Nothing you add to a battery is cheaper than water, and nothing added carelessly costs more. A flooded lead-acid battery drinks throughout its life — modestly if well charged, thirstily if hot or overcharged — and every litre that goes in stays in, along with everything dissolved in it. The cell is a closed loop with a long memory: it concentrates every sin, forgives none, and presents the bill months later as lost capacity and shortened life. This is the guide to what goes in the cell, why the battery drinks, and how disciplined watering works.
Why the battery drinks at all
Water leaves a flooded cell one main way: electrolysis. Near full charge, part of the charging current stops converting active material and starts splitting water into hydrogen and oxygen — the gassing that the charging guide describes and every equalising charge deliberately employs. The acid itself never leaves; only water does, which is why electrolyte grows STRONGER as the level falls, quietly accelerating corrosion in the exposed heat. Three things set the drinking pace: charging practice (the closer to correct, the less gassing), temperature (evaporation and reaction rates both obey Arrhenius), and age — as antimony migrates within the ageing cell, gassing gradually increases, so an old battery drinks faster than it did in youth, one of the honest signatures of advancing battery age. A watering log is therefore a diagnostic instrument: a cell suddenly drinking more than its siblings is raising its hand.
What water is allowed in
Only demineralised or distilled water — and the reason is chemistry, not fussiness. Tap, well and “clean drinking” water carry dissolved minerals, and the classic battery literature, from Vinal’s handbook onward, is blunt about what they do inside a cell. Iron is the notorious one: even traces set up wasteful internal reactions that raise self-discharge — the battery leaks its charge from the inside. Chlorides attack the positive plates and accelerate corrosion. Manganese, copper and organic matter each bring their own mischief. None of them ever leave: every careless litre adds its minerals to the acid FOREVER, and ten years of watering multiplies whatever the water carried. Battery makers therefore publish water-purity specifications — ours is available for the asking — and the practical rule is simple: buy packaged demineralised water of stated quality, store it in clean closed plastic (never metal, never an old acid container), and dedicate a plastic jug, funnel and filler to battery duty alone. RO water from a maintained plant generally serves; water from a rusty drum does not, whatever it started as.
The watering ritual — level, timing, and the two forbidden acts
The ritual is short. Check levels on a schedule — monthly is the classic flooded-battery rhythm, oftener in hot rooms — and top up to the maker’s level mark: plates and separators must never see air, yet headroom must remain, because electrolyte swells and stirs on charge and an overfilled cell spits acid over its own lid, where it feeds exactly the leakage currents and terminal corrosion a good walk-round exists to catch. Prefer topping up toward the end of charge, when levels read true and mixing is immediate — water added to a discharged cell sits in a layer and can mislead the hydrometer for hours. And the two forbidden acts, stated once more because the field keeps committing them: never add ACID to correct a low reading — low gravity is a charging problem or an ageing problem, and acid “corrections” poison the balance the maker built (the filling guide owns the one legitimate exception: replacing physically spilled electrolyte); and never add ANYTHING ELSE — no miracle powders, no rejuvenator bottles, nothing the myths guide has already buried. Water of the right purity, to the right level, at the right time. That is the entire art.
Watering at fleet scale
One battery is a jug; a forklift fleet or a hundred-cell substation bank is a system, and hardware exists for it. Electrolyte level indicators — fitted on our mining and traction ranges — show at a glance which cells ask for water, sparing the vent-plug parade. Watering guns with level-sensing nozzles fill to the correct level and stop themselves. Single-point watering systems plumb a whole battery’s cells to one filling port with float valves per cell — minutes per battery instead of an hour, and consistent levels the log can trust. The economics favour them quickly at fleet scale, because the alternative is not really “manual watering”; in practice it is SKIPPED watering, and the exposed-plate damage that follows. However the water gets in: log it. Date, cell, volume if you can. The battery keeps its ledger either way — better that you keep one too. Ask us for our water specification or for watering-system options on your fleet; both answers are free and both prevent expensive repairs.