This is how a battery works

Lead plate as an electrode that comes into contact with dilute sulfuric acid (electrolyte), under the influence of the elasticity of dissolution, releases positive ions into the electrolyte, i.e. electrically charged particles. Due to this, an electrical voltage is created between the lead plate and the electrolyte.

But in practice, one does not rely on this voluntary transfer of charged particles, but forces the battery to charge voltage. The effect is that the lead sulphate of the plates of a discharged battery is converted at the positive electrode into lead dioxide and at the negative electrode into spongy lead. At the same time, sulfuric acid is again formed in the electrolyte, and rising gas bubbles are an external sign of an almost completed charging process. When discharging, the reverse process occurs. The lead dioxide of the positive plate and the spongy lead of the negative plate are converted back into lead sulphate, while sulfuric acid is consumed and water is formed. Therefore, with discharge, the density of the acid decreases.