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.

Depends on how you're slicing it. By the total flux of energy batteries may be able to handle 80% of the "energy traffic" over the course of a day by smoothing out day/night cycles.

However that last 25% needs to be stored over long periods where the cost is prohibitive to store in current battery systems.