Calculating degraded battery capacity

Question

I'm working on a project where we want to predict the future battery degradation using the historical battery data. Now the battery capacity is an important value in this equation. I have done some research on the workings of batteries, battery degradation, and more, as I'm still quite new in this field.

This is the setup I'm working with: 4x Victron AGM deep Cycle batteries, 12V, 230Ah @ C20. Here is a link to the battery datasheet: https://www.victronenergy.nl/upload/documents/Datasheet-AGM-Super-Cycle-battery-EN.pdf (article number: BAT412123081).

Because the current capacity is not directly stored (as far as I could see), I saw that I could calculate it with either a constant current or constant power load. However, I don't have either in my situation.

Are there other ways I can get/calculate the battery capacity, or am I maybe doing something wrong?

Answer 1

Capacity is measured, not calculated. If it cannot be measured, it might be estimated (with some significant uncertainty).

Unless the load current is predictably repeatable, effective capacity changes from cycle to cycle. A cycle at about 1 A will result in a different effective capacity than a cycle at about 10 A, especially for lead acid batteries (Peukert's law, of course).

If you want to measure the nominal capacity (measured at C/20 current), there is no way around doing so than by discharging the battery over 20 hours at a constant current.