Low Cells. An unhealthy condition in a cell usually manifests itself in one of the following ways: Falling off in specific gravity or voltage relative to the rest of the cells, lack of gasing when charged, and color of the plates, either noticeably lighter or darker than those of other cells of the battery. When any of the above conditions are found in a cell, the cell should receive immediate attention, as a delay may mean serious trouble. The cell should be thoroughly inspected to determine if a short-circuit exists, either caused by some foreign substance, by an excess of sediment in the bottom of the tank, or by portions of the plates themselves. If such a condition is found, the cause should be immediately removed and, if the defect has been of short duration, the next overcharge will probably restore it to normal condition. If the defect has existed for some time, it is often necessary to give the cell a separate charge. This may be done by connecting it directly to the charging generator with temporary leads and thus bring it back to its normal condition. It is sometimes found necessary to replace the cell in order to restore the battery to its normal condition.

Sediment. The cells of the battery should be carefully watched to prevent the sediment which collects in the bottom of the jar or tank during use from reaching the bottom of the plates, thereby causing short circuits between them. When the sediment in the cell has reached within one-half inch of the bottom of the plates, it should be removed at once. With small cells using glass jars this can most easily be done directly after an overcharge by carefully drawing off the electrolyte without disturbing the sediment and then removing it from the jar. The plates and electrolyte should be replaced in the jar as soon as convenient to prevent the plates from becoming dry. If the plates are large and in wooden tanks, the sediment can most easily be removed by means of a scoop made especially for the purpose. The preferable time to clean the tanks is just before an overcharge.

Replacing Batteries. There comes a time in the life of nearly every central-office equipment when the storage battery must be completely renewed. This is due to the fact that the life of even the best of storage batteries is not as great as the life of the average switchboard equipment. It may also be due to the necessity for greater capacity than can be secured with the existing battery tanks, usually caused by underestimating the traffic the office will be required to handle. Again, it is sometimes necessary to make extensive alterations in an existing battery, perhaps due to the necessity for changing its location. To change a battery one cell at a time, keeping the others in commission meanwhile, has often been done, but it is always expensive and unsatisfactory and is likely to shorten the life of the battery, due to improper and irregular forming of the plates during the initial charge. The advent of the electric automobile industry has brought with it a convenient means for overcoming this difficulty. Portable storage cells for automobile use are available in almost every locality and may often be rented at small cost. A sufficient number of such cells may be temporarily installed, enough of them being placed in multiple to give the necessary output. By floating a temporary battery so formed across the charging mains and running the generators continuously, a temporary source of current supply may be had at small expense for running the exchange during the period required for alterations. Usually a time of low traffic is chosen for making the changes, such as from Saturday evening to Monday morning. Very large central-office batteries, serving as many as 6,000 lines, have thus been taken out of service and replaced without interfering with the traffic and with the use of but a comparatively few portable cells. One precaution has to be observed in such work, and that is not to subject the portable cells to too great an overcharge, due to the great excess of generator over battery capacity. This is easily avoided by watching the ammeters to see that the input is not in too great excess of the output, and if necessary, by frequently stopping the machines to avoid this.

Power Switchboard. The clearing-house of the telephone power plant is the power board. In most cases, it carries switches, meters, and protective devices.

Switches. The switches most essential are those for opening and closing the motor and the generator circuits of the charging sets and with these usually are associated the starting rheostats of the motors and the field rheostats of the generators. The starting rheostats are adapted to allow resistance to be removed from the motor armature circuit, allowing the armature to gain speed and increase its counter-electromotive force without overheating. The accepted type has means for opening the driving circuit automatically in case its voltage should fall, thus preventing a temporary interruption of driving current from damaging the motor armature on its return to normal voltage.

Fig. 417. Power-Plant Circuits
[View full size illustration.]

Meters. The meters usually are voltmeters and ammeters, the former being adapted to read the several voltages of direct currents in the power plant. An important one to be known is the voltage of the generator before beginning a battery charge, so that the generator may not be thrown on the storage battery while generating a voltage less than that of the battery. If this were done, the battery would discharge through the generator armature. The voltmeter enables the voltage of the charging generator to be kept above that of the battery, as the latter rises during charge. It enables the performance of several cells of the battery to be observed. A convenient way is to connect the terminals of the several cells to jacks on the power board and to terminate the voltmeter in a plug.

The ammeter, with suitable connections, enables the battery-charge rate to be kept normal and the battery discharge to be observed. In order to economize power, it is best to charge the battery during the hours of heavy load. The generator output then divides, the switchboard taking what the load requires, the battery receiving the remainder.

In systems requiring the terminal voltage of the equipment to be kept constant within close limits, either it is necessary to use two batteries—never drawing current from a battery during charge—or to provide means of compensating for the rise of voltage while the battery is under charge. The latter is the more modern method and is done either by using fewer cells when the voltage per cell is higher or by inserting counter-electromotive force cells in the discharge leads, opposing the discharge by more or fewer cells as the voltage of the battery is higher or lower. In either method, switches on the power board enable the insertion and removal of the necessary end cells or counter-electromotive force cells.