Beat the eggs with an electric mixer on medium-high speed for 2-3 minutes until pale and slightly thickened.
Gradually add the sugar while beating on medium-high speed, then continue beating for another 2-3 minutes until light and fluffy.
In a small bowl, stir the baking soda with the apple cider vinegar (it will fizz). Quickly add this mixture to the egg mixture and stir well to combine.
Add the cinnamon and nutmeg to the egg mixture and stir well to combine.
Gradually add the flour in 3 additions, folding gently with a spatula after each addition until the dough is smooth with no flour streaks.
Grease and flour a baking dish.
Pour half of the dough into the prepared dish, scatter the grapes evenly over the dough, then pour the remaining dough on top and smooth the surface.
Bake in a preheated oven at 180°C (356°F) for 30 minutes until the charlotte is golden brown.