Routine battery examinations divulge irregularities in the charging system as well as in the batteries. The principle method is to examine the electrochemistry of the battery through hydrometric electrolyte inspection. As previously discussed, this important examination cannot be accomplished with AGM or GEL batteries. Voltage readings alone require experience to interpret.
Hydrometric readings will uncover early warnings of overcharging or over discharging before batteries are damaged. The state-of-charge and reliability of a lead acid battery can best be determined by the specific gravity of the electrolyte measured directly with a common bulb-type hydrometer with a glass float. We do not recommend the ball float type hydrometer. Specific gravity is a unit of measurement for determining the sulfuric acid content of the electrolyte. The recommended fully charged specific gravity of marine batteries is 1.255 to 1.265 taken at 80°F. More than .025 spread in readings between fully charged cells indicates that the battery may need an equalization charge. If this condition persists, the cell is failing and the battery should be replaced. Since water has a value of 1.000, electrolyte with a specific gravity of 1.260 means it is 1.260 times heavier than pure water while pure concentrated sulfuric acid has a specific gravity of 1.835.
The following table illustrates typical specific gravity values for a cell in various stages of charge:
100% Charged = 1.285 - 1.290 Sp. Gr.
75% Charged = 1.240 - 1.245 Sp. Gr.
50% Charged = 1.195 - 1.200 Sp. Gr.
25% Charged = 1.150 - 1.155 Sp. Gr.
0% Charged = 1.115 - 1.120 Sp. Gr.
Temperature compensation of hydrometric readings is usually unnecessary unless the battery is extremely hot or cold, however, after hard charging or discharging, you may want to add or subtract points of Specific Gravity based on the table.
Do not apply hydrometer color-coding to readings taken from deep cycle batteries. These red-white-green markings are for "hot" automotive battery types. Also, hydrometer readings taken immediately after water is added to a cell is inaccurate. The water must be thoroughly mixed with the underlying electrolyte by charging, before hydrometer readings are reliable. In addition, do not assume a deep cycle battery will not take a charge because you have been charging it for a while and the float will not rise. If the battery has been fully discharged or partially sulfated it will require considerable charging or equalization before recovering. As electrolyte levels are reduced in the battery, it is important to add water to each cell. Note that only the water portion of the electrolyte evaporates, therefore, it is not necessary to add acid to a battery during maintenance. In fact, the addition of acid to an active battery will reduce its capacity and shorten its remaining life. Water should be added to cells after charging the battery. This will eliminate spillage due to expansion of electrolyte upon charging. Generally speaking, any water that is safe to drink is safe to use in a battery. Do not use water of a known high mineral content or stored in metallic containers. It is the metal impurities in the water that lower the performance of the battery. Distilled water guarantees purity.