Lead-Acid Capacity Loss
A lead-acid battery loses capacity for a handful of well-understood reasons. Knowing which one is at work is the difference between a recoverable battery and one that is already finished.
What causes capacity loss?
| Cause | What triggers it |
|---|---|
| Sulfation | Left undercharged / stored discharged |
| Grid corrosion | Overcharge, high temperature, age |
| Active-material shedding | Deep cycling, vibration, age |
| Water loss / dry-out | Overcharge (gassing), especially sealed batteries |
How does capacity loss happen?
All four reduce the amount of active material available to react, and most also raise internal resistance. Sulfation hardens the sulfate so it no longer converts back; corrosion converts the conductive grid into non-conductive oxide; shedding physically removes paste from the plates; dry-out removes the ion conductor.
What are the symptoms of capacity loss?
- Shorter runtime — the battery "dies" sooner under the same load.
- Lower measured Ah on a capacity test.
- Weaker cold start (lower CCA) from raised internal resistance.
- Battery won't reach or hold full charge voltage.
How is capacity loss measured?
Capacity is measured by a controlled discharge — the C20 test drains the battery at a fixed current (1/20 of rated Ah) until it reaches 10.5 V. The delivered amp-hours are the measured capacity. Compare that to the rated Ah to quantify the loss. See capacity testing.
How is capacity loss diagnosed?
A conductance/CCA reading and a specific-gravity check (flooded) separate the causes: uniformly low gravity and high resistance point to sulfation; visible corrosion or a bulged case points to corrosion/water loss; a low reading that recovers after a long slow charge suggests reversible sulfation.
How do you prevent capacity loss?
- Keep the battery charged (use the float stage); recharge promptly after use.
- Use the correct charger voltage for the battery type (flooded vs AGM vs gel).
- Avoid deep discharges and excessive heat; store cool and charged.
What can these methods not tell you?
Early sulfation can be partly reversed with a long slow charge or a controlled equalization (flooded only). Corrosion, plate shedding and dry-out are permanent — no charging or additive restores lost active material or electrolyte. When measured capacity falls to roughly 80% of rating, the battery is at its practical end of life.
World Battery Hub Analysis
My take: The single most useful fact about capacity loss is that most of it is preventable and some of it is silent — sulfation quietly builds over weeks of undercharge, then looks like sudden failure later.
Why: Treating "the battery died" as one event hides the real story: the damage was done earlier by a charging habit. The recoverable-vs-permanent distinction is what actually decides whether a battery is worth trying to save.
My view as an editor, not a purchasing guarantee.
Sources
- Battery Council International (BCI) — capacity-loss and C20 test references.
- Manufacturer maintenance guides (Yuasa, Varta, Exide) — sulfation, corrosion and water-loss diagnosis.
Last reviewed: 2026-09-16