Fig. 1,183.—Diagram showing three wire system with one dynamo and storage battery. A 220 volt dynamo charges a storage battery of corresponding pressure, which in turn subdivides the pressure and supplies a three wire system, the neutral wire of which is connected to the middle point of the battery as shown.
A voltmeter is required with a three-way switch to connect it to the dynamo, battery or lamps, and a circuit breaker must be inserted in the battery circuit in order that it may be opened when the current becomes excessive.
A discriminating cut out or reverse current circuit breaker is required between the dynamo and the battery to open the circuit when the charging current falls below a certain value, and thus avoid any danger of the battery discharging through the dynamo, if from any cause the voltage of the latter drop below that of the battery. This completes the ordinary measuring and circuit controlling apparatus employed with storage batteries.
Methods of Control for Storage Batteries.—As the external voltage of a storage battery varies with the amount of charge it contains and with the direction of the current, it is necessary to employ some means for compensating this variation in order to maintain a constant voltage on the line supplied by the battery. The various devices used for this purpose are as follows:
1. Variable resistances;
2. End cell switches;
3. Reverse pressure cells;
4. Boosters.
Fig. 1,184.—Diagram showing connections for ignition outfit. The charging switch has four indications—"Off," "Battery," "Dynamo" and "Charge." When engine is at rest switch is turned to "Off." The first turn brings it to "Battery," enabling the engine to be started. Next turn cuts battery off and puts "Dynamo" direct on engine. The next turn brings the switch to "Charge." Dynamo then charges the battery and surplus current is stored up. Next turn is "Off," which stops engine and disconnects battery from dynamo. Test the dynamo wires with test paper (negative makes mark). Put positive of dynamo to positive of battery. Dynamo should be regulated to charge at about four amperes.
The particular method selected will depend upon the size of the battery, the purpose for which it is used, the allowable limits pf current and voltage variations, the cost of the system, etc.
Variable Resistance.—Regulation by variable resistance may be used advantageously only with batteries of small capacity, and in small lighting plants such as those of yachts, where the space available for battery auxiliaries is limited, and where the cost of energy is so low that the loss of power in the resistance is not objectionable.