A separator keeps each positive plate from touching its neighbouring negative plate while letting acid and ions pass, and the best choice for a built cell is microporous rubber salvaged from old batteries, followed by glass fibre mat and woven polyester cloth. Wood veneer works as a last resort but must be soaked in acid first to leach out organic compounds, and it gives a short-lived cell. Cut every separator slightly larger than the plate and set the ribbed side against the positive plate.
Part of Battery Building and Maintenance
Materials that insulate adjacent battery plates from each other yet stay porous to the electrolyte, and how to test them.
A Separator Must Be Insulating, Porous And Acid-Proof All At Once
Plates sit a few millimetres apart in the acid. If they touch, the cell shorts. If the separator blocks ions, the cell will not pass current. If it dissolves, shreds or leaches organic compounds into the acid, the cell fails or self-discharges. The carrier and the lead-acid article together list these materials:
- Microporous rubber — best, salvaged from old batteries
- Microporous polyethylene — salvaged from modern batteries
- Glass fibre mat — excellent when available
- Woven polyester cloth
- Dense wood veneer (cedar or redwood) — last resort, short life
- Ceramic tile fragments — a historical option; Planté cells used them
- Glass rods or wooden dowels — spacers for the formation stage only, not for the finished cell
The carrier adds that the ribbed face of the separator goes against the positive plate, to allow acid circulation. The lead-acid article says to soak wooden separators in sulfuric acid first to leach out the organic compounds that would contaminate the electrolyte.
What You Need
- Separator material — salvaged or cut to size
- Scissors or a knife — no metal that will shed into the acid
- Dilute electrolyte — Electrolyte Preparation
- A glass dish — for the leaching soak
- A voltmeter — for the test cell
Method: Prepare And Test Separators
Step 1: Cut to size. Cut separators slightly larger than the plate so no plate edge reaches its neighbour.
Step 2: Leach wood. Soak veneer in dilute electrolyte (not concentrated acid, which chars wood; Sulfuric Acid lists wood among the materials concentrated acid degrades). Replace the liquid when it has turned brown. Repeat until a fresh soak stays clear. This repeat-until-clear rule is practical advice, not a corpus figure.
Step 3: Rinse synthetic materials in distilled water to remove loose fibre.
Step 4: Test in a small cell. Build one cell with the new separator and compare it with a cell using a trusted separator: measure the voltage under the same known load and the water use over time (Galvanic Cell Principles shows the internal resistance method). A rise in internal resistance or unusual gassing points to a poor separator.
Step 5: Place the separator ribbed side to the positive plate, and check there are no gaps (Cell Assembly).
Worked Example: How Many Separators For A Battery
A cell of 9 plates (4 positive, 5 negative) has 8 gaps and needs 8 separators. Six cells need 6 × 8 = 48. Cut a few extra for damage. If the plates are 12 cm × 10 cm and each separator is cut 5 mm larger on every side (the margin is assumed here, not from the corpus), a separator is 13 cm × 11 cm, 143 cm² of material each, and 48 of them need 48 × 143 = 6,864 cm² or about 0.69 m².
Common Mistakes
- Reusing a separator with holes or tears — a thin spot lets a paste fragment bridge the plates
- Putting the plain face to the positive — acid circulates less well
- Skipping the leach for wood — organic residue contaminates the electrolyte
- Leaving a rough grid edge — a sharp edge cuts through the separator (Grid Casting says to file off flash)
Safety
A failed separator shorts a charging cell
When a separator fails the plates touch, the cell shorts internally, current concentrates through the contact and the cell heats and gasses while the rest of the bank keeps pushing current into it. Hydrogen accumulates at the top of the case and a spark can ignite it. Check separators before sealing a cell, and watch the first charges for heat and uneven gassing (Hydrogen Gas Hazard). Glass fibre mat irritates skin and lungs (general knowledge, not from the other articles), so wear gloves and avoid breathing the loose fibre.
What’s Next
- Learn next: Battery Case Construction — the container the stack goes in
- Combine with: Cell Assembly — placing separators in the stack