What people often refer to as battery acid is infact pattasium carbonate. In side alkaline the electrolyte is often made from potassium hydroxide. When the potassium hydroxide leaks, it reacts with the carbon dioxide in air to make potassium carbonate. This is the hard, powdered substance that you find on the cathode of a battery. Interestingly it is only ever the cathode. This is because of the way that batteries are constructed. Hydrogen builds up in batteries over time and the seal on the cathode breaks, allow chemicals to escape.