Safety The grids, connecting strips and the battery

7.24.1 Safety The grids, connecting strips and the battery

posts are made from a lead alloy. For many years The following points must be observed when

this was lead antimony (PbSb) but this has now working with batteries:

been largely replaced by lead calcium (PbCa). The newer materials cause less gassing of the

● good ventilation electrolyte when the battery is fully charged. This ● protective clothing

has been one of the main reasons why sealed bat- ● supply of water available (running water

teries became feasible as water loss is consider- preferable)

ably reduced.

● first aid equipment available, including eyewash Modern batteries described as sealed do still ● no smoking or naked lights permitted.

have a small vent to stop the pressure buildup due to the very small amount of gassing. A fur- ther requirement of sealed batteries is accurate

control of charging voltage. Incremental changes over the years have made

7.24.2 Lead-acid batteries

7.24.3 Battery rating

the sealed and maintenance-free battery, now in

common use, very reliable and long lasting. This In simple terms the characteristics or rating of a may not always appear to be the case to some end

particular battery are determined by how much users, but note that quality is often related to the

current it can produce and how long it can sustain price the customer pays. Many bottom of the

this current. The rate at which a battery can pro- range cheap batteries with a 12 month guarantee

duce current is determined by the speed of the will last for 13 months!

chemical reaction. This in turn is determined by The basic construction of a nominal 12 V

a number of factors:

lead-acid battery consists of six cells connected in series. Each cell producing about 2 V is housed ● surface area of the plates;

in an individual compartment within a polypropyl- ● temperature; ene or similar case. Figure 7.54 shows a cut- ● electrolyte strength; away battery with its main component parts. The ● current demanded.

active material is held in grids or baskets to form The actual current supplied therefore deter- the positive and negative plates. Separators made

mines the overall capacity of a battery. The rating

Figure 7.54 Lead-acid battery

156 Advanced automotive fault diagnosis of a battery has to specify the current output and

7.25 Diagnosing battery

the time.

faults

Ampere hour This is now seldom used but describes capacity

how much current the battery is able to

7.25.1 Servicing batteries

supply for either 10 or 20 hours. The 20-hour figure is the most common. For

In use a battery requires very little attention other

example, a battery quoted as being

than the following when necessary.

44 Ah (ampere-hour) will be able, if fully charged, to supply 2.2 A for 20 hours

● Corrosion should be cleaned from terminals

before being completely discharged (cell

using hot water.

voltage above 1.75 V)

● Terminals should be smeared with petroleum

Reserve capacity A system used now on all new batteries

jelly or vaseline not ordinary grease.

is reserve capacity. This is quoted as a

● Battery tops should be clean and dry.

time in minutes for which the battery

will supply 25 A at 25°C to a final ● If not sealed, cells should be topped up with

distilled water 3 mm above the plates.

voltage of 1.75 V per cell. This is used

to give an indication of how long the

● Battery should be securely clamped in position.

battery could run the car if the charging system was not working. Typically a

44 Ah battery will have a reserve capacity of about 60 minutes.

7.25.2 Maintenance-free

Cold cranking amps Batteries are given a rating to indicate

By far the majority of batteries now available are

performance at high current output and

classed as ‘maintenance-free’. This implies that

at low temperature. A typical value of

little attention is required during the life of the

170 A means that the battery will supply this current for one minute at a

battery. Earlier batteries and some heavier types

do, however, still require the electrolyte level to

temperature of ⫺18°C at which point

the cell voltage will fall to 1.4 V (BS).

be checked and topped up periodically. Battery posts are still a little prone to corrosion and hence the usual service of cleaning with hot water if

These cold cranking amps (CCA) capacity rating appropriate and the application of petroleum jelly methods do vary to some extent: British standards,

or proprietary terminal grease is still recom- DIN standards and SAE standards are the three

mended. Ensuring that the battery case and in main examples!

particular the top remains clean will help to reduce the rate of self-discharge.

● BS 60 seconds The state of charge of a battery is still very ● DIN 30 seconds important and in general it is not advisable to ● SAE 30 seconds allow the state of charge to fall below 70% for

long periods as the sulphate on the plates can In summary, the capacity of a battery is the

harden, making recharging difficult. If a battery amount of electrical energy that can be obtained

is to be stored for a long period (more than a few from it. It is usually given in ampere-hours, reserve

weeks), then it must be recharged every so often capacity (RC) and cold cranking amps.

to prevent it from becoming sulphated. Recom-

mendations vary but a recharge every six weeks

A 40 Ah battery means it should give 2 A for is a reasonable suggestion.

20 hours. ● The reserve capacity indicates the time in min-

utes for which the battery will supply 25 A at

7.25.3 Charging

25°C. ● Cold cranking current indicates the maximum

The recharging recommendations of battery manu- battery current at ⫺18°C (0°F) for a set time

facturers vary slightly. The following methods, (standards vary).

however, are reasonably compatible and should not cause any problems. The efficiency of a bat-

A battery for normal light vehicle use may be tery is not 100%. Therefore the recharging process rated as follows: 44 Ah, 60 RC and 170 A CCA

must ‘put back’ the same Ah capacity as was used (BS). A ‘heavy duty’ battery will have the same

on discharge plus a bit more to allow for losses. Ah rating as its ‘standard duty’ counterpart, but it

It is therefore clear that the main question about will have a higher CCA and RC.

charging is not how much, but at what rate.

Engine systems 157

The old recommendation was that the battery the buildup of pressure. A rate of about five times should be charged at a tenth of its Ah capacity for

the ‘normal’ charge setting will bring the battery to about 10 hours or less. This is assuming that the

70–80% of its full capacity within approximately Ah capacity is quoted at the twenty hour rate, as

one hour. The table below summarises the charg-

a tenth of this figure will make allowance for the ing techniques for a lead-acid battery. Figures 7.55 charge factor. This figure is still valid but as Ah

and 7.56 show two typical battery chargers. capacity is not always used nowadays, a different method of deciding the rate is necessary. One way

Charging method

Notes

is to set a rate at a sixteenth of the reserve capacity, again for up to 10 hours. The final suggestion is

Constant voltage

Will recharge any battery in seven hours

to set a charge rate at one fortieth of the cold start

or less without any risk of overcharging

performance figure, also for up to 10 hours. (14.4 V maximum)

Constant current

Ideal charge rate can be estimated as:

Clearly if a battery is already half charged, half

1/10 of Ah capacity, 1/16 of RC or 1/40

the time is required to recharge to full capacity.

of cold start current (charge time of 10

In summary the ideal charge rate is determined

to 12 hours or pro rata original state)

from:

Boost charging

At no more than five times the ideal rate, a battery can be brought up to

● 1/10 of the Ah capacity;

about 70% of charge in about one hour

● 1/16 of the RC; ● 1/40 of the CCA.