Battery usable capacity over time

As with each charge/discharge cycle the usable capacity of the battery drops, how does it reflect on bms settings in terms of capacity and voltage?
I've 400Ah pack @48v, how does the bms know the capacity has dropped to say, 380Ah. I use daly BMS.

Solarguy said:

As with each charge/discharge cycle the usable capacity of the battery drops, how does it reflect on bms settings in terms of capacity and voltage?
I've 400Ah pack @48v, how does the bms know the capacity has dropped to say, 380Ah. I use daly BMS.

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It likely doesn't. BMSs that report a state of health have some algorithm that estimates cell degradation likely based on voltage/current/SoC behavior. I can only recall seeing this on "server rack" batteries, but these are "canned" batteries where the BMS can be customized for the cells.

I would assume that you need to periodically test the battery with a full cycle discharge test and determine its capacity with a DIY or a "generic" BMS like DALY, JBD, etc.