Learn how to safely build custom 18650 lithium-ion batteries in this comprehensive 20-minute video tutorial. Explore the fundamentals of 18650 cells, understand voltage, current, and capacity concepts, and master battery calculations. Discover tips for purchasing reliable cells, testing second-hand batteries, and constructing battery frames. Gain insights into nickel joining strips, spot welding techniques, and alternative assembly methods. Cover essential topics such as balance leads, terminals, charging, and battery management systems. Conclude with a practical example of a 3D printer battery and access valuable resources for further learning. Pay close attention to safety warnings and consult professionals before undertaking any battery-building projects.