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charged with the operation, care and maintenance of these batteries duly appreciate the foregoing analogy and bear in mind that there is a certain amount of “human nature” even in a storage battery. This done, it is believed that the sphere of usefulness of the storage battery will be further increased and that it will prove a stepping-stone to even greater achievements in the now rapidly developing art of electrical engineering in our naval service.
A very necessary routine practice in the care and maintenance of the storage batteries designed for the various duties of our naval service, and a practice which should be encouraged to the end that these batteries may always be maintained in the prime of condition and ready for instant duty, is known as the “ trickling charge," and it is the purpose of this article to attempt an explanation of the salient principles upon which this practice is based, as well as to describe the methods by which it may be applied to the storage batteries under regular service operating conditions on board ship and in the general naval service.
“TRICKLING CHARGE" DEFINED It is well at this point to define the term “trickling charge "; it may be defined as follows:
When the storage battery is connected across the electrical supply mains or bus-bars and the conditions obtain wherein the battery is at all times receiving just enough current to counteract local action and thus maintain it in a full charged condition, the storage battery is said to be receiving a “trickling charge." A fraction of an ampere only, the amount of which depends upon the size of the battery, is required for this “trickling charge," and, aside from the advantages obtained as a result of counteracting local action in the cell, the battery at all times has its entire capacity
available for instant use when required. In other words, the small amount of charging current passing or "trickling" through the battery is just sufficient to reduce the small layer of lead-sulphate (PbSO) normally formed in the plates as a result of the local action incident to the "internal or self-discharge” of the battery, and, furthermore, this small amount of charging current is not sufficient to cause any deleteri
ous effects through heating or undue gassing of the battery. In fact, when the "trickling charge” is properly conducted practically no rise in temperature is apparent, and there is only a slight amount of gas evolved, if any.
In order that a thorough understanding may be had as to the object of the “trickling charge," it is well in the beginning to consider the prime constituents of the lead-acid storage battery cell and the fundamental equation of the reactions which take place in this cell during the cycle of charge and discharge.
FUNDAMENTAL EQUATION OF THE LEAD-ACID STORAGE BATTERY
CELL The active constituents of the lead-acid storage battery cell are as follows: (a) Positive plate; lead-peroxide (PbO,), which is of a
velvety “chocolate" brown color. (b) Negative plate; finely divided sponge lead (Pb), which is
of a “ battleship" gray color. (c) Electrolyte; dilute sulphuric acid (H,SO), consisting
of chemically pure sulphuric acid diluted with pure
distilled water. The generally accepted fundamental equation for the normal chemical action which takes place in this cell may be thus indicated as follows:
Pb+PbO, +2H,SO,; cell in charged condition.
2PbSO4+2H,0; cell in discharged condition. Therefore, in combining the above, the fundamental equation of the complete reaction is written as follows:
Pb+PbO, +2H,SO4 (Ž) 2PbSO4+2H,O. In other words, the conventional sign ) indicates that this reaction is completely reversible; that is, reading this equation from left to right (→) denotes the action which takes place during discharge of the cell, and reading from right to left (), that which takes place during charge.
It is, therefore, apparent from the above equation that during discharge the acid radical, SO,, of the electrolyte combines with the active materials of the positive and negative plates and converts both of these plates into lead-sulphate (PbSO.). More
over, during charge the lead-sulphate is reduced by the charging current and the acid radical returned to the electrolyte, the active materials of both plates being accordingly restored to their original states; that is, to sponge lead and lead-peroxide.
SELF-DISCHARGE OF AN IDLE BATTERY It is an established fact that if a fully charged or a partially charged battery is allowed to stand idle long enough it will eventually become completely discharged of its own accord. This is manifested by a reduction in the cell voltage, a drop in the specific gravity of the electrolyte and the formation of leadsulphate in the positive and negative plates. In other words, although the circuit connecting the terminals of the battery has not been closed during the idle period and, consequently, no current drawn from the battery, the acid radical of the electrolyte has nevertheless combined with the active materials of both sets of plates, converting them into lead-sulphate in the same manner as though the battery had been subjected to a regular useful service discharge.
