A battery case must hold sulfuric acid, carry the weight of the plates and liquid, and let hydrogen escape from the top. Glass, hard rubber, salvaged polypropylene battery boxes and stoneware with a known lead-free glaze all hold the acid, while iron, steel, copper, aluminium and bare wood do not. Any salvaged vessel must have its previous contents identified and removed before acid goes in, because a residue can react with the acid.
Part of Battery Building and Maintenance
Choosing and preparing an acid-resistant container for a lead-acid cell from the materials a settlement has.
Only A Few Materials Survive Sulfuric Acid
The carrier lists the options for a case: hard (vulcanized) rubber, which it rates ideal; lead-lined wooden boxes, a historical method; glass jars, fragile but perfectly acid-resistant; glazed stoneware crocks; and pitch-lined wooden boxes, which it calls temporary because pitch degrades in acid. The lead-acid article adds hard plastic and notes that conventional car battery containers of hard rubber or polypropylene are excellent salvage (Lead-Acid Construction).
What to keep out: Sulfuric Acid lists iron or steel (rapid corrosion), copper or brass (rapid dissolution), aluminium (a violent reaction that makes hydrogen) and wood or organic materials (charring and degradation) as unacceptable. Container Selection adds that dilute sulfuric acid in a zinc-lined vessel produces hydrogen that can rupture or ignite it.
Glaze needs a special check. Chemical Vessels says never to use leaded or unknown glazes for acids, because acid leaches lead out of the glaze faster than water does. A crock with an unknown glaze puts lead into the electrolyte and then into whatever it touches.
What You Need
- A container of a suitable material — salvaged battery box, glass jar or lead-free glazed stoneware
- A lid with a vent — the lid keeps dust and contact out but must not seal; hydrogen must be able to escape from the top
- A tray or secondary container — to catch a leak
- Knowledge of what the container held — name it before you use it
Method: Prepare A Case
Step 1: Identify past contents. If you do not know what the vessel held, do not use it for acid. Fuel, fertiliser, pesticide, cleaner and bleach residues can react with sulfuric acid; acid on a hypochlorite bleach residue gives chlorine gas, a hazard described in Gas Hazards (the reaction itself is general knowledge, not from the other articles). Do not repurpose a vessel just because it is the right size and shape; the previous contents decide whether it is safe.
Step 2: Clean and test. Rinse thoroughly with water, then check for cracks by filling with water and watching the outside for dampness for a day.
Step 3: Size to the stack. Internal size is the plate stack plus 10 mm clearance each side and 20 to 30 mm below the plates for sediment (Cell Assembly).
Step 4: Provide a ledge or ribs to hold the plates off the floor, made of a material that does not conduct and resists acid.
Step 5: Fit a vented lid and place the case in a tray.
Worked Example: Weight Of The Liquid
Specific gravity is density relative to water, and water is about 1 kg per litre, so electrolyte at 1.265 weighs 1.265 kg per litre. A case that holds 3 litres of electrolyte therefore carries 3 × 1.265 = 3.8 kg of liquid before the plates are counted, and the plates add a good deal more, since lead is very dense (11.34 g/cm³, general knowledge, not from the other articles). Glass and thin stoneware crack under uneven loads and knocks, so support the case on a level, padded base and do not lift a filled cell by its rim.
Common Mistakes
- A sealed lid — hydrogen builds up inside the case and the first spark ruptures it
- A cracked case or hairline in glaze — acid weeps, then eats the surface below
- Wooden boxes with pitch only — the carrier calls this temporary
- Treating “the right size” as “safe” — salvaged vessels have histories (Spill Response lists the cleanup if one leaks)
Safety
A cracked case spills acid, and a sealed one holds explosive gas
A leak puts sulfuric acid on floors, wiring and skin, so stand every cell in a tray large enough to hold its whole volume and keep baking soda to hand for spills (never on skin). A cell sealed without a vent cannot release the hydrogen it makes on charge, which rises and collects at the top of the case until a spark ignites it between 4% and 75% of the air (Hydrogen Gas Hazard). Never use a former fuel, gas or chemical container for acid without establishing and removing its previous contents.
What’s Next
- Learn next: Terminal and Bus Bar Connections — joining plates and cells
- Combine with: Cell Assembly — putting the stack in the case