Today the Arduino based spot welder arrived: DIY Arduino Battery Spot Welder Prebuilt Kit V4 - Malectrics (it arrived quickly, because the seller is also located in germany)
I also purchased a 12V 50A Lead Acid Car battery locally.
This is the arduino based control board:
The arduino basically controls the pulses to do the spot welding. with the knob you can set the desired pulse length.
I purchased the board, a (3D printed) case and the welding cables/probes.
It took me about 20 minutes to put everything together.
I have also ordered a BMS that is capable of managing 13 cells in series and a discharge current of 300A. This is the one: 8 S 24 S smart display 2,4 V oder 3,2 V oder 3,7 V 300A lithium batterie schutz bord ausgewogene management system BMS Coulomb Zähler|display 6s|counter displaydisplay counter - AliExpress
Additionally I ordered 3 sets (6 pieces total) of these plastic parts: 5*13 (5P13S) 18650 batteriehalter und reinem nickel Für 13 S 48 V 10Ah 15Ah lithium akku 5P13S halter Fabrik direkt versorgung|18650 battery holder|battery holderbatteries battery holder 18650 - AliExpress
I will attach three of these together (with some glue and by spot welding the nickel strips) which will result in my 13S15P Setup. I bought the plastic part w/out the nickel strips, as I wanted to purchase them separately:
Freies Verschiffen 18650 batterie reiner nickel streifen 18650 zelle nickel band 0,15*27*5000mm nickel gürtel verwendet für 18650 batteriehalter|belt belt|belt freebelt b - AliExpress
I still need to buy the 18650 cells, however I am waiting for some quotes first (I need 195, but I will be buying 200, to have 5 as a spare).
The charger that I am planning to buy is the Cycle Satiator. It is quite versatile, so it will be able to charge the 12V car battery as well as the 48V OpenPPG battery (once it is fully assembled).
I will post more pics once I receive all the material and can start building.