BATTERY Batteries are provided with side, top stud and top post type terminals (Figure 3) for various vehicle applications. MT-23244 Figure 3 - Types of Battery Terminals LOW MAINTENANCE BATTERIES These are conventionally vented lead-acid batteries requiring normal periodic battery maintenance. A small amount of antimony has been added to the lead to improve grid strength.  The antimony tends to cause "gassing" (breaking down of water into hydrogen and oxygen gases) during battery operation, thereby lowering the electrolyte level.  Hence, periodic addition of water to the battery cells is required. "Fleetrite" and "International" low maintenance batteries are provided with easily accessible cell filler caps. MAINTENANCE FREE BATTERIES Maintenance free batteries do not require the addition of water during normal service life.  This is due to the fact that maintenance free batteries utilize calcium rather than antimony to improve grid strength.  The advantage of calcium is that it greatly reduces the tendency for the battery to gas at normal charging voltages. Consequently, very little water is used. Maintenance free batteries are not sealed.  All batteries (including maintenance free) generate gases, especially during charge.  While the volume of gases produced by the maintenance free battery is reduced by more than 75%, there are small vent openings to allow this gas to escape of the battery is turned upside down or placed on its side "Fleetrite" and "International" maintenance free batteries have concealed cell caps which can be removed to permit battery testing and addition of water if required. ELECTROLYTE AND SPECIFIC GRAVITY The electrolyte in a lead-acid storage battery is a dilute sulfuric acid solution.  The sulfuric acid in the electrolyte is one of the necessary ingredients in the chemical actions taking place inside the battery.  It supplies the sulfate which combines with the active material of the plates.  It is also the carrier for the electric current as it passes from plate to plate.  When the battery terminals are connected to an external load, the sulfate combines with the active materials of the positive and negative plates forming lead sulfate and releasing electrical energy. SPECIFIC GRAVITY Specific gravity is a unit of measurement for determining the sulfuric acid content of the electrolyte.  The recommended fully charged specific gravity of most 12- volt batteries today is 1.265 corrected to 26.7^oC 80°F).  A battery with a fully charged specific gravity of 1.265 contains an electrolyte with approximately 36% sulfuric acid by weight or 25% by volume.  The remainder of the electrolyte is water.  Pure (concentrated) sulfuric acid has a specific gravity of 1.835.  Water has been assigned a value of 1.000.  Therefore, electrolyte with a specific gravity of 1.265 means it is 1.265 times heavier than pure water. The state-of-charge of a battery can be determined by the specific gravity of the electrolyte.  The specific gravity can be measured directly with a hydrometer (Figure 4). A hydrometer is a bulb-type syringe which will extract electrolyte from the cell.  A glass float in the hydrometer barrel is calibrated to read in terms of specific gravity. The lower the float sinks in the electrolyte, the lower its specific gravity. MT-23245 Figure 4 - Battery Hydrometer Figure 5 graphically illustrates the relationship between gravity readings and the combination of the sulfate from the acid with the positive and negative plates for various states of charge.  The black dots represent the sulfate radical.  A fully charged battery has all of the sulfate in the acid.  As the battery discharges, some of the sulfate begins to appear on the plates.  The acid becomes more dilute and its specific gravity drops as water replaces some of the sulfuric acid.  A fully discharged battery has CTS-4111B - Page 5