A fully charged battery will completely discharge itself in approximately 100 days if allowed to remain idle without receiving a freshening charge during this period. However, the degree of acid concentration in the electrolyte and the temperature to which the battery is subjected are governing factors in the time element required for a battery to become discharged through self-discharge, high acid concentration and high surrounding temperatures tending to lessen the time element necessary for a complete self-discharge as outlined above.
FACTORS WHICH PRODUCE SELF-DISCHARGE There are several factors which are in various degrees responsible for the internal or self-discharge which takes place in an idle storage battery. These factors, when considered either individually or collectively, are, in battery parlance, usually referred to under the general term “ local action.” Chief among these several factors may be stated the following:
1. Impurities in the electrolyte. 2. Impurities in the materials composing the grids, and
3. Local couples formed in the manufacture of the positive
plates. 4. Local couples formed in the manufacture of the negative
plates. 5. Leakage of current between the cell terminals as a result
of moisture grounds, etc. Each of the above factors may be briefly commented upon as, follows:
Impurities in the Electrolyte.-As a general rule any metallic impurities present in the electrolyte will cause a loss of charge at the negative plates. During charge such metallic impurities are deposited upon the negative plates where they 'form innumerable local couples with the active materials of these plates, with the consequent result that in the presence of the electrolyte discharge takes place, thus liberating hydrogen at the negative plates and with a loss of charge at these plates.
Such metallic impurities include antimony, arsenic, copper, iron, platinum and tin. Iron is in general the most active and destructive of the above-mentioned impurities, for due to the fact that the ions of this metal can exist in two different stages of oxidation, each stage of which is capable of being converted from one to the other, these ions continually oscillate from one group of plates to the other, when the cell is placed on open circuit, thus causing a consequent loss of charge at each group.
It requires only a comparatively small amount of iron in a cell to completely discharge it in a very short while when the cell is left on open circuit. Therefore, great care should be exercised when operating the storage battery that iron is prevented from entering the cell, such as through using electrolyte or water which contains iron, dropping into the cell iron nuts, bolts, washers, nails, tools, etc., or through any other cause. Furthermore, all iron which enters a cell from time to time is cumulative in effect, as none of this metal is lost by electrolytic decomposition or liberated in a gaseous state, as is the case with certain other impurities.
Impurities in the Materials Composing the Grids, and Defective Grid-Casting.–The alloy used in casting the grids of the storage battery cell consists of lead and antimony. If these metals are not refined to a very high degree the other metallic impurities contained will set up small local couples in the
presence of the electrolyte, thus causing a loss of charge of the plates. Also, if the lead-antimony alloy is not a homogeneous mixture or if there are segregations of pure antimony and pure lead in spots with blow-holes or shrinkage cracks in the casting as a result of improper cooling or insufficient mixing of the alloy before pouring into the molds, other local couples are formed, which accounts for a further loss of charge of the plates.
Local Couples Formed in the Manufacture of Positive Plates.--As outlined above, the grids are composed of leadantimony alloy, whereas the active material of the positive plates consists of lead-peroxide. We thus have a couple formed by the lead-peroxide and the grid in the presence of the electrolyte, which results in a certain amount of discharge of the positive plate, the amount of which depends upon the surface contact area between the positive active material and the grid. However, the discharge from this cause is of comparatively short duration, since a layer of lead-sulphate is eventually formed between the grid and the active material of the positive plate, thus forming an insulating medium which prevents further discharge.
Also, another source of internal or self-discharge of the positive plates, especially in the Planté type, is the failure to remove all of the forming agents which were used in forming the plates. If these plates are not thoroughly cleared of all such forming agents, the loss of charge from this cause is likely to prove quite appreciable in amount.
Local Couples Formed in the Manufacture of Negative Plates.-As in the case of the positive plates, we have in the negative plates local couples formed by the lead-antimony alloy grid in contact with the sponge lead active material, and in the presence of the electrolyte a certain amount of discharge takes place in the negative plates from this cause. Also, as was described in the preceding paragraph relating to the positive plates, a thin insulating layer of lead-sulphate is similarly formed between the negative grid and the active material of this plate, thus preventing a further loss of charge from this cause.
Another loss of charge at the negative plate is due to the local action which takes place between the various materials used for obtaining porosity, increasing conductivity and the various expanders used in the manufacture of these plates.