TYPES OF ACCUMULATORS The Tudor Cell.—The Tudor accumulator is designed pri marily for use as a stationary battery and is suitable for lighting and power requirements generally where a large capacity and high efficiency are required. The positive plates are made of pure lead and cast in one piece with thin vertical ribs and provided with slot-shaped intervening pieces. The ribs are intersected at inter vals by shorter horizontal supports to give the plates rigidity in both directions. By means of an improved Plante process an adherent and homogeneous coating of peroxide is formed on the large surface provided by these plates. As the coating of active material on the surface is thin, plates can be charged and dis coiled into a rosette (cf. fig. 2). The rosettes have sufficient spring to fix themselves in the holes of the lead plate, but are keyed in position by a hydraulic press. The plates are then "formed" by passing a current for a long time. The negative plate consists of a grid with thin vertical ribs, connected horizontally by small bars of triangular section. The bars on the two faces are "staggered," that is, those on one face are not opposite those on the other. The grid is pasted with a lead oxide paste and after wards reduced; this is known as the "exide" negative.
The larger sizes of negative plate are of the "box" type, formed by riveting together two grids and filling the intervening space with paste (cf. fig. 3).
Figs. 16, 17 and 18 show the negative plate which consists of a grid of hard antimonial lead supporting and contain ing strips of active material which is ap plied to the grid in the form of paste.
To prevent contact between the grids through buckling or distortion, thin dia phragms of prepared and treated wood are applied on each side of the positive plate as in fig. 19.
A vehicle battery consists of a number (often 4o to 8o) of cells, coupled by con necting straps. At a normal discharge rate of 5 hours the capacity of the large units ranges from 3.5 to 4.8 amp. hours per lb. of cell according to the design.
At an average voltage of 2, this corre sponds to from 7.o to 9.6 watt hours per lb. of the active material employed.
2. Never leave the cells discharged, if it be avoidable.
3. Give the cells a special equalizing charge once a week.
4. Make a periodic examination of each cell, determining its e.m.f., density of acid, the condition of its plates and freedom from growth.
two methods can be adopted. In private installations it may be disconnected and charged by one or two cells reserved for the pur pose ; or, as is preferable, it may be left in circuit, and a cell in good order put in parallel with it. This not only prevents the faulty one from discharging, but keeps it supplied with a charging current till its potential difference is normal.
Every battery attendant should be provided with a hydrometer and a voltmeter. The former enables him to determine from time to time the density of the acid in the battery. The voltmeter should read up to about three volts. Some form of wooden scraper should be supplied to remove any growth from the plates. The scraping must be done gently, with as little other disturbance as possible. By the ordinary operations which go on in the cell, small portions of the plates be come detached. It is important that these should fall below the plates, lest they short-circuit the cell, and therefore sufficient space ought to be left between the bottom of the plates and the floor of the cell for these "scalings" to accumu late without touching the plates, and they should be disturbed as little as pos sible. The liquid must be kept above the top of the plates. Experience shows the advisability of using distilled water for this purpose. It may sometimes be neces sary to replenish the solution with some dilute acid, but strong acid must never be added to the cell.
The chief faults are buckling, growth, sulphating and disinte gration. Buckling of the plates generally follows excessive dis charge, caused by abnormal load or by accidental short-circuiting. At such times asymmetry in the cell is apt to make some part of the plate take much more than its share of the current. That part then expands unduly, as explained later, and curvature is produced. The only remedy is to remove and press it back into shape as gently as possible. Growth arises generally from scales from one part falling on some other—say, on the negative. In the next charging the scale is reduced to a projecting bit of lead, which grows still further because other particles rest on it. The remedy is, gently to scrape off any incipient growth. Sulphating, the for mation of a white hard surface on the active material, is due to neglect or excessive discharge. It often yields if a small quantity of sulphate of soda be added to the liquid in the cell. Disintegra tion is due to local action, and there is no ultimate remedy. The 5. If any cell shows signs of weakness, keep it off discharge till it has been brought back to full condition. See that it is free from any connection between the plates which would cause short circuiting; the frame or support which carries the plates some times becomes covered by a conducting layer. To restore the cell, end can be deferred by care in working, and by avoiding strains and excessive discharge as much as possible.
