Soak it in diluted acid.

Yep. Rinsing only removes loose debris. That white, crusty calcium scale is chemically bonded to the cell plates, and you need an acid soak to dissolve it.

The usual approach is a 4:1 water-to-muriatic acid mixture, or you can buy a commercial cell cleaner (same stuff, just more expensive). Submerge the cell for about 10-15 minutes until the bubbles stop, then rinse thoroughly. Wear gloves and goggles—no shortcuts here.

Don’t scrub with a wire brush or metal tool. That damages the catalytic coating and kills the cell. And never add water to acid; always add acid to water.

Do this once or twice a season, and your cell will last years longer.