - a very big deal. 

>exactly what parts I need
A base BMS will have leads (small size wire) to connect to each pack in series so it can monitor/track the voltage.  These same leads can be used to balance the packs relative to each other if the BMS has a balance feature.  A lot of BMSs have a balance feature but not all.   A BMS will typically have a way to mesure the temperature of the battery - should include some temp probes.  A BMS will have some way - either internal relay or you have to buy an external relay - to shut off power flowing in/out of the overall battery.   Finally, a more expensive BMS may have a bluetooth -> android phone app or a screen so you can see what its doing / set various parameters. 

There are many youtubes on BMSs...   Here's 3 from the same person to give you an idea of the range of simpler/cheaper BMSs...
Here's a youtube on a base BMS to give you an idea..  (its for LifePo4 but info applies to Lithium-ion)...
Here's a youtube on a more expensive one with it's own screen ...
Here's a youtube on a phone based BMS... 

>The LED indicators on this powerwall would indicate if a battery has failed so that it can be easily replaced; 
If you buy a BMS with visual monitoring - that is a common way to know if the battery is healthy - e.g. check the BMS status. 
For the dirt-cheap / LED solution - you can buy an LED (or even a cheap volt meter) and power it by the output of your battery (down stream from the BMS) and if the BMS trips the LED will go out.  Of course the LED will drain power the entire time its on so you might want to add a switch so you can turn the switch on to check the LED when you're looking and then back off to avoid power drain when you're not looking Smile.     
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RE: Solderless Powerwall Project - Simplest & Cheapest Design? - by OffGridInTheCity - 07-21-2020, 09:36 PM

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