It’s not the battery being used for seasonal storage but the difference in rates of battery degradation.

Suppose a normal discharge cycle costs 10c/kWh and a very deep discharge costs 100c/kWh to access that last 10%. That means there are energy reserves unacceptably expensive for normal operations, but it also means simply operating batteries efficiently automatically creates reserve capacity which is the entire point of dedicated seasonal storage.

Put another way if you design a 1TWh battery for daily use it’s going to have a 0.1TWh reserve capacity just sitting there.

This isn’t a lot of power across a full season, but it address seasonal storage in terms of short term abnormal peaks like heatwaves.

PS: Proponents of seasonal storage argue for seasonal deficits in production rather than short term gaps. However, the daily variability of renewable production promotes significant excess generation capacity.

Yes, what you're saying works. But only for short term outlier events, not storing power from summer for winter.

We can’t predict seasonal demand that accurately so trying to use a finite reserve for anything but outlier events is risky. Plan for a large deficit and the infrastructure you create to fill that reserve is sitting around in the off season reducing the size of your deficit. Plan for a small one and your estimate may be wildly inaccurate.

What’s left for “seasonal storage” is to cover gaps in production from extreme outlier events using surplus production. That is useful, but also largely covered by battery power as I just covered.

Granted if someone comes up with cheap enough seasonal storage it might have a place, but that’s a possibility not a guarantee.

If you use Ammonia as fuel, you can ship it worldwide in an energy crunch.