Battery Cell Voltage and Specific Gravity Testing: Float Surveys

What it is

This is the basic condition survey of a stationary battery bank that is held on charge by its charger. With the charger energized and the battery in float mode, the voltage of the whole string is measured at the terminals and the voltage of each individual cell or multi-cell monoblock is measured across its own posts. On vented lead-acid cells, the electrolyte is sampled to read specific gravity, which is the density of the electrolyte compared with water, and the electrolyte temperature is taken; on vented nickel-cadmium cells the electrolyte level is checked instead, since density says little about charge there. A pilot cell, chosen as representative of the string, may be read more often than the others, and the temperature of negative posts or container surfaces may be recorded on valve-regulated batteries, where the electrolyte is not accessible. The charger float and equalizing set points are checked at the same time, because the cell readings only mean something when the battery is being charged at the correct voltage. The survey applies to flooded lead-acid, valve-regulated lead-acid and vented nickel-cadmium batteries, and to the battery side of direct-current charger systems.

Why it is done

A battery bank is a series string, so the weakest cell sets the performance of the whole. One cell that has drifted low in voltage, or has sulfated, dried out or developed an internal short, can limit the capacity of the string long before the rest show any sign of age. Cell voltage deviations show up early in float surveys, and specific gravity tells about the state of charge and the health of the electrolyte in vented cells. A string held at the wrong float voltage ages prematurely: too low and the cells slowly sulfate, too high and they gas, lose water, corrode their grids and, in valve-regulated cells, risk dry-out and thermal runaway. Individual cells that are outliers also hint at developing problems such as cracked jars or bad connections. Because the readings are quick, non-destructive and taken without interrupting service, they are the most common way to keep an eye on a station battery, and a recorded history allows weak cells to be corrected or replaced before an outage proves they are inadequate.

The principle

Each cell on float is held by the charger at a voltage slightly above its open-circuit voltage, so a small trickle of current flows to offset self-discharge and keep the plates fully charged. In a healthy string every cell sits close to the same float voltage, and the readings cluster tightly. A cell with a lower reading is not being kept fully charged, often because of internal leakage or sulfation, while a cell with a high reading may be losing electrolyte or developing higher internal resistance. The other cells in the series string compensate, since the charger holds the string total constant. In vented lead-acid cells, the specific gravity of the sulfuric acid electrolyte falls as the cell discharges and returns as it recharges, so it indicates state of charge, and it is temperature dependent, being read and corrected to a reference temperature. In nickel-cadmium cells the electrolyte density changes little with charge, so voltage and electrolyte level are the main indicators. Temperature affects float voltage requirements, which is why charger settings may be compensated for temperature and why negative post temperatures are watched on sealed cells.

How it is performed

The survey is conducted with the battery on its normal float charge, and the charger output voltage is read first from a calibrated meter at the battery terminals. A calibrated digital voltmeter is then used to measure each cell or monoblock across its posts, being careful to maintain a consistent measuring point and to make firm contact. Readings are taken in sequence along the string so the positions can be tracked. On vented cells, a hydrometer or refractometer is used to draw an electrolyte sample, the specific gravity is read, and a thermometer in the electrolyte gives the temperature for correction; the electrolyte level is also checked against the marks on the jar. Valve-regulated and sealed cells are checked for voltage and for the temperature of the negative post or container, using a contact thermometer or an infrared device, while the visual condition is noted for swelling, leaks and corrosion. The pilot cell, if designated, is read as well. Precautions include eye and skin protection against electrolyte, no open flame or sparks near vented cells, insulated tools, and care not to short across cell posts. The test never disconnects the battery from its load.

What is recorded

The record identifies the battery, its type and how many cells or units it has, and the date, the instruments used and the person taking the readings. It documents the charger float and equalizing set points, the string voltage at the terminals and the individual cell or monoblock voltages in order along the string. For vented cells it records the specific gravity readings and the electrolyte temperature and level, and for sealed cells the post or container temperatures. The pilot cell results are noted, together with observations about the condition of jars, covers, posts and connections, and any cells that stand out from the others.

How results are judged

Float voltage readings are judged against the manufacturer's recommended float setting, adjusted for temperature, and against the spread among cells in the string, in both cases with limits taken from the battery maker and the governing documents. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 set out expectations for cell voltages and, on vented cells, for specific gravity corrected to the reference temperature and for electrolyte level. A cell that falls well below the string average, or whose voltage drifts across successive surveys, is flagged for follow-up such as an equalizing charge, a capacity test or replacement. Specific gravity is compared with the nominal value for the cell type and with the other cells and the pilot cell, and the temperature of sealed cells is compared with ambient and with their neighbors. A reading that is unusual but stable is treated differently from one that is moving. The actual limits belong to the named standards and the manufacturer, so consult them rather than relying on a rule of thumb.

Variations

At acceptance, the survey sets the initial values after the battery is first charged, documenting the baseline that later readings will be compared with. In maintenance it is repeated on a regular schedule, with a more detailed survey at longer intervals, and sooner after a discharge, an equalizing charge or a charger problem. Flooded lead-acid cells are the only ones that allow direct hydrometer readings, valve-regulated batteries rely on voltage and temperature, and nickel-cadmium cells use different voltage levels and generally a different electrolyte check. Charger float and equalize settings are verified together with the cells because they govern what the readings should be. Monoblock batteries are measured per unit rather than per cell. In hot rooms, temperature compensation and negative post temperatures take on more importance. The survey is often paired with intercell connection resistance and ohmic readings, which add information about connections and internal condition.