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Battery-Charging Methods—Currents and Voltages-Electrolytic Rectifiers
-Vibrator Rectifiers—Mercury Arc Rectifiers–Rotary ConvertersRheostats-Lamp-Bank Resistance-Charging Precautions—Charging Vehicle Batteries—Winter Care of Automobile Storage Batteries.
The equipment to be used in charging storage batteries depends entirely upon the type and size of batteries to be charged, the current voltage and character available for charging, and the individual characteristics of the batteries themselves. Storage batteries can be charged only with direct current, i. e., that which flows always from the same direction. It is evident that the use of alternating current, if the mains were attached directly to the battery, would result in rapid changes in the interior of the cells, and as the flow in one direction would tend to neutralize that in the other, the plates would depreciate very rapidly. If alternating current is the only kind available, this must be transformed or rectified into direct current. All cells cannot be charged at the same rate. The greater the capacity of the battery and the higher its discharge rate, the greater the amperage of the current that can be used in charging. While the voltage of a storage battery made of certain materials will not vary with the size, the amperage or current output increases with the plate size and number. A leadplate storage battery no longer than a thimble will have just as high voltage as one as big as a barrel. It will be evident, however, that if too much current is passed through a small cell it will be injured, whereas too little current passed through a large eell will have but little effect on changing the character of the plates.
There are two general methods in use for charging either the sulphuric-acid-lead batteries or the alkali-nickel-iron batteries used in the various commercial applications. The first method, and the 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
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
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 varieś very much, the charging rate may be too low for some cells and too high for others. These disadvantages 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 arç 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