In usual service, with the normal rate, it is advisable to stop the discharge at 1.90 volts per cell. If the discharge rate is considerably less than normal, the voltage should not be allowed to fall as low as 1.90 volts per cell, for the reason that with a very low rate of discharge the voltage will not begin to fall off until the limit of capacity is almost reached. The fall in specific gravity of the electrolyte also serves as an indication of the amount taken out and is in direct proportion to the ampere-hour discharge, thereby differing from the drop in voltage, which varies irregularly for different rates and degrees of discharge. For this reason, under ordinary conditions, the fall in specific gravity is to be preferred in determining the amount of discharge. The actual amount of variation in the specific gravity of the electrolyte between a condition of full charge and a complete discharge is dependent upon the quantity of solution in the containing vessel compared with the bulk of the plates. When cells are equipped with the full number of plates, the range will be about 35 points (0.035 sp. gr.); for instance, if the maximum specific gravity reached on the preceding overcharge is 1.209, the extreme limit beyond which the discharge should not be carried is about 1.174. If the cells have less than the full number of plates, this range in specific gravity is proportionately reduced, except in the case of the “pilot cell,” which should be equipped with a device for displacing the excess electrolyte. The available capacity is temporarily reduced at low temperatures; with a return to normal temperature the capacity is regained. The battery should preferably be charged at the normal rate. It is important that it should be sufficiently charged, but the charge should not be repeatedly continued beyond that point. Both from the standpoint of efficiency and life of the plates the best practice is the method which embraces what may be called a regular charge, to be given when the battery is from one-half to two-thirds discharged, and an overcharge to be given weekly if it is necessary to charge daily, or once every two weeks if the regular charge is not given so often. The regular charge should be continued until the specific gravity of the pilot cells has risen to within five points of the maximum, as shown on the last previous overcharge. For example, if on the previous overcharge the maximum is 1.210, then on the following regular charges the current should be cut off when the specific gravity of the pilot cell reaches 1.205. The pilot cell method of noting the end of charge should not be used with a battery unless all the cells are approximately in the same condition. With an old battery whose plates are not uniform, readings should be taken on each cell to determine the end of charge.