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Battery Faults

WHAT MAKES BATTERY FAULTY?

 

The troubles that are mainly met with in lead-acid cells are: lagging cells, sulfation of the plates, short circuits, corrosion of the grids, treeing and buckling of the positive plates, shedding of the active materials, contamination of the electrolyte.

 

 Lagging cells in lead acid batteries

All the cells in a battery should always remain in the same state of charge or discharge.

lf only one of the cells in a battery will discharge earlier than the others, the eflìciency of the battery will be determined by this cell.

Such a cell limits the capacity of the battery because during discharge its voltage will drop to the final value ahead of any of the other cells.

If the discharge of a battery is continued after the voltage of such a lagging cell has dropped to the final permissible value, this cell may rather quickly discharge to zero, while the other cells in the battery still have a voltage higher than the final value and remain in a state of charge. In this case, the discharge current of cells that retain their charge, in passing through the lagging cell, will begin to act on the lagging­cell plates like a charging current. As a result, lead dioxide will be formed on the negative plates of the lagging cell, while lead forms on the positive plates. The final result will be a reversal of polarity of the plates, following which the voltage of the battery will drop considerably. This makes it clear why the discharge of a battery must be stopped as soon as the voltage of any of the cells has dropped to the limited final value.

The laggin of a cell may be identified by the following signs: the density of the electrolyte in a cell during a discharge of the battery is found to be lower than that in the other cells, and does not remain within a permissible limit; the voltage of the cell at the end of a charge is the lowest in value, while the temperature of the electrolyte during the charge may rise higher than that in the other cells.

During a discharge of the battery, the rapid drop in voltage of the lagging cell will limit the capacity of the battery. However, if the ampere­hours obtained during the control discharge are close to the guaranteed value, or to the ampere-hours obtained during the previous control discharge, the is considered is fit for service, although the discharge was interrupted because the voltage of only one cell has dropped to its final value. In a battery that is fully fit for service, the difference between the voltages of the cells at the time a discharge is discontinued will not exceed 0.2 V.

Timely detection and the remedying of lagging cells can only be attained when the density of the electrolyte is closely watched.

The density of the electrolyte in the cells after the latter are brought up to a working condition is corrected so that it will not differ in any given battery by more than 5kg/m3 from the established value for the given climatic conditions.

If during a routine charge of the battery all the usual signs of the end of the charge are observed (constant electrolyte density and voltage, and abundant gassing in all cells over a period of two hours), but the density of the electrolyte in some of the cells remains less than is normally required by more than 10 kg/m3, it is necessary lo discontinue the charge for one hour and then renew it for another two hours. lf the density of the electrolyte in the cells after this rises to within the limits of 10 kg/m3 from the required value, the battery may be put back into service. However, if the electrolyte density in some of the cells still remains too low, it can be considered that these are lagging cells. Such batteries must be subjected lo several cycles of charge and discharge to more completely convert the lead sulfate into active materials, and also decide on the necessity for any repairs.

If several batteries are operated on some given unit or machine and are connected in parallel or in series, it is important to closely watch that all of the cells in these batteries remain in the same condition.

When the capacity of the cells in one of the series-connected batteries is too low, the duration of battery dscharge is limited by the capacity of the lagging cells. If the discharge is continued it is possible that the lagging cells may have their polarity reversed.

In this connection it should be noted that the danger of polarity reversal is especially great when operating a repaired battery which, along with the old cells, contains cells provided with new sets of plates.

 

SULFATION PLATES IN LEAD-ACID BATTERIES

As is known, when a cell is discharged lead sulfate is formed on the positive and negative plates. This sulfate, during the subsequent charge of the cell, will readily be converted into active materials. The trouble called “sulfation of the plates" results from a certain discharged condition of the plates due to which they become covered with a layer of less-soluble lead sulfate which does not readily revert to an active material within the usual period of time required with a charging current of normal value.

The lead sulfate which appears in conditions of normal discharge consists of small crystals which are uniformly distributed and form a porous mass that is highly conductive. When a cell containing such a sulfate is charged again, the sulfate is readily converted into lead dioxide and lead. However, if the discharge of the cell is carried out too deeply, the active material of the plates is almost completely converted into lead sulfate which, in some cases, changes over from a small-crystal to a large-crystal structure.

