Charging a lead-acid cell past full splits water into hydrogen and oxygen, so the level falls while the acid stays behind, and the cure is distilled water, never acid and never tap water. Check the level monthly and top up until the liquid stands 10 to 15 mm above the plate tops, not to the brim. Each ampere-hour of gassing current removes about 0.34 g of water from a cell.
Part of Battery Building and Maintenance
Topping up the electrolyte of a flooded lead-acid cell with distilled water, and estimating how fast it is lost.
Water Leaves The Cell As Gas, And The Acid Stays
When the plates are full, extra charging current electrolyzes the water in the electrolyte (Lead-Acid Chemistry). The hydrogen and oxygen escape; the sulfuric acid does not. That is why the lead-acid article stresses that water is consumed during charging, not acid, and why adding acid to a cell with a low level makes the electrolyte too strong and corrodes the plates. A plate that stands above the liquid dries out, oxidizes and cannot work.
Use only distilled water. The carrier says tap water’s minerals poison the plates, and Rechargeable Batteries specifies distilled water, meaning condensed steam. The level target is 10 to 15 mm above the plate tops (carrier), and the energy-storage article says fill to just above the plates, not to the brim, because gassing throws a little liquid out of an over-full cell.
What You Need
- Distilled water — condensed steam (Distillation covers a still; collect clean condensate in glass)
- A clean glass or plastic funnel, or a squeeze bulb — no metal
- A level mark on the case or a clear gauge stick
- A tray, rags and baking soda — for drips
Method: Top Up A Cell
Step 1: Check the level monthly (carrier), more often if the bank is cycled daily or equalized.
Step 2: Look at the plates. If any plate top is dry, add water at once and then charge; the dry part has been damaged by air.
Step 3: Add water slowly until the liquid is 10 to 15 mm over the plate tops. Do not fill to the brim.
Step 4: Mix. General knowledge, not from the other articles: water poured onto denser acid floats on top until the cell is charged, so it is best added shortly before or during a charge and not after, when the cell is cooling and mixing is slowest.
Step 5: Wipe and log. Wipe any water off the top of the cell, and record the water used per cell.
Worked Example: How Fast A Cell Loses Water
At 0.34 g of water per ampere-hour of gassing: 1 faraday splits half a mole of water, 9 g (general knowledge: 18 g/mol, 2 electrons per molecule) and 26.8 Ah = 1 faraday, so 9 ÷ 26.8 = 0.336 g per ampere-hour. Suppose a cell receives 10 Ah more than it gives back on each cycle, and cycles 30 times a month: 10 × 0.336 = 3.4 g (3.4 mL) per cycle, and 30 × 3.4 = 101 mL per month.
In a case with an assumed internal plan area of 10 cm × 14 cm = 140 cm², a 10 mm fall in level is 140 cm² × 1 cm = 140 mL at most, since the plates occupy some of the space. So 101 mL a month is roughly a 7 mm fall, comfortably inside the 10 to 15 mm margin. The check interval therefore needs to be monthly or shorter, and the rate rises with overcharge, so a cell that uses much more than this has too high a charging voltage (Float Charging).
Common Mistakes
- Adding acid to raise a low level — the acid does not evaporate; adding it makes the electrolyte too strong
- Tap or well water — dissolved minerals contaminate the electrolyte
- Filling to the brim — gassing pushes acid out of the cell
- Letting plates dry — the dry zone loses capacity permanently
Safety
A topping-up job puts acid and a possible hydrogen pocket in your face
Electrolyte splashes when water is poured, and the headspace of a recently charged cell holds hydrogen that a spark can ignite between 4% and 75%. Wear splash goggles, stand back from the opening, keep flames and cigarettes away, and use insulated tools around the terminals (Acid Handling, Hydrogen Gas Hazard).
What’s Next
- Learn next: Desulfation Techniques — when a low level has let a plate sulfate
- Combine with: Maintenance and Reconditioning — the full routine