(81) Dry Batteries.
Dry batteries are by far, the most convenient and economical form of primary battery to use, for there is no fluid to slop and leak, the first cost is low, the output is large for the weight, and last but not least, the cell can be thrown away when exhausted without great monetary loss. This does away with the expense and annoyance of changing wet cells, a factor that will be appreciated by those that are far from a source of chemical supplies. Since the advent of the automobile the use of dry cells has extended so that they may be obtained in almost any country town or village.
While the cell is not dry, strictly speaking, the solution is held in such a way that it cannot slop around in the cell nor leak out of the seal. The only fault of a dry cell is its tendency to deteriorate with age because of the constant contact of the electrolyte with the electrodes.
The negative electrode of the dry cell (zinc) is in the form of a cup which serves as a containing vessel for the electrolyte and the depolarizer.
The electrolyte is usually composed of a solution of ammonium chloride, with a small percentage of zinc sulphate, this fluid being held by some absorbent material such as blotting paper, or paper pulp.
The electrolyte is applied to the electrodes by means of the saturated blotting paper, which is also used to line the zinc container, thus providing insulation between the electrodes.
A rod of solid carbon which forms the positive electrode is placed in the center of the container, and the space between the rod and the zinc is packed solidly with granulated carbon, the blotting paper lining preventing contact of the zinc with the carbon.
Pulverized manganese dioxide is mixed with the granulated carbon for a depolarizer.
Brookes Four Cylinder Gasoline Engine Direct Connected to Dynamo.
After the zinc container is filled with the electrolyte and pulverized carbon, the top of the container is closed hermetically by means of sealing wax. Granulated carbon is used for it presents a large surface to the electrolyte, reduces the internal resistance of the cell, and therefore increases the current output of the battery.
As soon as the battery starts generating current, polarization begins, with the liberation of hydrogen gas. If the cell is discharged at a high rate, the manganese dioxide will be unable to absorb all of the gas, and consequently pressure will be erected within the cell. The greater the rate of discharge, the greater will be the amount of hydrogen set free, and the higher the pressure.
If a short circuit exists for any length of time, the pressure of the excess hydrogen will speedily ruin it, as the cell will puff up, or even burst under the pressure. If the rate of discharge be kept so low that all of the gas will be absorbed by the manganese, as soon as generated, the cell will furnish a steady current until the elements of the cell or the electrolyte are exhausted.
The steady current limit, or non-polarizing limit is about one-half ampere and if long life of the cell is expected, the current drain should be less than this amount. A good spark coil will develop a satisfactory spark on a quarter to one-half ampere, so that the demand of a good coil is well within the safe limits of battery capacity. The voltage of the average dry cell when in good condition is 1.5 volts on open circuit. When the cell is old or exhausted, the voltage falls rapidly when any demand for current is made on the cell, and the voltage also varies with the rate of current flow, the voltage decreasing with an increase of current.
As there is not much difference in voltage between a new and old cell when on open circuit, it will be seen that the ammeter giving the current output will give a more accurate determination of the condition of the battery. The voltage is independent of the size of cell.
The battery showing the greatest amperage is not necessarily the best for general use, as cells having an unusually high current capacity are generally short lived. The strong electrolyte used in high ampere batteries causes them to burn out or deteriorate rapidly when not in use.
Under ordinary conditions, a correctly proportioned No. 6 ignition cell should show a current of from fifteen to twenty amperes on short circuit when the cell is new, although higher results may be obtained safely with some makes of cells.
While the voltage is the same for all sizes of batteries, and depends on the material used in the construction, the amperes increase with the size of the cell, and the area of the electrodes.
If a cell does not show more than ten amperes on short circuit, it should be thrown out and another substituted for it, as the cell is liable to go out of commission at any minute when reaching this point of exhaustion.
A small battery testing voltmeter or ammeter should be in the kit of every gas engine operator using a battery for ignition, as the exact condition of a vital part of the power plant can be determined quickly and with accuracy. For dry batteries an ammeter is preferable; for storage batteries a voltmeter must be used.
When buying dry batteries insist on having new, fresh cells, as any battery depreciates in value with age. Never take a cell without testing it, as it is the practice of dealers to work off their old stock on unsuspecting customers. Examine the battery closely for the makers’ dates, and if the battery is several months old, it is probable that the electrolyte is dried up or that the electrodes are wasted through long continued local action. As heat stimulates chemical action in the cell, they should be stored in a cool place to retard the wasting action as much as possible. Under all conditions, the cell should be kept dry, since the moisture that is deposited on the cell forms a closed circuit for the current which soon exhausts the battery. Cold retards chemical action in the cell and consequently reduces the output in zero weather to such an extent that starting is frequently impossible.
Multiple cylinder engines exhaust a battery quicker than those with a single cylinder, as there are more current impulses in a given time and consequently more current is used. A battery may be compared with a bottle that holds a certain quantity of fluid. If the water is allowed to drip out slowly it will last for a long time, but if allowed to flow in a continuous stream will soon be exhausted.
With badly designed or poorly adjusted spark coil, the demand on the batteries is greater than with one that is in proper condition. An engine that runs continuously exhausts a battery faster than one that is run at long intervals. Always open the battery switch when the engine is to be idle for any length of time, as the engine may have stopped with the igniter in contact, allowing the battery to expend its energy uselessly.
Test batteries immediately after a run, as the batteries will recover after standing a while, and will show a fictitious value.
A weak, partially exhausted battery will cause a poor spark that will result in misfiring or a loss of power. It is poor economy to attempt running an engine on a weak battery. An engine may run on a weak battery for a short time, and then gradually decrease in speed until it comes to a full stop. Misfiring is generally in evidence as the engine dies down. In case of an emergency, weak batteries may be made to run an engine of an automobile or boat to its destination, by stopping the engine frequently and allowing the batteries to recuperate during the idle periods. A battery that is temporarily weakened by hard service or by a temporary short circuit will usually revive or partially recover its strength if allowed to “rest” for a short time until the hydrogen is absorbed by the depolarizing material. The life of a dry cell can be extended for a few hours by punching a hole in the sealing wax on the top of the battery, and pouring water, or a solution of water and sal-ammoniac into the cell. This will reduce the internal resistance and increase the current. The batteries will run under these conditions for a short time only, and new cells should be procured at the earliest possible moment. No old cell can be made as good as new by any method. Never drop the cells on the floor nor subject them to hard usage mechanically, for if the active material is loosened, the current output will be reduced. A short circuit through a closed switch with the engine stopped or a loose dangling wire will put the cells beyond repair.
If the binding screw on the carbon electrode does not make good contact with the carbon, tighten it to decrease the resistance. Fasten the connecting wires firmly under the binding screws and keep the connections clean.
In the absence of an ammeter, a rough estimate of the condition of the cell may be made by fastening a short wire tightly in the zinc binding post, and touching the carbon surface lightly and intermittently with the free end of the wire. When contact is made with the free end of the wire, a small puff of smoke will arise and a red spark will be seen if the cell is in good condition.
Sometimes the contact made on the carbon will produce only a small black ring on the surface of the electrode. This indicates a battery that is nearly exhausted, and one which is good for only a few more hours of service.
When a number of cells are connected together in such a way that they collectively form a single source of current, the group is called a battery, and the resulting voltage and amperes of the group depends on the way in which the cells are interconnected.
It is possible to connect the cells of a battery in such a way that total voltage of the group or battery is equal to the sum of the voltages of the individual cells. A battery connected in this manner is said to be connected in series. While the voltage of a battery is increased, by series connection, the number of amperes is the same as that given by a single cell, the same current flowing through the set.