If the farmer does not desire to go to the additional expense of automatic regulation, there are cheaper plants, requiring attention for charging. These plants are identical with those described above, except they have no regulators. With these plants, when the battery runs low (as is indicated by dimming of the lights) it is necessary to start the engine, bring it up to speed, adjust the dynamo voltage to the proper pressure, and throw a switch to charge the battery. For such plants it is customary to run the engine to charge the battery twice a week. It is necessary to run the engine from 8 to 10 hours to fully charge the discharged battery. When the battery approaches full charge, the fact is evidenced by so-called "gassing" or giving off of bubbles. Another way to determine if the battery is fully charged is by means of the voltmeter, as the volts slowly rise to the proper point during the process of charging. A third way, and probably the most reliable is by the use of the hydrometer. The voltage of each cell when fully charged should be 2.5; it should never be discharged below 1.75 volts. Many storage battery electric light plants on the market are provided with a simple and inexpensive circuit breaker, which automatically cuts off the current and stops the engine when the battery is charged. The current is then thrown from the dynamo to the house service by an automatic switch. If such a circuit breaker is not included, it is necessary to throw the switch by hand when charging is begun or ended. Since the principal item of first cost, as well as depreciation, in a storage battery electric light plant is the storage battery itself, the smallest battery commensurate with needs is selected. Since the amount of current stored by these batteries is relatively small, electric irons and heating devices such as may be used freely on a direct-connected plant without a battery, are rather expensive luxuries. For instance, an electric iron drawing 400 watts an hour while in use, requires as much energy as 20 tungsten lamps of 16 candlepower each burning for the same length of time. Its rate of current consumption would be over 13 amperes, at 30 volts; which would require a larger battery than needed for light in the average farm home.