When this is so, if organic surface­active agents are present in the cell, adsorption of these substances will take place on the surfaces of the lead sulfate crystals on the negative plates. This will lower the rate at which the crystals will be able to go into solution. It was found that the internal electrical resistance of the active-material layer increases abruptly in such cases. The sulfate particles completely cover the conductive active material and thus stop the passage of current through the plate. The potential at which gassing starts during charging also changes. The conversion of the portion of the sulfate carrying a film of adsorbed substances to lead dioxide and lead becomes impossible in these conditions, and the cell loses part of its capacity, and on very heavy sulfation may lose almost all its capacity. 

The causes that lead to the formation large crystals of lead sulfate may be: systematic, excessively deep discharges of the cells; regular undercharging of the cells; cells are left for long periods in either the semi-charged or semi-discharged condition; low level of the electrolyte in the cells.

The signs of sulfation of the plates are:

1. a decrease in cell capacity. The capacity in most cases is limited by the negative electrode. When this is so, the potential of the negative electrode on discharge at the 10-hour rate, after 5 to 8 hours, reaches the value of 0.4 to 0.6 V relative to a cadmium test electrode;

2. an electrolyte density less than the normal value;

3. a high cell voltage at the beginning and end of charge (up to 3 V). The potential of the negative electrode rapidly acquires a negative value after the charge begins;

4. gassing begins much earlier during the charge of the cell;

5. the positive plates have an abnormal colour (lightbrown, sometimes with white spots);

6. an abnormal condition of the negative plates. The active material of the negative plates has increased in volume and is seen to bulge from the pockets. A white deposit of sulfate is usually visible on the negative plates.

There are several methods of restoring the capacity of sulfated cells: a long charge of the cells with a small current; charging of the cells in distilled water; discharging of the cells with a small current; charging with a heavy current for 1 to 2 hours; cycling with polarity reversing.

 

SHORT CIRCUITS IN LEAD ­ACID BATTERIES

Short circuits may occur within a cell as a result of damage to one or several separators between the positive and negative plates; because of excess accumulation of sediment in the bottom of the cell container, or because of “treeing”, the growth of dendrites in the lead sediment. Dendrite formation may be due to two causes: (1) the loosened particles of the active material raised by the gassing during a charge settle on top of the plates and form bridges over the separators; (2) the grid material contains certain constituents, cadmium, for example, that facilitate the formation of dendrites at the sides and bottom of the plates.

Grids of pure lead also have a tendency to form dendrites in the direction from the negative to the positive plates. The presence of antimony in the grid material somewhat neutralizes this tendency. The kind of dendrite formation that may occur is influenced by the surface­active agents that find their way in the expanders included in the negative-plate active material. The signs of short-circuits inside the battery are: continuous decrease in electrolyte density, notwithstanding the fact that the battery is receiving a normal charge; rapid loss of capacity after a full charge; a low open­circuit voltage.

To remedy this condition it is necessary to dismantle the cells, remove all the sediment accumulated in the bottom, wash out the container, replace the old separators and remove any dendrites from the plate.


Corrosion of positive plate grids in lead-acid batteries.

While a cell is being charged, the lead sulfate which has been formed directly from the grid material as a result of local action is also converted into lead dioxide. This process, called the forming of the grid, although it somewhat weakens the grid, does not shorten its normal service lile.

Premature destruction of the positive plate grid takes place when the lead dioxide becomes separated from the lead grid surface and the electrolyte fills the space between them.

Long-continued overcharging causes oxidation of the positive grid, reduces the cross-section of the grid bars and eventually brings about complete destruction of the grids.

It should be borne in mind that the premature forming process may become accelerated if the charge is conducted at a temperature exceeding 45°C.

The grids of positive plates that have been subjected to this “overforming” may easily be detected by checking the colour of their fracture. If the fracture is seen to have a brown colour, it is an indication that the grid lead has become converted to lead dioxide. Such grids are brittle, and the positive plates may be easily broken by hand.

