Lead-acid batteries (flooded and VRLA/AGM/Gel) usually fail for predictable, preventable reasons. This article organizes the common failure modes, explains the electrochemistry in plain language, and outlines field diagnostics and maintenance practices to extend service life.
- What it is: Float or equalize voltages set too high (often above ~2.30 V per cell at 20–25 °C) causing excess gassing and heat. VRLA cells are especially vulnerable.
- Damage pattern: Water loss and dry-out (VRLA), accelerated positive-grid corrosion, active-material shedding, and risk of thermal runaway.
- Field clues: Warm case on float, rising water consumption (flooded), abnormally high cell voltages, increasing ohmic values.
- What it is: Float set too low, infrequent equalize, or chronic partial state of charge.
- Core mechanism — sulfation: Hard, coarsened \(\mathrm{PbSO_4}\) crystals resist reconversion, shrinking reactive area.
- Field clues: Low capacity despite “full” indicators, sluggish recharge, cells floating lower than peers.
Overall cell reactions (idealized):
$$
\textbf{Discharge:}\quad \mathrm{Pb} + \mathrm{PbO_2} + 2,\mathrm{H_2SO_4} \rightarrow 2,\mathrm{PbSO_4} + 2,\mathrm{H_2O}
$$
$$
\textbf{Charge:}\quad 2,\mathrm{PbSO_4} + 2,\mathrm{H_2O} \rightarrow \mathrm{Pb} + \mathrm{PbO_2} + 2,\mathrm{H_2SO_4}
$$
- Mechanism: The positive grid slowly oxidizes, forming a thick, brittle corrosion layer that reduces conductor cross-section and can lift posts.
- Effect: Higher internal resistance \(R_\text{int}\), hot spots, uneven charge acceptance, and capacity loss.
- Mechanism: Cycling and gassing detach newly formed \(\mathrm{PbO_2}\) from the grid; shed material becomes electrically inactive.
- Effect: Reduced available capacity and early voltage drop under rated load.
- Mechanism: Corroded straps or lifted posts add connection resistance and imbalance.
- Effect: Local heating and misleading string-level readings.
- Causes: Chronic overcharge, high ambient temperature, or missing temperature compensation.
- Effects: Reduced ionic pathway, rising \(R_\text{int}\), local heating, and runaway risk.
- Causes: Case cracks, vent defects, external contamination.
- Effects: External shunts, charge imbalance, and safety hazards.
- Signature: One block shows abnormally low voltage while neighbors rise to absorb the charger’s total voltage; heat and gassing spike in the “healthy” cells.
- Signature: One block reads abnormally high (no current sharing); string charge current collapses.
- Healthy float: Cells cluster around \(2.25{-}2.30\ \mathrm{V/cell}\) at 20–25 °C.
- Warning signs: Wide spreads, drifting outliers, or rapid movement relative to peers.
String voltage across \( N \) series cells:
$$ V_\text{string} = \sum_\text{i=1}^{N} V_i $$
A positive feedback loop: higher temperature increases charge current at fixed voltage, which raises \(I^2R\) (Joule) heating; drier separators (VRLA) increase \(R\), which further raises heat.
Runaway intuition:
$$
P = I^2 R, \qquad \frac{dR}{dT} > 0 ;\Rightarrow; \frac{dP}{dT} = I^2 \frac{dR}{dT} > 0
$$
- Excessive float/equalize voltage
- Missing temperature compensation
- High ambient temperature
- Shorted cells or poor heat dissipation
- Keep float within spec (≈ \(2.25{-}2.30\ \mathrm{V/cell}\) at 20–25 °C)
- Use temperature-compensated charging
Typical compensation slope per cell:
$$
V_\text{float}(T) \approx V_\text{float}(25^\circ\mathrm{C}) + \alpha,\big(T-25^\circ\mathrm{C}\big), \quad \alpha \approx -3~\mathrm{mV/cell/^\circ C}
$$
- Respect charge-current limits
- Maintain ventilation and replace suspect cells early
- Idea: Internal elements behave like a network of resistors and capacitors; changes correlate with capacity and faults.
- Metric: Conductance \(G\) (AC method) is often used and trends downward with degradation.
Conductance (conceptual):
$$
G = \frac{I_{\mathrm{ac}}}{V_{\mathrm{ac}}},\cos\varphi
$$
- How to use: Track each cell’s ohmic value over time; trend analysis beats single absolute thresholds. Cells trending below ~80% of the site-specific reference should be watched; below ~60% often indicates high risk.
- Method: Discharge with a controlled load to the specified end-voltage at the rated time (e.g., C10 to 1.80 V per cell). Log block voltages and temperatures.
Capacity calculation:
$$
%\ \mathrm{Capacity} = \frac{T_s}{T_a}\times 100
$$
where \(T_s\) is the actual discharge time and \(T_a\) is the standard/rated time.
- Float voltage: Hold within manufacturer spec (≈ \(2.25{-}2.30\ \mathrm{V/cell}\) at 20–25 °C).
- Temperature compensation: Verify sensors and set points; a fixed float in hot rooms is a frequent root cause.
- Equalize: Apply judiciously to counter mild sulfation; avoid chronic over-equalizing.
- String balance: Periodically log cell/block voltages and ohmic values; act early on outliers.
- Thermal control: Ensure airflow; avoid hot spots; watch VRLA for dry-out symptoms.
- Charge-current limits: Many standby systems recommend \(\le 0.1,C\) to limit heating stress.
Lead-acid batteries rarely “just fail.” The usual culprits are incorrect float/equalize settings, heat, missing temperature compensation, and slow-creeping defects like sulfation, corrosion, dry-out, and strap/post problems. Tight voltage control, temperature-aware charging, routine voltage/ohmic trending, and periodic capacity tests catch issues early and extend service life significantly.
