HBerman

There is a note in my AMG EQEX4 manual to drain the battery down to 20% every 6 months and then to lock the car overnight for battery reset. What is the purpose? What happens if I don't do this regarding battery life/capacity?

bytemaster0

It recalibrates the BMS so it knows where low pack voltage is. Most helpful if you regularly drive the car to near 0%. Won't harm the battery if you don't do it, but it's useful to have your BMS be more accurate. I do it every 6 moths - bring the battery down to 15-18%. I set a charging pause from say, 10PM until 5AM, plug it in before I go to bed, and then let it resume charging after that so the car isn't drained completely before I have to go somewhere. If you never did this, then after a year or two you might risk running out of energy just prior to 0%. With the above steps it's easy to do, so I just do it around the 6 month mark and it's been pretty happy about it.

nosnoop

It's for cell balancing primarily.
Over time, the battery pack would develop imbalance when different cells have different level of state of charge (SOC). And the overall battery capacity would be limited to the weakest cell in the battery pack.

Cell balancing would transfer energy from cells with higher SOC to those with lower SOC until all of them have the same SOC.
That's why you need to park it overnight below 20%.
Some cars do top balancing when the car is fully charged.
Some others like Mercedes do bottom balancing when the battery pack is low in charge.

bytemaster0

Cell balancing can only happen during charging though. The <20% state is passive and doesn't involve charging, so it should only affect BMS low pack calibration. Balancing happens high-end and requires power input.