Curing hardens and bonds the paste to the grid, and formation then converts it electrically, so the positive plates become brown lead dioxide and the negative plates gray sponge lead. Form at a constant current of one twentieth of the target capacity in amperes, for a 50 Ah cell 2.5 A, in very dilute sulfuric acid for one to three days. A plate that is under-formed gives poor capacity, so test the finished cell rather than trusting the colour.
Part of Battery Building and Maintenance
Curing pasted plates and forming them by charging into the positive and negative active materials of a lead-acid cell.
Curing Bonds The Paste, Formation Makes It Active
A pasted plate is not yet a battery plate. During curing the paste chemically bonds to the grid, hardens and changes colour slightly (Plate Pasting). Formation is the first charge: it converts the cured paste to lead dioxide on the positive plates and sponge lead on the negative plates, using the reverse of the discharge reaction (Lead-Acid Chemistry). The carrier calls formation the single most important step, because under-formed plates mean poor capacity.
The Corpus Gives Two Curing Recipes And Three Formation Acids
Curing: the carrier stacks plates with spacers at 35–50°C and 90%+ humidity for 24–72 hours. The lead-acid article cures at room temperature for 24 hours, then at 60°C for 12 hours to dry without cracking. These are different ideas, one humid and one dry, and the corpus does not reconcile them. Trial plates from the same batch, cured both ways and then formed identically, show which your paste prefers: cracking, flaking and capacity are the measure.
Formation acid: the carrier uses specific gravity 1.050, the lead-acid article 1.050–1.100, and Rechargeable Batteries 1.15. All three are very dilute compared with the 1.265 working strength. The lead-acid article says the dilute acid reduces heat during the initial high-resistance formation charge. Use the weakest of the three first.
Formation time: 24–48 hours (carrier), 48–72 hours (lead-acid article), and the rechargeable article adds three to five discharge-recharge cycles until full capacity develops. The rate is C/20 in both of the first two.
What You Need
- Cured plates — positives and negatives, labelled
- A glass, ceramic or hard-rubber vessel — large enough to hold the plates with spacers (Battery Case Construction)
- Insulating spacers — the lead-acid article uses glass rod spacers or wooden dowels for formation
- Dilute sulfuric acid — Electrolyte Preparation
- A DC source with current limiting — Constant Current Charging
- A voltmeter and an ammeter — Metering
Method: Cure And Form
Step 1: Cure. Stack the pasted plates with spacers so air reaches every face, in warm humid air for the first recipe or room air then gentle heat for the second. Do not rush: plates that crack here crack in the cell.
Step 2: Set up the formation vessel. Alternate positive and negative plates with spacers in dilute acid. Confirm which plate connects to the source’s positive (Polarity Check).
Step 3: Charge at C/20. For a target capacity of 50 Ah this is 2.5 A (carrier). Hold the current steady for the stated time.
Step 4: Watch the colour. Positives turn dark brown and negatives gray. The lead-acid article treats gassing as a sign that formation is nearly complete.
Step 5: Discharge and recharge. Discharge through a resistor, then recharge, and repeat three to five times (Rechargeable Batteries).
Step 6: Raise the acid to the working strength of 1.265 (lead-acid article) once the plates are formed, then assemble the cell (Cell Assembly).
Worked Example: How Much Charge Formation Passes
At the C/20 rate for a 50 Ah target: I = 50 ÷ 20 = 2.5 A. For 24–48 hours: charge = 2.5 A × 24 h = 60 Ah to 2.5 A × 48 h = 120 Ah. That is 60 ÷ 50 = 1.2 to 120 ÷ 50 = 2.4 times the rated capacity. The excess is expected: the first charge of an unformed plate is inefficient, and the corpus gives no efficiency figure. Late in formation most of the current splits water. At the 0.42 L of hydrogen per ampere-hour of water electrolysis (Water Electrolysis), a 2.5 A current that is entirely gassing makes 2.5 × 0.42 = 1.05 L of hydrogen per hour.
Common Mistakes
- Reversing the connections — check polarity before switching on, since the plates would be formed the wrong way
- Forming in strong acid — the corpus uses very dilute acid to limit heat
- Judging by colour only — a uniformly coloured plate can still be under-formed; check capacity on the finished cell by discharging it to 1.75 V per cell
- Skipping the cycles — the rechargeable article asks for three to five
Safety
A formation tank makes hydrogen for a day or more
Formation runs for 24 to 72 hours, and in its late stage the current splits water. The hydrogen rises and collects at the highest point of the room, and a spark from a switch, a loose clip or an unplugged lead ignites it between 4% and 75% in air. Form outdoors or under a roof open at the top, switch the supply on and off at a distance from the tank, and tighten every connection before powering up. The acid, though dilute, still burns eyes (Hydrogen Gas Hazard).
What’s Next
- Learn next: Cell Assembly — putting formed plates into a cell
- Combine with: Specific Gravity Control — adjusting the acid after formation