If the battery is full - motor regen could be shut off and only mechanical braking can be used. This how most electric cars work. When the battery percentage is close to 100% the regen is disabled and you can only use mechanical braking. A more complex way is through heat dissipation using a brake resistor or similar. However i don't know if this has ever been done in practice because the regeneration would generates a lot of kW.

Thanks Jordan, very insightful. Laser welding seems to be the most versatile method.

Ultrasonic welding or laser welding are the two most commonly used welding processes. Many pros and cons to each process but it can all depend on the thickness of your busbars. Laser welding is more suited when your battery pack has a high amperage output, thus needing thicker busbars. In relation to dissimilar metals, again laser welding is more suited for joining dissimilar metals but you may need to nickle plate the copper bus bars to prevent galvanic corrosion - someone else might be able to chime in on this... The material thickness will depend on the amps your pack is outputting. For the joint design - if using a laser welder, typically it is set to weld a predefined circular shape.