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Electric Storage Battery Company, who issue the complete discussion in their Bulletin No. 156. These instructions do not apply to all types of lead-plate batteries, however, but refer to the latest vehicle types made by this company.
To the user of lead acid storage batteries in self-propelled vehicles, the steady improvement of recent years is hardly noticeable to the eye, although there has been an increase in the watt-hour capacity of the battery per unit of space and weight,
10 20 30 40 50 TH 10 20 30 40 50 2Hr. 10 20 30
Fig. 52.-Theoretical Variation in Charging Rate When the Rate in
Amperes Equals the Ampere-Hours Out of the Battery.
and also in the serviceable life of the battery. There has also been a marked advance in the permissible rates of charge and discharge, which has added so much to the flexibility of operation as to permit the use of much smaller batteries than would have been considered a few years ago in the same service. The following article explains the permissible rates of discharge and the behavior of batteries under operating conditions which a few years ago were considered prohibitive:
It can be safely said that high rates of discharge are in no
way detrimental to modern lead battery plates. Batteries of the vehicle type are in regular operation under conditions in which practically all their work is done at rates which would empty the battery in ten minutes, and the same batteries would be sold to operate at the three-minute rate if there were a commercial demand for such operation. For such high rates of discharge, extra heavy terminals are provided to carry the current, no other changes being required.
It is well known that when discharged continuously at a constant rate the available ampere-hour capacity of a battery is a function of the rate of discharge, the available capacity being lower at the higher rates. This reduction in available capacity at the higher rates of discharge is due to depletion of the acid in the pores of the plates. The rate of this depletion is the difference between the rate of absorption by the plates of the acid that is in the pores of the plates and the rate at which this acid is renewed by diffusion with the other acid in the cell. It is the limit of this available acid that limits the capacity of the battery at high rates of discharge, and not any limitation in the plates themselves. It is, therefore, impossible to damage the plates by overdischarge at high discharge rates. In fact, very low rates of discharge should receive more careful consideration than very high rates.
In general, a battery may be charged at any time when a charge will be useful and at any rate which will not cause the temperature of the battery to exceed 110° F. and which will not cause the cells to gas freely except at low rates of charge. If these conditions can be watched no further directions or limitations need to be considered. As it is not always possible to watch these conditions, several methods of charging have been worked out which reduce the amount of attendance required while charging, and which permit the selection of the most economical way to charge the battery under any particular set of local conditions, while assuring that the above limitations will not be exceeded.
A general rule for determining the maximum permissible rate of charging a battery is: The charging rate in amperes must never exceed the ampere-hours out of the battery. Any method of
charging that keeps the charging current within this limit will not overheat the battery or cause it to gas. In applying this rule it is not necessary to reduce the charging rate below the “finishing” rate recommended by the battery manufacturers. ampere-hour meter is used on the vehicle, so arranged as to indicate the ampere-hours out of the battery, it also indicates at all times the maximum permissible charging rate. It will be noted that the maximum charging rates are no longer a function of the
Fig. 53.-Freezing-Points of Battery Electrolyte.
size of the battery or its relative state of discharge, but depend only on the actual state of discharge. The curve in Fig. 52 shows in percentage the theoretical variation in charging rate and also in state of charge if a battery were charged strictly in accordance with this rule, and this represents the method by which a battery may be safely charged in a minimum time in regular operation.
It is evident that very wide latitude for proper charging is offered from which to select the best way to charge a battery under any given local conditions. If the vehicle is equipped with an ampere-hour meter the readings of this meter may be taken as the basis for selecting a charging rate which may be used for a particular length of time, so that at the end of that time the charging rate will be at the maximum permissible rate, at which time the rate should, of course, be reduced. It follows from the general rule for charging that, if R=permissible charging rate, D=ampere-hours out of the battery at the start of the charge (reading of the ampere-hour meter) and T=time in hours until the current can be adjusted, then R=D: (+T)=maximum permissible charging rate for T hours, and at the end of T hours the charging rate will equal the reading of the amperehour meter. This value can again be divided by 1+T for the new charging rate, and so on until the charge can be finished at the finishing rate. If it is desired to charge the battery rapidly, the time I should be taken as short as possible. For convenience Table I, given in Fig. 51, calculated from this formula, is given. This table is of use not only in the charging-room, but also for the determination of the best manner for charging vehicles under any contemplated conditions and for the selection of charging equipment to meet the requirements of these conditions.
Ampere-hour Meter Indications as Basis for Charging: It should be carefully noted that if an ampere-hour meter is made the basis for charging a battery, care must be taken to be sure that the meter indicates as nearly as possible the real state of charge of the battery. An accurate record of the ampere-hours discharge from a battery does not give an accurate measure of the ampere-hours necessary to fully recharge it, for there are certain variable losses in the battery which the ampere-hour meter cannot
measure. In fact, there is no accurate way to predetermine exactly how many ampere-hours charge may be necessary to fully charge a battery, nor is it necessary in ordinary service that the battery be really completely recharged daily. An ordinary clock is not an accurate instrument for measuring time, yet if it is set correctly occasionally it is sufficiently accurate for ordinary purposes. It is the same with an ampere-hour meter. It is necessary that a battery be fully charged occasionally, say, once a week, if the battery is subjected to hard daily use, as on a commercial truck, and this furnishes an opportunity to set the ampere-hour meter.
A battery is fully charged only when all the sulphate has been driven out of the plates into the electrolyte, and this is most easily indicated by the specific gravity of the electrolyte. As long as sulphate is being thrown out of the plates into the electrolyte during charge, the specific gravity of the latter must continue to rise, and when the rise stops the battery is fully charged. Most battery manufacturers recommend that a battery be given such a charge (called an equalizing charge), regardless of the indication of the ampere-hour meter, once a week or once in two weeks. When it is known that the battery is full, the charge is discontinued and then the meter is set to indicate a full battery, and the meter is then a sufficiently accurate indicator of the state of battery charge to be used for a week or two weeks until another equalizing charge is given the battery, when the meter should again be set.
Ampere-hour meters require cleaning and regulation at intervals as does a clock, and if they are treated in this manner they are of great assistance in the proper handling of a battery. These meters are frequently furnished with a contact-making device, so arranged as to interrupt the charging circuit when the meter indicates that the battery is fully charged, and this is a valuable protection again unnecessary charging and gassing of the battery during ordinary operation. This tripping device should, of course, be disconnected during the equalizing charge.