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The last discharge rate is three amperes, and there will be required a battery of sufficient size to furnish 60 ampere-hours at a three-ampere rate. This being less than the eight-hour rate, we require a battery having a normal rating of 60 ampere-hours. By referring to the tables given in the Gould catalog it will be seen that Type M-307 will give 7.5 amperes for eight hours, or 60 ampere-hours at normal rating. Battery required, 60 cells, Gould Type M-307.

The above example shows a condition where the full normal capacity of the battery is used in carrying the load. Under some conditions this is not possible, i. e., where the latter part of the discharge is at a high rate; and it is advised that the battery company check the size of battery before the final decision.

Methods of Operation.—The principal function of a storage battery in small plants being the furnishing of current for a considerable period of time, such as the night load of a residential plant, the operation of the battery consists of cycles of charge and discharge covering practically the capacity of the battery. The problem of operation, therefore, resolves itself into two parts:

First-Voltage control during discharge: Under ordinary operating conditions, it is desirable to maintain practically constant voltage on the lighting circuits—hence, as the voltage of a storage battery varies during discharge, various methods have been developed to compensate for the changes in the battery voltage. A fully charged battery which, standing idle, shows about 2.1 volts per cell, will show while discharging at the eighthour rate about 1.8 volts at the latter part of the discharge, and somewhat less at higher discharge rates. For isolated plants one of the following methods is usually employed: (A) Resistance control. (B) End-cell control.

Second_Control of charging current: To fully charge a battery it is necessary to raise the voltage, as the charge proceeds, from about 2.2 volts at start to approximately 2.62 volts per cell at the completion of the charge. This is usually accomplished in one of the following ways: (A) Normal voltage charge. Resistance control. (B) High-voltage charge directly from generator. (C) Shunt-booster charge.

As the selection of a proper lighting battery should not be undertaken without consulting the battery maker, any reader wishing more involved, technical explanation of any of the methods described can obtain same by consulting the engineers of whatever battery maker he may select. Typical lighting batteries of Gould manufacture are shown in Fig. 67, which outlines the construction of the glass jar, glass tank and wood-tank types.

Details of Installation.—Gould storage batteries for light and

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Fig. 68.-Typical Isolated Lighting Battery for Medium-Capacity

Plant.

power plants are usually installed either in glass jars, glass tanks or lead-lined wooden tanks. The smaller type of cells are installed in glass jars resting on a bed of sand contained in a glass or wooden sand tray. The sand tray is supported by four glass insulators placed under the corners of the tray. Cells of medium capacity are usually installed in tanks of pressed glass, no sand trays being used, the glass tank resting on the glass insulators aird separated thereirom wy a small cushion of either lead or rubber to keep the hard surfaces out of contact. Cells of the glass

jar (or glass tank) type are the easiest to install, as the plates are grouped at the factory. The plates of each cell are burned to common cross-bars and terminal straps, the negative plate to one cross-bar and the positive to the other, forming respectively the negative and positive “groups.” The two groups with separators form what is known as the "element.” The cells of the glass jar iypes are joined by bolting the lead terminal straps together by means of lead-covered brass bolts.

Lead-lined wooden tanks are often used for plants of medium size and always for those of large size. Plates of this type of cell are grouped at the place of installation by "lead-burning” the positive plates of one cell and the negative plates of the adjoining cell to a common busbar. All types of cells are usually installed with the supporting insulators resting on wooden stringers, these stringers having been previously painted with two or three coats of acid-proof paint. Cells of the smaller types which are not too heavy are generally installed on two-tier wooden racks in order to save floor space. The larger cells are installed in one tier, the wooden stringers being supported by vitrified brick set upon the floor or by another set of glass insulators resting on vitrified tiles. A typical installation of glass jar cells joined to form a mediumcapacity battery is illustrated at Fig. 68. In this, the battery is in a special room prepared for it, and the cells rest on wooden stringers, as previously described.

Switchboard and Fittings. It will be evident that the switchboard is an important detail of the storage battery lighting plant. The Type E switchboard used with some of the Electric Storage Battery Company outfits is shown at Fig. 69, the view at the right showing the face of the switchboard, while that at the left shows the method of installing the switchboard in connection with the kattery supporting platform. The electric-current generator can also be fastened to the platform and belt-driven from the internalcombustion engine ordinarily used as power. In one corner, the ampere-hour meter dial is shown as an inset. Its use will be more fully described in proper sequence. The switchboard illustrated is utilized in connection with low-voltage house-lighting plants, and is simple, yet complete. The general arrangement has been

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Fig. 69.-Typical Isolated Lighting-Plant Switchboard, Showing Fittings Needed.

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worked out with great care by competent engineers who are thor-
oughly conversant with small-plant switchboard requirements as
the result of experience gained in the installation and operation
of great numbers of such plants.

The arrangement permits:
1. Lights to be run from battery only, or

2. Lights to be run from generator while generator is also charging the battery, or

3. Lights to be run from generator and battery in parallel, the battery assisting the generator, or

4. Lights to be run from generator only.

In the ordinary operation of the plant, no hand manipulation of switches or any part of the switchboard apparatus is at any time necessary except adjustment of the generator current by means of the generator field rheostat. Hand manipulation of switches is superseded by the automatic operations of the automatic cut-in and cut-out switch. This switch is simple, durable and reliable. It has no adjustments, because it needs none. Made in two sizes: 30-ampere, 32-42 volts, and 60-ampere, 32-42 volts. Panel of black oil-finished slate, 1 x 2314 x 18 inches, on heavy strap-iron frame. Generator and battery switches are double-pole, single-throw, with enclosed fuses. Ampere-hour meter connected in battery discharge and charge circuit. Ammeter connected in battery discharge and charge circuit. Pilot-light socket connected across generator leads. Automatic switch connected in generator circuit, automatically closes generator circuit to line when generator voltage rises to proper value, and opens on small reverse current. Generator field rheostat furnished only as extra and to purchaser's specifications. Standard board includes provision for mounting front of board type of rheostat. Ground-detector lamp sockets for testing for grounds.

Utility of Ampere-Hour Meter.—The ampere-hour meter, which is a feature of these switchboards, is of great practical value. It is to the storage battery what a tank gauge is to a water tank. It indicates at all times the current in ampere-hours taken out of the battery. The hand moves from “Full” toward “Empty" when the battery is discharging, and from "Empty" toward “Full” when

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