Contamination of the electrolyte by organic acids brings about rapid destruction of the grids; particularly heavy corrosion is caused by acetic acid. Chloride contamination of the electrolyte also causes corrosion of the grid.

A sign of grid corrosion is a reduced number of amperehours obtained from the battery on discharge at the 10 hour rate. The capacity is always limited by the positive electrode.

Cells containing plates destroyed by corrosion are no longer fit for service. Usually, corrosion of the grids is a sign of long service of the given cells.

 

 Bulging and buckling of positive plates.

 

 If the service conditions have been abnormal, the positive plates will be found to change in size, buckling will also be observed. These are the result of lack of uniformity in the rates of charging and discharging over the entire area of the plates. Buckling usually takes place during charges with currents of high density, short circuits, during overcharges, and because of failure to hold the temperature within permissible limits during a charge. The growth in size of the plates is due to gradual corrosion of the grid because the lead dioxide resulting from corrosion occupies a larger space than the grid lead from which it is formed. There are sometimes cases when the plates change their dimensions as much as several centimetres.

 

 Shedding of the positive active material.

 The shedding of the active material from the positive plates is one of the causes of premature failure in service of lead-acid cells. The essence of this trouble is that tiny crystals and grains of lead dioxide smaller than 0.1 micron (one tenth of one thousandth of a millimetre) become dislodged from the plates. The shedding mainly takes place at the end of a charge and the beginning of a discharge. Till recently, the explanation was that shedding is due to: volumetric variations of the material on the electrode during its operation, free gassing at the electrode during overcharges, and operation of the cells at high temperatures. 

The shedding of active material from the positive plates has been investigated by many electrochemists. It has been established that the temperature of the electrolyte and current density during the charge do not have an important bearing on the service life of the active material. It is the conditions of discharge that essentially affect the service life of the active materials.

lncrease in concentration of the electrolyte, reduction in temperature, and increase in current density during discharge greatly attribute to the rate of destruction of the active material.

For example, a reduction in the density of the electrolyte from 1,200 to 1,100 kg/m3 increases the service life of the active material some 8 to 10 times, and is the most essential factor. A three-fold reduction in the discharge current density lengthens the service life about 50 per cent, while an increase in temperature from 25 to 50C on discharge increases the service life of the active material more than 2 to 2.5 times.

It has been shown recently that the shedding of the active material is the result of the appearance of crystals of lead dioxade with a different form of crystalline structure.

One of the ways of increasing the service life of the active material is to introduce into the cell, after it has been in operation for 70 to 100 per cent of its guaranteed service life, about 0.5 to 1.0 per cent of a suitable reducing agent, for example, hydroxylamine sulfate (suggested by I. I. Koval). The purpose these agents serve is chemical reduction of lead dioxide to lead sulfate, from which, during a subsequent charge, is formed an active material which possesses a strong structure. However, this method has yet to be more widely tested.

 

Contamination of the electrolyte

Contamination of the electrolyte by impurities, especially by salts of the metals and organic substances, will greatly accelerate corrosion of the grids. The measures that must be taken to prevent contamination are simple and amount to preparing the electrolyte only from battery­grade sulfuric acid and distilled water.

In those cases when sulfuric acid of the technical grade is accidentally used to prepare the electrolyte, the active material, as well as the grids of the positive plates, due to presence in this acid of various impurities, are often destroyed even after the first charge.

 

This also occurs in those cases when, to prevent freezing of the electrolyte, alcohol is added to it.

Only use distilled water which is known to be pure to prepare the electrolyte and never use drinking water, it always contains compounds of iron, chlorides, nitrates (salts of nitric acid) and other substances which may destroy the active material and plate grids and lead to an increased self-discharge of the cells.

 

 Increased self-discharge.

Discharge of a cell which takes place while it remains open-circuited is called self-discharge.

When batteries are in service, cases arise where normal and increased rates of self-discharge may be observed.

A self-discharge, though inevitable, should not exceed a rate established as normal.

