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one most widely followed, is called the constant-current system. The other system, which has only received attention lately, is called the constant-potential method. Two other schemes are used also which are modifications of the two previously named general

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Fig. 37.–Battery-Charging Outfit, Having Field-Coil Rheostat.

methods, one known as the multiple voltage system, the other as a fixed resistance method. The constant potential method is said to offer a number of advantages. There is less evaporation of electrolyte and less shifting of rheostats is needed. Any form of battery may be charged by means of a fixed resistance connected in series. When the battery counter electromotive force is nearly

equal to the voltage of the line, this method approximates the general characteristics of the constant potential system. If the voltage of the battery is considerably less than that of the charging current, the characteristics will approximate the constant-current charging method. It may be stated that the variation in the current used for charging is inversely proportional to the difference between the maximum counter voltage of the battery and the potential or voltage of the supply circuit.

The usual method of charging batteries in garages is to con

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Fig. 38.-Direct-Current Battery-Charging Outfit, With Rheostat in

Line Between Battery and Main-Line Switch. nect them up in series. In order to meet the requirements of the best battery-charging practice it is stated that they should never be charged in series unless they are all composed of the same general type, size and capacity of cells, and that all of the batteries are in the same state or condition of discharge. The disadvantages of this method are that the batteries that have not been discharged so much as the others are apt to be overcharged, and if the battery capacity varies very much, the charging rate may be too low for some cells and too high for others. These disadvan

tages do not apply in charging vehicle batteries where the individual cells are of the same size, type and capacity. The only way the series charging can be carried on is by using a compromise charging rate and carefully testing the various batteries from time to time to make sure that they will be removed when properly charged. The types and sizes of cells used in automobile starting, lighting and ignition batteries do not vary as much as might be expected, and if a compromise charging rate is intelligently selected, it is a method that gives fairly good results in practice, though it is theoretically wrong.

When a direct current of 110 volts potential is available Edison batteries composed of 60 cells or lead-plate batteries of 40 to 44 cells may be charged directly from the line by means of a rheostat to regulate the amount of current passing through the batteries, which is placed in series with each battery. If the service is of higher voltage, a motor generator or rotary converter set. may be used. If alternating current only is available, various types of rectifiers are needed. If only one battery is to be charged and if the voltage of the generator can be adjusted by means of a rheostat connected in series with the field coils of the dynamo, as shown at Fig. 37, then no rheostat will be needed between the battery and the dynamo, because the charging current can be kept to the proper value by varying the dynamo voltage. When a mercury arc rectifier is employed in battery charging, the charging current can be regulated by control dials and a rheostat is not needed. The various types of rectifiers suitable for use with alternating current are to be described in proper sequence. Rotary converters may be used with either direct or alternating current, depending entirely upon the type of motor used. It is evident that the dynamo of such a combination must always be of the direct-current type, though its output will vary according to the power of the actuating motor, the method of winding, and wire used in field and armature coils. The motor may be a direct-current type, wound for high voltage, or it may be a type wound to operate on alternating current.

The arrangement of the essential parts of a typical batterycharging system where the current value is altered by a dynamo

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Fig. 39.—How to Wire Rotary Converter Charging Outfit, With Ammeter Rheostat and Voltmeter,

Showing Front and Rear of Switchboard.

field-coil rheostat is shown at Fig. 37. A shunt-wound generator is employed, and the main leads from the armature brushes are connected to the lower poles of a double-pole knife-switch. The battery is connected to the upper portion of the switch, an amperemeter being placed in circuit as indicated. The hinges of a small double-throw, double-pole switch are connected with a voltmeter. The amperemeter and voltmeter should be of the permanent type. Before throwing in the charging switch it is possible to read the voltage of the battery, and also by throwing the switch to read that of the charging generator so that it may be adjusted to a slightly greater voltage. The main switch is then closed and the rheostat used to raise the voltage sufficiently to drive a suitable charging current through the battery. With a system of this kind a circuit breaker or automatic overload switch should be included in the main line to protect the apparatus in case of accidental short circuit. An underload circuit breaker should also be provided to shut off the battery if the current falls to such a point that the battery will discharge through the generator. These are not shown in the simplified wiring diagram, neither are the fuses that prudence dictates should be used.

The Westinghouse Vibrator Rectifier is an inexpensive form of apparatus to charge small batteries from ordinary lighting circuits. The device, which is shown at Fig. 42 A, reduces the voltage of the lighting circuit to the proper value by the use of a small double step-down transformer, and rectifies this reduced voltage to the uni-directional voltage necessary for battery charging by electrically operating switching mechanism. The transformer serves the double purpose of diminishing the line voltage for the battery to be charged and also for providing a return path for the direct current. The charging current flows from one end of the secondary winding, and after passing through a regulating resist

nce passes through a pair of contacts, which are closed automatically and at the proper time, and out from the center point of the armature to the battery, from which it returns to the neutral point of the transformer. During the next half cycle the voltage in the transformer secondary is reversed in direction and the other pair of contacts is closed and the voltage is applied to

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