« ПретходнаНастави »
Leakage of Current Between Cell Terminals.-Although, properly speaking, loss of discharge from this cause is not due to local action in the strict meaning of the term, it is nevertheless included here since it accounts for quite an appreciable amount of loss of charge in an idle storage battery cell if such a condition is allowed to exist sufficiently long without rectifying it; in fact, the loss of discharge through this cause is in some cases equal to, if not greater than, the combined loss of charge due to the other factors outlined above, provided the leakage of current between the terminals is of protracted duration.
METHOD OF CONDUCTING “TRICKLING CHARGE” Having considered the effects of the various factors of local action in producing self-discharge of the idle storage battery cell, the object of the "trickling charge” in reducing to a minimum the effects of this local action, as well as maintaining the battery in a fully charged, healthy condition is, therefore, readily apparent.
As was explained in defining the term "trickling charge” in the early part of this article, only a fraction of an ampere of current is sufficient to counteract this local action, the amount of the current depending upon the type of the battery in respect to the size and the number of plates installed in the cells.
LAMP-BANK METHOD A very satisfactory and simple method of conducting the “ trickling charge” and one which is very conveniently applied on board ship is known as the Lamp-Bank Method, and consists in connecting lamp-banks in series with the battery and the charging busses of the ship's main supply lines, the number of lamps used depending upon the following:
(a) Type of battery; size and number of plates in the cells. (b) Number of cells in the battery. (c) Voltage of the charging busses.
The function of the lamp-banks is that of a resistance to absorb the excess voltage in the main charging line over that required for the small amount of “trickling charge" current passing through the battery.
Fig. I contains an illustration of the equipment and necessary connections required for conducting a “trickling charge” by the lamp-bank method on navy type storage batteries.
The lamp-banks connected in series with the battery and the main charging busses are plainly shown in this illustration, as is also the double-pole snap-switch used for cutting on or off the
trickling charge” current, as desired. The direction of the current in passing through the battery on charge is as indicated by the arrows in the drawing. In this regard, as in all other cases of charging storage batteries, it is essential that only direct current be used for this purpose and that the positive terminal of the battery be connected to the positive charging bus and the negative terminal of the battery to the negative bus. To do otherwise will result in serious harm to the battery.
In conducting the “trickling charge” by the lamp-bank method, the life of the lamps will be increased if the arrangement of the lamp-banks is such as to reduce the voltage sufficiently to cause the lamps to burn at a low incandescence. Also, as a general rule, on account of their high efficiency and long life, tungsten filament lamps should be used, if obtainable, in preference to carbon filament lamps, as they afford a finer degree of current and voltage regulation than the carbon filament lamps. However, if the conditions are such that it is not practicable to use tungsten filament lamps, carbon filament lamps may be used.
The advantage in using lamp-banks as a resistance, instead of using regular commercial resistance units in conjunction with a low-reading ammeter, rests in the fact that lamp-banks at all times afford a reliable visual indication that current is “trickling” through the battery, whereas, the needle of the ammeter does not present so striking an indication of the charging current; in other words, as long as the lights are burning it is definitely known that current is passing through the battery, and anybody on watch in the vicinity, whether he be a coal-passer or an ordinary seaman, can tell when the charging current is on or off.
At navy yards, shore stations and regular battery service stations, where the organization is such that someone is in constant attendance with the storage batteries on charge, commercial resistance units may well be used in connection with ammeters and voltmeters, as at such places proper facilities are at hand for using at all times accurately calibrated instruments, etc.
Standard commercial resistance units of identical rating as standard size lamps may now be obtained; these resistance units are also designed to screw into the standard incandescent lamp sockets.
All storage batteries designed for stand-by circuits on board ship, as well as spare submarine cells kept in the battery service stations on submarine tenders, should be maintained in a charged condition by the "trickling charge” method. Also, spare submarine batteries stored at navy yards for emergency installation can be kept in good serviceable condition and with comparatively little cost of upkeep and maintenance by this method, and its practice should be encouraged.
COMPUTING THE TRICKLING CHARGE” RATE The number of positive plates contained in the cell constitutes the basis for computing the "trickling charge” rate for a given battery installation. For the portable types of storage batteries used in the naval service and having positive plates 1 inch in thickness a trickling charging rate of .025 ampere per positive plate has been found to be sufficient to counteract local action and maintain the plate in a fully charged, healthy condition. Thus, if such a cell contains n positive plates, the “trickling charge” rate for this cell would be n X.025 ampere.
For all portable type cells having positive plates less than 4 inch in thickness .0125 ampere per positive plate, or one-half of the 4-inch plate rating, should be used in absence of any other specific rating designated by the battery manufacturer. In respect to this feature, the navy specifications for portable types of storage batteries require that each storage battery manufacturer submit detailed drawings of every type cell supplied on government contracts; in addition to containing detailed drawings of parts for the information and use of the battery service stations and operating personnel in ordering spare parts, making repairs, etc., these drawings also specify the number and size of the plates installed in the cells. For submarine type cells the battery manufacturers also supply the required “trickling charge ” rate for
Therefore, in order to ascertain the “ trickling charge ” rate for a given cell or battery installation, it is only necessary to con
sult these detailed drawings supplied by the battery manufacturers. The required “trickling charge” rate should also be found on the metal name-plate attached to the cell tray.
It has also been found that the charge voltage of a cell through which is passing the required amount of "trickling charge current averages from 2.15 to 2.20 volts. Therefore, when calculating the “trickling charge ” rate for a given installation if 2.15 volts per cell is used the results will be sufficiently accurate for practical application.
Example.—The storage battery charging busses on board ship are connected across the 115-volt supply mains, and it is desired to place a set of auxiliary lighting batteries on
“trickling charge”; each cell of this battery contains nine plates, four positive and five negative, and the entire battery consists of 12 cells, all of which are connected in series.
Find the "trickling charge rate for this battery, and the amount of the resistance to be placed in series with the battery in order to conduct the "trickling charge” at the required rate.
Solution.—This type of cell conforms to the navy standard for these batteries and contains four 1-inch positive plates. Hence, the “trickling charge” rate for this cell is :
4X.025 ampere=.I ampere. Also, since there are 12 cells connected in series, the counter electromotive force produced by this battery when on “trickling charge" at the required rate is:
12 X 2.15 volts=25.8 volts. Therefore, 115-25.8=89.2 volts, which must be absorbed by a resistance placed in series with this battery. Now, by Ohm's law:
R R=892 ohms, the amount of the resistance to be inserted in series with the battery in order to allow a trickling charge of .I ampere to pass through the battery.
Therefore, in order to translate this resistance in terms of lamp-banks it is only necessary to select lamps of such rating and to so combine them that the value of the resistance offered by the entire lamp-bank will be 892 ohms; various combinations of lamps may be utilized for such a lamp-bank. For the particular problem outlined above it has been found that a bank consisting of three 25-watt metallic filament lamps placed in series with each
or .I =