If you follow your chemistry you'll find the reason you add water is because the acid has turned into lead sulfate.. this happens by large current draws across the cells. This causes boiling around the plates which releases the hydrogen gas. The acid water solution is designed to cool these plates as well as provide electron transfer. Natural evaporation occurs as well since evaporation is based around exciting electrons. A cup of water in a room will evaporate even when the room is 50 degreesF. When you add water and then apply electricity, you will increase the acidity and cause the lead sulfate process to reverse. This is why you never have to add acid (in theory). A little cup like at your office water cooler works great. If you squeeze it then it forms a perfect funnel for pouring water into those little holes. Holds almost enough water to top off a cell. DI water is not recomended as it will not support ion transfer but actually inhibit it.
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