Specific gravity, read with a hydrometer, is the best single measure of how charged a lead-acid cell is: a full cell reads about 1.265 and a flat one about 1.100 because discharge turns acid into water. You can build a hydrometer from a weighted float and calibrate it by weighing a known volume of electrolyte against the same volume of water, which needs no reference chart. Read each cell separately, since one weak cell hides inside a good battery voltage.
Part of Battery Building and Maintenance
Measuring electrolyte density to find a lead-acid cell’s state of charge and to adjust its acid strength.
Discharge Dilutes The Acid, So Density Falls With Charge
Discharge uses sulfuric acid and makes water (Lead-Acid Chemistry), so the electrolyte gets lighter as the cell empties and heavier as it recharges. The carrier tabulates this at 25°C:
| State of charge | Specific gravity | Cell voltage |
|---|---|---|
| 100% | 1.265 | 2.10 V |
| 75% | 1.225 | 2.08 V |
| 50% | 1.190 | 2.04 V |
| 25% | 1.155 | 1.98 V |
| 0% | 1.100 | 1.75 V |
The other sources give slightly different ends: Charging Systems has charged cells at 1.265–1.280 and discharged at 1.100–1.150, and Rechargeable Batteries gives 1.26–1.28 charged and 1.12–1.15 discharged. The last table row’s 1.75 V is the loaded cutoff that the Rechargeable Batteries article gives for a discharged cell, not a resting voltage. Three corpus voltage tables also disagree on what a resting 12.0 V battery means (25% in two, roughly 50% in Metering), which is the reason to use specific gravity and not resting voltage for charge state.
The steps between rows are not equal (0.040, 0.035, 0.035, 0.055), so interpolate between rows rather than using a straight line. The lead-acid article adds a diagnostic: cells that read below 1.200 after a full charge indicate sulfation damage.
A Hydrometer Is A Float Whose Depth Measures Density
General knowledge, not from the other articles: a weighted float of mass m and uniform cross-section A sinks until the liquid it displaces has the same weight, so its submerged length L = m ÷ (A × density). The depth is therefore proportional to 1 ÷ specific gravity. The lead-acid article’s recipe is a glass tube with a lead weight at the bottom and a paper scale inside, calibrated against known solutions.
What You Need
- A glass tube, sealed at both ends, with lead shot or a lead weight sealed in the bottom so it floats upright
- A paper scale sealed inside the tube
- A glass cylinder tall enough for the float to ride freely
- A flask or small bottle with a mark, and a scale for weighing
- A glass or plastic syringe or squeeze bulb for drawing electrolyte without metal parts
Method: Calibrate And Read
Step 1: Mark the water point. Float the tube in distilled water and mark the water level on the stem. This is 1.000.
Step 2: Make a second point by weighing. Fill a marked flask with distilled water to the mark and weigh it, then empty, dry and fill to the same mark with your electrolyte and weigh again. Specific gravity is the electrolyte’s mass divided by the water’s mass. Float the tube in that electrolyte and mark the level.
Step 3: Place the other marks. Because depth goes as 1 ÷ SG, space the marks by the arithmetic below, then verify with a third weighed sample.
Step 4: Read. Draw electrolyte into the cylinder, let the float settle without touching the wall, and read at the liquid surface with your eye level with it. Correct for temperature using your own method: read the same sample at two temperatures with your own hydrometer and note the difference, because the corpus gives no correction figure.
Worked Example: Where The Marks Go
Suppose the water mark is 100 mm below the top of the stem. In SG 1.265 liquid the submerged length is 100 ÷ 1.265 = 79.05 mm. In SG 1.100 liquid it is 100 ÷ 1.100 = 90.91 mm. The two marks are 90.91 − 79.05 = 11.86 mm apart for a span of 1.265 − 1.100 = 0.165 in specific gravity, so 11.86 ÷ 0.165 = 72 mm per unit, or about 0.072 mm for 0.001. A 0.005 division then needs a mark every 0.36 mm, which a fine pen on a paper scale can reach, but a thinner stem gives longer spacing: halving the cross-section area at the same float mass doubles the submerged length and the spacing.
Common Mistakes
- Reading the voltage instead — voltage changes little until the cell is nearly flat
- Reading just after charging — gas bubbles cling to the float and read wrong; wait until the cell stops gassing
- Reading only one cell — measure each separately, as the lead-acid article says
- Using a metal-tipped syringe — acid dissolves it and contaminates the cell
- Adding acid to correct a low reading after discharge — low specific gravity after discharge is normal; recharge first
Safety
The sample is acid and the cell mouth may hold hydrogen
Electrolyte dripped from the hydrometer burns skin and eyes, and a freshly charged cell holds hydrogen in its headspace that a spark can ignite. Wear splash goggles and gloves, hold the instrument over a tray, keep flames, cigarettes and unprotected switches away from open cells, and wipe drips with water (Acid Handling). Never use a flame to look for gas.
What’s Next
- Learn next: Electrolyte Preparation — trimming the acid you just measured
- Combine with: Float Charging — using the reading to judge charging