Reusing fritter batter the next day is generally not recommended. The batter's leavening agents lose effectiveness, and the flavor can deteriorate.
Fritter batter typically contains ingredients like baking powder or baking soda to help it rise and create a light, airy texture when fried. These leavening agents are most effective when freshly mixed. Over time, they lose their potency, resulting in fritters that are dense and flat. Additionally, the batter can absorb flavors from the refrigerator, negatively impacting the taste of your fritters. The wet ingredients can also start to break down the dry ingredients, leading to a less desirable consistency. For best results, always prepare fritter batter fresh just before frying.
If you absolutely must save fritter batter, store it in an airtight container in the refrigerator and use it within 12 hours. Be aware that the results may not be as good as with freshly made batter. You might need to add a tiny pinch more of baking powder to try and revive the leavening.