Specific gravity is the density of the electrolyte compared with water — and in a flooded lead-acid battery it is the most honest state-of-charge gauge you own, because the acid is not a bystander in the reaction; it is a participant. As a cell discharges, sulphuric acid is consumed into the plates and the electrolyte thins toward water; on charge, the acid is returned. So the density of the liquid is a direct chemical record of how much of the reaction has run. The standard reference states it plainly: during discharge, specific gravity decreases in proportion to the ampere-hours removed (Linden, Handbook of Batteries). Why the acid participates at all is the story of where two volts comes from; what the liquid actually is belongs to the battery acid guide. This page is about the gauge — how to read it, and when it lies.
The straight line everyone is searching for
The relationship people want as a “SoC chart” really is close to a straight line: the reference literature plots state of charge against specific gravity as a linear relationship, tight enough that Linden publishes a simple regression for it. What we will not do is print one universal chart, because the line’s endpoints belong to the design. Linden’s “typically about 1.28” for a fully charged cell is exactly that — typical: tubular, flat-plate, stationary and traction designs are filled to different gravities, and makers deliberately choose lower gravities for service in high-temperature climates (Linden). The honest instruction is one sentence long: get the full-charge and discharged gravity values for your battery from its maker’s datasheet, and read your hydrometer against that line — not against a chart from someone else’s battery. Ours are on ours; the downloads page is where they live.
Reading it without fooling yourself
The tool is a hydrometer — a float in a syringe, graduated in density — and the craft is all in not being fooled. Draw and return the sample gently, read at eye level on a level float, and log the value with the cell number and the electrolyte temperature, because density moves with temperature and hydrometers are calibrated at a reference point; the maker’s documentation carries the correction. Wear the eye protection and gloves the safety guide insists on — you are handling acid, however routinely. Read every cell, not a lucky sample: the information is less in any single number than in the spread, which is why the reading belongs inside the routine our maintenance walk-round lays out, written down every time. A pilot cell tells you about a pilot cell.
When the gauge lies
Three caveats keep the gauge honest. After charging, wait. Fresh charge leaves the acid unevenly mixed — dense acid low, weak acid high — so a reading taken straight off charge reports the layer the sampling tube reached, not the cell; stratification is a whole subject of its own, and gassing at the end of a proper charge is part of how the cell stirs itself. After topping up, wait longer. Added water floats on the surface until charging mixes it in, so the cell reads weaker than it is — top up, charge, then trust the number; when and how to water is the walk-round’s business. And know when the gauge simply does not apply: in a sealed VRLA battery there is no free electrolyte to draw — state of charge there is judged by voltage and test methods instead, which is the territory of our state-of-charge guide and battery testing. Open-circuit voltage does track electrolyte strength — Linden tabulates rest voltages falling with gravity — but it is the indirect cousin, and it needs the same discipline: rested cell, known reference.
What the numbers are trying to tell you
Read as a fleet, the gravities are a diagnosis in progress. All cells low together: the bank is undercharged or working harder than its charger expects. One cell drifting below its neighbours, reading after reading: that cell is asking for attention — the drift pattern that our how batteries die guide traces to its endings. Gravity that never recovers to the datasheet’s full-charge value despite a proper charge: sulphation or water mismanagement in the history. None of these verdicts comes from one reading; all of them come from the log. If your log is showing you a pattern you cannot name, send us the readings — cell numbers, dates, temperatures — and we will read them with you. It is the cheapest diagnosis in the industry, and it is free.