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.
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.
So rather than having a singe battery doing nothing for 364 days a year and getting used one day you have a battery bank which gets discharged slightly more 1 day a year and recovers that deficit over some time period.
For redundancy reasons you want excess generation capacity should something happen which would most of the time allow for a full charge soon afterwards.
PS: individual wind locations also tend to get more power on specific time of the year which can offset seasonal issues.