Lead-acid batteries

One lead-acid cell contains:

Nominal cell voltage ≈ 2 V. Industrial strings stack many cells for 24 V, 48 V, and higher DC systems.

Multiple plate sets interleaved increase surface area → higher ampere-hour capacity without changing chemistry.

Specific gravity compares electrolyte density to water (water = 1.000). Fresh acid for service often reads about 1.215 – 1.280 depending on application and manufacturer.

A hydrometer (or refractometer in some shops) samples electrolyte from each cell. Higher acid concentration → higher gravity → more charged.

As the cell discharges, acid is consumed and water forms → gravity drops. That is why gravity is a classic state-of-charge indicator — when chemistry is still healthy.

With a load connected:

1. Negative plate: lead atoms lose electrons → Pb²⁺ ions combine with sulfate (SO₄²⁻) → lead sulfate (PbSO₄) coats the plate.
2. Positive plate: lead dioxide gains electrons; oxygen leaves the compound; Pb²⁺ combines with sulfate → PbSO₄ on the positive plate too.
3. Electrolyte: two hydrogen ions (H⁺) react with liberated oxygen → water forms; acid weakens.

Both plates trend toward lead sulfate; electrolyte gets more dilute. Terminal voltage falls under load as internal resistance and chemistry shift.

Charger positive to battery positive, negative to negative. Charger voltage must exceed battery terminal voltage to push current into the cell.

During charge:

• At the negative plate, hydrogen forms; sulfate returns to acid; PbSO₄ converts back toward lead.
• At the positive plate, water breaks down; oxygen recombines with lead dioxide chemistry; PbSO₄ converts back toward PbO₂.
• Sulfuric acid strength rises again — hydrometer reading climbs toward fully charged values.

Stop or taper charge per manufacturer — “more amps forever” is not better.

General guideline: charge current ≤ 1/10 of ampere-hour rating unless data sheet says otherwise (80 Ah → ≤ 8 A).

Overcharge risks:

• Excess hydrogen (highly explosive) — no sparks, flames, or unvented arcs near batteries on charge
• Heat — many designs limit about 110 °F (43 °C) on the case
• Sulfate flakes breaking off plates, falling to the bottom → permanent capacity loss; flakes touching plates → shorted cell

Mechanical shock and high charge rate can shed sulfate the same way. Sealed gel types immobilize electrolyte and use pressure-relief valves (~35 psi) but still need correct charge voltage.

Ampere-hour (Ah): often measured at 20-hour rate at 80 °F — e.g., 4 A for 20 h = 80 Ah.

Cold-cranking amps (CCA): max initial current at 0 °F (−18 °C) for engine starting — not the same as Ah.

Hydrometer: quick per-cell state-of-charge if plates are healthy. Low gravity after full charge → sulfated or damaged plates.

Load test: apply about 3 × Ah as test current; 12 V battery should stay above 80% of nominal (≥ 9.6 V on 12 V) for 3 minutes. Reveals weak cells better than open-circuit voltage alone.

Lead-acid remains default for engine start, UPS, and many lift trucks because of cost, surge capability, and recyclability.

Situation. A forklift battery room smells sharp; charger still in bulk mode overnight. Morning: one cell boils dry, gravity never reaches spec, truck dies mid-shift.

Diagnosis path. Overcharge raised temperature, gassed hydrogen and oxygen, shed sulfate, and stratified electrolyte. Hydrometer after a “full” charge still read low — classic damaged plate or shorting debris, not “needs more amps.”

Applied lesson. Follow Ah/10 charge guidance, monitor temperature, equalize only when manufacturer allows, and load-test before blaming the truck controller.