Station Battery Capacity Curve

Where have you looked so far?

Dp you have ANY USEFUL INFORMATION TO PROVIDE?

You could ask if all that you know is that it is a battery.

Google has it but you have to tell it the specifics like the brand, type, capacity, ratings, etc. of the battery, even Google can't read minds...yet.

The total output of the battery (Ampere-hours) depends upon the current drawn by the substation during outages. This will include circuit breaker charging motors, protection relays, indicator lamps, etc. The greater the current drawn the shorter the time that the battery is available for use.

Am I over-simplifying this or do I not understand the question?

You need the specific exide battery model used. Exide makes station batteries from small 12 volt models used usually in a bank of 10 which aren't great to large 2 volt cells in a bank of 60 or 62 that are really great. They are good at answering questions (or were a few years ago at least) so contact them. They will provide all the answers you need. There are generic values for state of charge vrs voltage in general for lead acid batteries but the specific gravity is the best way to measure state of charge if that is what you want.

Try the battery manufacturer's websites, that's where I got mine when I last needed them for an application. If not, contact the battery manufacturers directly.