I've been looking into building a DIY energy storage system for my electric vehicle project, and I've come across the idea of using 18650 cells to create a custom battery pack. It seems like a popular option given their availability and price.

However, there are a few aspects I'm unclear about:

Configuration and Capacity: What's a good configuration for balancing capacity and voltage? I've seen terms like "10s4p" thrown around, but I'm not entirely sure how to interpret these or what would work best for an EV application.
Safety Concerns: I've read about the risks of overheating and thermal runaway with these cells. What are some recommended safety measures? Are there particular types of 18650 cells that are more reliable or safer for these applications?
Battery Management System (BMS): How critical is it to have a BMS, and any suggestions on reputable brands or models that work well with a 18650 setup?
Assembly Tips: Any advice for assembling the pack itself? I've heard spot welding might be necessary, but I’m wondering if there are other methods. Also, what kind of housing should I consider for protection?
Future-proofing: With technology rapidly advancing, how easy is it to upgrade or replace 18650 cells in such a custom setup if needed down the line?

Thanks in advance for any insights or advice! Would love to hear about any personal experiences or lessons learned from those who've gone down this path.