Normal self-discharge of a cell takes place due to several causes. The grid of the positive plate is not fully in contact with the lead dioxide and the electrolyte occupies the spaces left free between the grid and the lead dioxide. Because of this a difference in potential is created between the lead grid and lead dioxide, or in other words, a local cell which is in a state of discharge is formed.

The discharge of this local cell is accompanied by conversion of the active material into lead sulfate and thus hampers further discharge of the local cell. This explains why there is the considerable decrease in self-discharge from day to day when the battery is allowed to stand idle.

The negative plate grid, which is made of an alloy of lead with antimony, and the negative-plate active material containing sponge lead, represent two electrodes between which a difference in potential that causes self-discharge is created.

Metal impurities which can only be removed with great difficulty and are always present in the materials from which the plates are made, and the impurities contained by the electrolyte, are also causes of normal self-discharge. Another cause of normal self-discharge is that the density of the electrolyte at the bottom of the plates is always a little greater than that at the top of the plates.

Since the potential is dependent on the density of the electrolyte, a potential difference is created between the upper and lower parts of the plates, this leading to self-discharge.

If a film of electrolyte appears on the internal surface of the cell cover it forms a contact bridge between the terminal post of the groups of plates; this also may be a cause of self-discharge.

Batteries in which separators of mipor or miplast are used, when left to stand idle for 30 days, should have a normal self-discharge of not more than 21 per cent of their 10 hour rate capacity.

Let us consider the causes of excessive self-discharge.

During careless filling of electrolyte into the cell and violent gassing while charging, the external surface of the cell may become wetted by spilt electrolyte. This will greatly increase the rate of self-discharge. The rate of this self-discharge (or leakage) in some cases exceeds 5 to 10 per cent of battery capacity per day, due te which the battery may be discharged in 10 to 20 days.

This form of self-discharge may be detected with a voltmeter. One lead of the voltmeter is tightly held against the battery terminal, the other is held against the surface of the battery where traces of spilt electrolyte may remain. If the pointer of the voltmeter deviates from zero, it shows the existence of a current path for self-discharge.

Lead-Acid Cell and Battery Troubles and Their Remedies

 

PROBLEM

CAUSE

REMEDY

1. The battery has low capacity

1. Plates worn because of long service

Replace battery

2. Shedding of active material from positive plates

Replace battery

3. Systematic undercharge

Carry out a long overcharge cycle (equalize)

4. Contamination of electrolyte

Replace electrolyte, wash out cells

5. Sulfation of plates

Carry out desulfation charging

6. Leakage of current, heavy self discharge

Check cell containers, clean and dry the cells

7. Battery is used at a low temperature

Lag the battery to reduce the loss of heat, slightly increase the density of electrolyte

2. No voltage or practicly no voltage across cell terminals

Short cuircuit, high leakage of current, sulfation

Carry out desulfation charge, if does not help replace the battery

3. Abnormal increase in temperature of electrolyte during charging

1. Excessive charging current

Discontinue charge and decrease charging current

2. Short circuit in cell

Replace battery

3. Heavy sulfation

Carry out desulfation charge

4. The electrolyte has abnormal colour, cell contains much sediment

Shedding of active mass

Remove shedding by washing. Charge and discharge with normal current

5. Density of electrolyte is low at the end of charge, no gassing is observed

Short circuit in cell

Replace battery

6. Abnormal and premature gassing during charging

1. Sulfation

Carry out a desulfation charge

2. Large charging current

Change to normal value of current

3. Charge is carried out at too low temperature

Warm up battery

7. Heavy gasing during discharge

Dirty electrolyte

Change electrolyte

8. Abnormal colour of plates, presence of white spots on top parts of plates

1. Sulfation

Carry out desulfation charge

2. Contamination of electrolyte

Change electrolyte, wash out cells

3. Excessive lenth of service

Replace battery

9. Destruction of positive plates

1. Long term overcharges

Adjust charging rate of the cell to avoid overcharging

2. Contamination of electrolyte

Change electrolyte, wash out the cells

3. Excessive lenth of service

Replace the battery

Electrolyte is contaminated by chlorides or acids

Check and change electrolyte, wash out the cells