High discharge rates are not harmful to a battery in that with a properly designed cell a direct short circuit can be applied across the terminals without doing harm. Generally speaking the life of a battery is cut down more by improper charging than by any amount of discharging at high rates. Discharging at high rates should not be confused with over-discharging at high rates, that is, to continue to take current from a battery after it has become nominally discharged. A battery in a discharged condition, or in an over-discharged condition will sulphate very rapidly, and every effort should be made to prevent a battery from lying in a dis charged condition any longer than is absolutely necessary. If a battery is discharged below 1•76 volts, lead sulphate causes an in crease in volume of each of the plates, setting up strains in them and tending to buckle them and cause the active material to crack and drop off. The injurious results at voltages below this value arise because then the pores contain water. The chemical reaction is altered and oxide or hydrate is formed which will partially dis solve, to be changed to sulphate when the sulphuric acid subse quently diffuses in. But formed in this way it will not appear mixed with the active masses in the electrolytic paths, but more or less alone in the pores. In this position it will obstruct the pas sage and isolate some of the per oxide. Further, when forming in the narrow passage its disruptive action will tend to force off the outer layers.
When cells have been allowed to remain any length of time in a discharged condition, the sulphate becomes very hard and brittle, and when the battery is placed on charge, small quantities flake off and fall to the bottom of the jar.
This results in loss of active material and gradual disintegra tion of the plates. The positive plate is affected to a greater ex tent than the negative as it has less mechanical strength, conse quently positive plates have a shorter life than negative. The more porous the active material the greater depth the acid will penetrate to and the greater will be the capacity of the battery.
The current must be distributed equally over the surface of each plate in order to ensure an equal life of all parts, otherwise some portions would be worked more than others.
Regulation 6f the potential dif ference is managed in various ways. More cells may be thrown in as the discharge proceeds, and taken out during charge, but this method often leads to trouble as some cells get unduly discharged, and the unity of the battery is disturbed. Sometimes the num ber of cells is kept constant for supply, but the P.D. they put on the mains is reduced during charge by employing regulating cells in opposition. Both these plans have proved unsatisfactory, and the battery is now prefer ably joined across the mains in parallel with the dynamo. The cells take the peaks of the load and thus relieve the dynamo and engine of sudden changes as shown in fig. 21. Here the line cur rent (shown by the erratic curve) varied spasmodically from o to 375 amperes, yet the dynamo current varied from i oo to i so amperes. only (see line A). At the same time the line voltage (535 volts normal) was kept nearly constant. In the late evening the cells became exhausted and the dynamo charged them. Extra, voltage was required at the end of a "charge" and was provided by a "booster." Originally a booster was an auxiliary dynamo worked in series with the chief machine and driven in any con venient way. It has developed into a machine with two or more exciting coils, and having its armature in series with the cells (see fig. 22). The exciting coils act in opposition, the one carry ing the main current sets up an e.m.f. in the same direction as that of the cells, and helps the cells to discharge as the load rises. When the load is small, the voltage on the mains is highest and the shunt exciting current greatest.
Batteries should be kept free from dirt and corrosion. Ter minals should be occasionally re moved and covered with vaseline to maintain good contact. Most troubles occur after a battery has finished its useful life, the length of which depends upon the de sign, the quality of the materials of which it is constructed, the care given to it and the relation of the current put in to that taken out ; the useful life of a battery should be from 4 to 5 years. In commercial practice, however, a smaller battery working at a higher rate is generally employed in order to economize in weight, size and cost and the useful life of such a battery is between and 3 years, in which case the battery requires greater attention.