I assume they used to fire up the coal plant to fill gaps but now use the battery which stores excess energy generated.

Edit:

The plant’s 185 megawatts of instantaneous discharge capacity match what the old coal plant could inject into the grid, though the batteries react far more quickly, with a 250-millisecond response time. Instead of generating power, they absorb it from the grid, ideally when it’s flush with renewable generation, and deliver that cheap, clean power back in the evening hours when it’s desperately needed.

Seems pretty clear to me.

Coal is also energy storage, well, all fossil fuels. That's their primary advantage, on-demand easily accessible stores of energy.

If only they didn't simultaneously pollute and cook the earth when used...

in terms of usage though, they are quite similar. Coal serves on-demand power, whereas renewables generate power at times that don't always align with demand. Batteries can take the role of a coal plant if the renewables already generate sufficient energy, just at the wrong times.

No, the plant is full of primary batteries! 4.2 million AA cells!

All our "energy production" just converts energy from one state into another. Like a battery.

A dam is often referred to as a giant gravity potential energy battery.