Lfp is a option, rightFinished battery or cells only?
Salvaged cells 18650 or LFP bricks, every country has a different market availability for this kind of stuff. Sometimes even laws making it hard (you are from Germany, right? Recycling laws
New cells, I would definitly recommend to go LFP. The 280Ah bricks seem to be handy these days (ca. 100 Euro/kWh)
Finished systems, you can find chinese manufacturers on Alibaba. LFP 5kW/24V packs. They will build it as you ask for. Probably somewhere around 1000 Euros for such a pack shipped to your door.
Any particular reason why 24V? If you are later planing to expand your system, go for 48V. Lower currents make things much easier.
this is really a good idea with the 18650s I actually wanted to do it that way. When I tried to buy cheap cells, I quickly noticed that I can't find anything under 2.20 € per cell at the moment. then i thought for the money i can also order lifepo4 from aliexpress. but this suggestion with the 18650 cells from the ring batteries is really great I've seen it on the site too BYD LiFePO4 24v Battery Pack with 3-4 kWh for 295$If you're willing to build a 24v@200ah 18650 battery... you could do this with 7s67p 3000mah cells. That's 469 cells at 3000mah each (or a few more cells at lesser mah each). It so happens that Battery Clearning House is selling RING packs - and they have 2 x 3000mah cells in each one. So if you bought 3 of these - https://www.batteryclearinghouse.com/products/lot-of-300-panasonic-ncr18650a-cells-in-ring-packs - (3 * $135) you'd have enough cells. I just processed 1600 of these in last 6 months and they were 99.9% 3000mah / great cells for low amp/cell (powerwall) type applications.
3 x $135 = $405 for 600 cells. *This is pretty cheap in the grand scheme of things*
OPUS(testers), wire, solder, holders, etc - $300?
BMS - $200?
Just to give you a baseline - I think you could build a 24v@200ah powerwall battery for <$1000 if you're willing to invest some time.