A device for measuring the specific gravity (relative density) of a liquid, such as the electrolyte of a lead-acid battery or the glycol mixture in the cooling system
. It normally consists of a calibrated float contained in a glass cylinder into which a sample of the liquid is induced. The amount of water contained in the battery
acid reflects the state of charge of the battery
as a whole. The greater the water content, the lower the charge. The water content can be measured by sucking a small quantity of acid up from the battery and into a hydrometer. Inside the main tube is a small float, with a scale marked on one side. The acid level will be lower down the float if the water content is small, higher up the float if it is comparatively great. The hydrometer gives an extremely accurate but simple guide to the condition of your battery
. A reading on the float of 1.280 shows a cell in good condition, if it falls as low as 1.1 50 the cell is flat. Be sure to check all the cells. If one gives a significantly different reading to the others it is almost certain that the battery
has a major fault.
Also see: How it Works - The Car Battery