Water quality is critical for flooded lead-acid battery maintenance. Using the wrong water can cause battery failure within months. Here is what you need to know.
Why Water Quality Matters
Impurities in water (chlorides, iron, organic matter, silica) contaminate the electrolyte, accelerate self-discharge, promote grid corrosion, and reduce cycle life. Even small amounts of contamination accumulate over hundreds of charge cycles.
Acceptable Water Types
- Distilled water: Purified by boiling and condensation. Removes virtually all impurities. The standard for battery watering.
- Deionized (DI) water: Passed through ion exchange resin. Equally pure, sometimes cleaner than distilled. Fully acceptable.
- Reverse osmosis (RO) water: Membrane-filtered. Acceptable if TDS below 10 ppm. Check with battery manufacturer.
Unacceptable Water Types
- Tap water: Contains chlorine, fluoride, minerals — will damage batteries
- Well water: May contain iron, bacteria, minerals — not recommended
- Rainwater: Collects atmospheric contaminants — not recommended without testing
Water Filling Best Practices
- Always fill after a full charge, never before (electrolyte expands during charging)
- Fill to the level indicator (usually 1/2 inch above plates)
- Never overfill — electrolyte expands during discharge and can spill
- Use a battery watering gun or filling tube for safety
- Check monthly — evaporation and electrolysis consume water, especially in hot climates