Speaking as someone who's extremely tempted to build his own e-bike from scratch, and also as someone who adores the idea of user-repairable hardware components

Speaking as someone who's extremely tempted to build his own e-bike from scratch, and also as someone who adores the idea of user-repairable hardware components, the battery of my e-bike is the one and only component that I do not trust to build myself. As much as possible, I want a tested and certified battery from a known, respected, susceptible-to-lawsuits manufacturer. The peace of not worrying that my battery is going to catch fire and explode while charging overnight is worth it.