Battery Technology Reference · Last reviewed 2026-09-16
| Specific gravity | State of charge |
|---|---|
| 1.265 | 100% |
| 1.225 | ~75% |
| 1.190 | ~50% |
| 1.155 | ~25% |
| 1.120 | Discharged |
Typical values at room temperature; temperature correction applies.
The chemistry is why: during discharge, sulfuric acid is consumed to form lead sulfate and water, so the acid dilutes — the density falls. During charge the reaction reverses, the acid reconcentrates, and the density rises. Specific gravity is therefore a direct read on how much of the discharge has been reversed — the chemical equivalent of the resting-voltage chart, but measured inside the cell.
The hydrometer needs free liquid, so it works only on flooded batteries. In VRLA designs the electrolyte is immobilised (absorbed in the mat or gelled), so specific gravity cannot be sampled — which is one reason sealed batteries are read by voltage and conductance instead.
My position: Specific gravity is the most honest charge measurement in lead-acid because it reads the chemistry itself rather than an electrical proxy — and its flooded-only limitation quietly explains why sealed batteries lost this direct window.
Why I think so: Voltage and conductance are proxies; the acid density is the reaction. The hydrometer's decline from mainstream practice is a convenience trade, not a technical improvement — worth remembering when diagnosing a flooded battery precisely.
This is the author's editorial view, not a purchasing guarantee.
Return to Battery Technology Reference · World Battery Hub. Informational, not purchasing advice.