FIRE APPARATUS MAINTENANCE
THE ELECTRICAL SYSTEM and accessories constitute the most important group of operating units on our fire fighting and auxiliary vehicles. Their importance warrants special effort since maintenance records indicate they require close attention. For this reason our discussion will be in greater detail than just an outline of inspection procedure.
Maintenance of the electrical system starts with the inspection and maintenance of the battery or batteries. In a dual or two-battery system, only one battery is in the active circuit to receive a charge from the generator or alternator and to supply the current needs. The second battery (B) is on standby. Both batteries must be inspected, and this includes changing the selector switch so that every 10 days the standby battery is placed in the active circuit and the active battery is placed on standby.
First, the top of the battery must be cleaned to prevent current leakage and corrosion. Dirt, acid and moisture accumulation must be removed. This is doubly important on a 12-volt battery because of the higher voltage. The current discharge is relatively small when the top is dirty, but the discharge is continuous for 24 hours of every day and contributes to battery discharge and plate sulfation.
Clean the top of the battery with a solution of common baking soda or household ammonia, followed by a clear water rinse. Be sure the vent caps are tight before starting the cleaning operation. Copper sulfate, the green corrosion from the terminals, must not be allowed to get inside any cell; it will cause discharge of the negative plate.
The battery posts and terminals should be cleaned, lightly greased, and the terminals tightened.
If there is any evidence of lifting of a cell cover, it is usually caused by plate swelling from overcharging or sulfation. The charging rate should be immediately checked and if found too high, the voltage regulator adjusted to the correct rate. See your manual for the correct procedure required to make this adjustment. Records indicate more batteries fail from overcharging than any other cause. If the charging rate is found to be correct, the trouble is probably plate sulfation, which means a short battery life. Replacement of the battery is recommended.
The battery hold-down clamps should be inspected for tightness. Be sure they are not too tight. If too tight, they will distort and probably crack the battery case.
The next important inspection is the level of the electrolyte. Be sure it covers the plates in each cell to the recommended depth. If water is added to bring the level up to proper depth, DO NOT OVERFILL. If the level is allowed to drop below the top of the plates, the exposed part of the active material will dry out and harden, permanently reducing the battery capacity. The battery requires this periodic attention to replace the water lost by the electrical action when the battery is being charged. During charging some of the water is converted to the basic elements of hydrogen and oxygen, as evidenced by the bubbles of gas released. Only pure hydrogen and oxygen are released, and if the water used to refill is not pure (distilled), the minerals and other impurities remain and accumulate in the electrolyte to cause eventual premature battery failure.
If water has been added, the next step of inspection must be delayed for checking the charge condition of the battery until the water and electrolyte have thoroughly mixed.
The proper equipment for checking the charge condition of a battery is; (1) Hydrometer; (2) thermometer; (3) opencircuit voltage tester.
In a fully charged automotive battery, the normal specific gravity reading for the electrolyte is 1.265 to 1.280, although a reading as high as 1.310 may be obtained under certain conditions. These are readings for electrolyte at a temperature of 80 °F which is the standard temperature adopted by battery manufacturers. As the temperature of the electrolyte increases, the volume increases and the specific gravity decreases. To compensate for the increased temperature (above 80°F), add .004 to the specific gravity reading for each 10°F above 80°F. For example, if the hydrometer reading is 1.250 (indicating normally a 3/4 charge) and the temperature of the electrolyte in 120°F, as it can well be in the summer immediately after a good charge rate, the specific gravity as corrected will be 1.250 + .016 = 1.266 … for a full charged battery. Any more charge would be detrimental to the battery. When a battery is on charge, three successive readings alike at 1-hour intervals indicate the battery is fully charged. When charging a battery, never allow the temperature of the electrolyte to exceed 130°F. Excessive water loss usually indicates the charge rate is too high or overcharging.
When the temperature of the electrolyte is less than 80°F, the density and specific gravity increases and the volume decreases; so .004 is subtracted for each 10°F below 80°F to obtain the corrected specific gravity reading. For example, a hydrometer reading of 1.265 (normally full charge) with an electrolyte temperature of 40 °F would have a corrected specific gravity of 1.265 — .016 = 1.249 . . . which is an actual 3/4 charge.
The relation of specific gravity to the proportional charge condition of a battery is approximately as follows:
The above specific gravity readings are based on electrolyte of 1.265—1.279 used in the battery. Batteries for other services or operated under other conditions may use a different specific gravity. Batteries used in tropical climates, where no freezing problem exists, generally use an electrolyte of 1.225 specific gravity.
Battery manufacturers generally recommend that when the charge conditions of a battery shows a specific gravity reading of 1.215, the battery should be placed on charge to fully recharge if the active life of the battery is to be utilized. No battery should be allowed to remain over 30 days in a partial charge condition as sulfation of the plates will occur with performance permanently impaired. We have noted the inspection form which we are following (page 42, FIRE ENGINEERING, February 1965), specifies 1.200 specific gravity as a minimum. We believe the recommendation of the battery manufacturers (1.215) will assure longer battery life.
Any battery having a difference in specific gravity readings between cells of more than .025 should be immediately replaced.
The open-circuit voltage tester is used also to determine the condition of charge of a battery. Some testers are calibrated for an equivalent specific gravity of 1.280. When used on a battery which is fully charged at 1.270, the reading will need to be adjusted to add .01 volt to obtain the correct reading. Be sure your tester is calibrated for the electrolyte you are using or make corrections for the difference. When the cell voltage reads 2.08 (the equivalent specific gravity is 1.220), the battery should be placed on charge. When the cell voltage differs more than .05 volts, the battery should be replaced.