There's many ways to judge what "80%" is. The batteries will cycle many times every week. They will have much more energy flowing through them in a year than the storage we need for dunkelflaute would.

Yes, in terms of amount of energy stored at full capacity, batteries would not get us to 80%. But those last 20% have reduced requirements for charge/discharge rates, effiency, cost/kWh, which makes many other solutions viable.