It is possible to test the state of charge of a lead acid battery (or battery bank) with a multimeter simply by measuring the voltage however this is not always an accurate indicator. More accuracy is provided by measuring the specific gravity of the electrolyte in the battery (assuming that the battery is not of the sealed type).
Measuring the Specific Gravity of a Battery
Specific gravity is measured with a hydrometer. Simply draw the battery acid into the hydrometer so that the float is not touching the sides, top or bottom of the barrel. Take the reading with your eyes level with the surface of the drawn up liquid, and then subtract 0.004 for each 5°C above or add 0.004 for each 5°C below 25°C. Do this for each of the cells of the battery.
Below is a table of the specific gravity measurements to be expected at 75°F.
| State of Charge | Specific Gravity |
| 100% charged | 1.265 |
| 75% charged | 1.239 |
| 50% charged | 1.200 |
| 25% charged | 1.170 |
| Fully discharged | 1.110 |
Buy a Hydrometer
Hydrometers are available from many home brewing suppliers but may not be suitable for measuring battery electrolyte – you need to find one with a specific gravity range of approximately 1.000 to 1.300. Take a look on eBay UK where this search battery hydrometer usually brings up a good selection of suitable hydrometers.