Specific gravity, as read at the end of the duty cycle, is an excellent indication of the amount of battery capacity that has been used. Battery manufacturers provide graphs for all batteries in their product lines to translate specific gravity into depth of discharge.

The manner in which one manufacturer presents this data is given in the section "Discharge as a Function of Specific Gravity." For flat-plate batteries in this product line, fully-charged cells have a specific gravity of 1.285 +/- 0.005. For tubluar-plate batteries in this product line, the full-charge specific gravity criterion is 1.310 +/- 0.005. As the cells discharge, electrolyte specific gravity reduces at slightly different rates for different-sized plates.

While overdischarging is detrimental, overcharging is not beneficial; overcharging can in no way increase the amount of battery work capability. Modern, electronically-controlled battery chargers comprise refined controls to ensure that the battery is safely charged to the proper level within the allotted charging time. Chargers can, however, get out of adjustment. Any of the following trouble signs indicate that the charger might be in need of adjustment.

Determining When Battery Is Fully Charged Three indicators are used to determine if the battery has received its proper charge. First of these is voltage stabilization. During charging, voltage across the battery will continue to rise slowly. When the battery is fully charged, the voltage will level off.

Another indicator is leveling off of the rise in specific gravity. Specific gravity will continue to increase as the battery absorbs charge, but as the battery approaches full charge, there will be no further increase in specific gravity. In fact, there might even be a decrease in specific gravity, because of the temperature rise caused by continuing to charge a fully-charged battery.

The third indicator of full charge is leveling off of the charge current, as indicated on the charger ammeter or readings on a computerized control display.

Battery manufacturers usually supplement the specific gravity/percent discharge graphs for their product lines with simplified state-of-charge tables. The battery should retain its nominal full-charge specific gravity value throughout its service life. A gradual reduction in specific gravity can be expected, though, if the battery is exposed to the practice of overwatering or adding water before charge.

In the twilight of its life, a battery can experience internal short-circuiting and plate deterioration, which might be identified with reduced specific gravity. In such cases, it might be difficult to exceed 1.240 specific gravity after charging.

An equalizing charge is a low-rate charge applied to ensure that all cells attain full charge with no appreciable difference between cells. It is simply a continuation of the regular charge, applied at a low rate for an additional three hours. Applying an equalizing charge once a week is an effective means of maintaining electrical balance within the battery, thereby prolonging battery life.

An effective battery maintenance program requires that attention be given to the charging equipment. Charging-rate controls and cutoff controls should always be checked before placing a battery on charge. A battery that is regularly overcharged requires frequent addition of water, and finishes its charge at a higher-than-normal temperature. If a battery is using an excessive amount of water, or if it develops excessive temperatures during charging, the charge rate should be lowered. This usually means adjusting the end-of-charge rate to somewhere between a normal finish rate and one-half the normal finish rate.

If proper care is taken in charging batteries and adding water, there is no reason batteries should not remain clean and dry. In dusty atmospheres, however, it might be necessary to clean off dust and dirt accumulations by brushing or blowing with low-pressure compressed air. It is essential to ensure that all vent plugs are securely in place any time that cleaning is undertaken.

If a battery is constantly wet, if might be due to overfilling or overcharging. Because spilled electrolyte corrodes steel, copper and other metal surfaces it contacts, electrolyte spills on a battery or overflow from inside a cell should be neutralized immediately with a solution of one pound of baking soda to a gallon of water. The solution should be applied with a paintbrush until all fizzing stops, taking care to work the solution under connectors, and brushing out grime. Residue should then be cleansed from the battery by rinsing with water from a low pressure hose.

A certain amount of internal loss of water in cells is normal. The lost water should be made up at the end of the charge, and never at the beginning of a charge. If water is added at the beginning of a charge, overflow might occur during charging.

Subsequent replacement of the lost electrolyte with water results in dilution and an imbalance of acid and water. The specific gravity of the cell is lowered, resulting in less cell capacity.

It is very important that makeup water be either distilled or deionized water, or, if tapwater, certified by chemical laboratory testing as acceptable for battery use.

When adding water, it is better to underfill than overfill. If an automatic filling system is used to add water, it should be checked periodically to ensure that it is adjusted to terminate fill at the proper level. All cells of a battery should use about the same amount of water. If one cell is using more than the others, it might be losing water through a leaky jar or through the jar-to-cover seal. The leak should be located and repaired.

Electrically-powered vehicles in multi-shift service usually require more than one battery per vehicle. Each battery should be given a unique identity by painting an identifying number on the battery tray, and a service log should be kept for each battery.

One cell in each battery should be identified as the pilot cell, with pilot cell designation rotated periodically among all cells of the battery.

Condition of the pilot cell can be presumed to represent condition of the battery as a whole. Pilot cell hydrometer readings should be taken and logged for each duty cycle. Logged information should contain the date of reading, before-charge and after-charge specific gravities, and addition of water.

Several times a year, a full set of voltage and specific gravity readings should be taken at the end of an equalizing charge. If voltage and specific gravity readings are uniform from cell to cell, the battery is in good condition. If variations in specific gravity exceed about 20 points, or if voltage varies by more than about 0.15 volts per cell, there is reason to suspect that the battery is in trouble, and the reason for the variation should be investigated.

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