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EXPLANATION.-In the left-hand column find the figure nearest to the ampei e-hours discharged from the battery; follow across to the column headed by the available time. The figure at this intersection is the current to be used.

EXAMPLE.—Ampere-hour meter reading, 103 ampere-hours discharged; time available for boosting, one hour. Start at 100 in the left-hand column; follow across to the column headed 1 hour and find 50, which is the current to be used.

Fig. 51.–Table of Charging Rates.

rators, after having been in service, will not stand much handling
and had better be thrown away. If it is thought worth while to
keep them they must be immersed in water or weak electrolyte,
and in reassembling the electrolyte must be put into the cells im-
mediately, as wet wood separators must not stand exposed to the
air for any unnecessary moment, especially when in contact with
plates. Storage batteries always should be stored in a dry place,
preferably in one where the temperature will never fall below 40°
Fahr. Storage-battery solution or electrolyte varies greatly in
density between the points of complete charge and complete dis-
charge. When completely discharged the electrolyte of the aver-
age battery has a specific gravity of 1.14, and a sulphuric acid
solution of 1.14 specific gravity has a freezing-point of about 10°
Fahr. Therefore, if a completely discharged battery is allowed to
stand where it is exposed to extremely low temperature it is quite
possible for the electrolyte to freeze and the cells to be injured in
consequence. However, as already pointed out, a battery for other
reasons must not be allowed to stand in the discharged condition
for any length of time. With increasing charge the density of the
electrolyte increases until, when the charge is complete, it attains
1.28 specific gravity. The freezing temperature of the solution
drops very quickly as the specific gravity increases, somewhat as
follows:
Specific

Freezing-Point
Gravity.

Degrees.
1.14

+10
1.16

+ 5
1.175
1.20

-16
-36

4

1.225
1.25
1.28

—60
---85

Consequently, there is no possibility of a storage battery being injured by freezing in this latitude if it is kept in a fair state of charge. The freezing-points of electrolyte of different specific gravities are also shown in graphic chart at Fig. 53.

Charging Vehicle Batteries of Lead-Plate Type.—The following extracts on modern electric vehicle batteries are reproduced from an article by J. H. Tracy and with the permission of The 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,

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10 20 30 40 50 TH 10 20 30 40 50 2Hr. 10 20 30

Time

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

If an

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

[graphic]

Freezing Point Deg. Fahr.

- 100)

1.050

1.100

1.250

1.300

1.150 1.200 Specific Gravity

Fig. 53.-Freezing-Points of Battery Electrolyte.

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