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.
The loss-mode table
| Mode | What it is | Reversible? |
|---|---|---|
| Sulfation | Crystals locking active material | Early only (see desulfation) |
| Shedding | Active material falling off plates | No — mechanical loss |
| Grid corrosion | Positive grid oxidising with age/heat | No — end-of-life process |
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.
What buyers should ask
- Is the capacity loss from sulfation, shedding, corrosion or a specific abuse, and is it reversible?
- Has the battery been tested for actual capacity, or is 'it dies fast' the only evidence?
- Would better charging discipline or a maintainer have prevented the loss mode?
- Is the loss gradual (age) or sudden (a short or a dropped cell), which points to different causes?
The bottom line
Capacity loss is a symptom with a specific cause — sulfation, shedding or corrosion — and the cause decides whether it is preventable or reversible. Diagnose the loss mode before replacing, or the next battery dies the same way.
Capacity loss: which modes you can reverse
| Mode | Reversible? | Recovery route |
|---|---|---|
| Sulfation | Partly — early only | Full absorption charge, repeated if caught early |
| Grid corrosion | No | Prevention only: charging-voltage discipline |
| Active-material shedding | No | Prevention: vibration control, quality plates |
| Water loss (flooded) | Yes | Top up with deionized water, then recharge |
The table turns the usual conversation around: capacity loss is a diagnosis, not a verdict — one row is often fixable, and knowing which row you are in decides whether the battery gets a charger or a replacement.
The Editor’s View
My position: Every capacity-loss complaint deserves one charge attempt before a purchase decision — the sulfation row of the reversibility table is real, and skipping it replaces a £0 fix with a £100 battery.
Why: The failure modes that dominate — sulfation and water loss — are the two a charger and a bottle of deionized water can actually treat. A buyer who cannot tell the reversible rows from the irreversible ones is shopping blind.
Editorial interpretation, not purchasing advice.
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