Absorbent glass mat (AGM) batteries have electrolyte which is solidified by an absorbent glass fleece material made of boron silicate. This acts as a separator between the electrodes and absorbs the free electrolyte (like a sponge). Its purpose is to promote recombination of the hydrogen and oxygen given off during the charging process. The fibreglass matt absorbs and immobilises the acid in the matt but keeps it in a liquid rather than a gel form. In this way the acid is more readily available to the plates so allowing faster reactions between the acid and the plate material and allowing higher charge/ discharge rates as well as deep cycling.
This construction is very robust and able to withstand severe shock and vibration and the cells will not leak even if the case is cracked. The battery is sealed with a cover which includes the sealing plugs and the gas vent. Unlike maintenance-free batteries, AGM batteries are not equipped with a built-in condition indicator. AGM batteries generally belong to the VRLA function type and are employed in applications where cycle capacity, cold start and leak protection need to be high. They have a very low self-discharge rate of 1%– 3% per month. The disadvantages of the AGM battery are that they are expensive and not suitable for engine compartment mounting due to temperature effects.
3.2.5 Gel
These batteries are similar in operation to the VRLA type in that the gases are recombined during charging. The electrolyte is solidified to a gel mass by the addition of silicic acid to form sulphuric acid. The phosphorus acid contained in the electrolyte increases the charge/discharge cycle capacity and therefore offers favourable conditions for recharging after deep discharge. The battery is sealed by a battery cover which contains non-removable sealing plugs and a gas
vent channel. These batteries are not equipped with a condition indicator.
The advantages of this battery technology are mainly due to the solid electrolyte (which means there is no possibility of acid spillage), the high charge/discharge cycle capability and the fact that they are maintenance free. The disadvantages include poor performance for cold starts, high price and poor suitability for installation in the engine compartment (due to elevated temperatures). Therefore they are normally used in special applications and not for general automotive application.
3.2.6 Hybrid
These batteries were an interim development. The negative plates are made of calcium alloy and the positive from antimony alloy. This substantially reduces gassing and thereby water consumption compared with traditional battery types. However, due to the antimony component that still exists in this battery technology, they cannot meet the extreme demands for low water consumption that are currently a feature of the automotive passenger car sector. Only maintenance- free batteries can achieve this.
A number of different battery technologies have been developed over the years. In the future, battery technology is likely to change dramatically with the introduction of hybrid vehicles to meet emission regulations
Modern battery types should not be fast charged and must be handled according to manufacturer instructions
In many batteries there is no access to the cells or electrolyte. Some have built-in hydrometers and condition indicators K ey P oints K ey P oints
3.3
BATTERY MAINTENANCE
3.3.1 Battery testing
There are three basic checks which should be performed in order to establish the condition of the battery for its application. These are:
● visual inspection
● specific gravity check ● load test.
The first indication that a unit is failing is when the starter is operated with a cold engine. Under these severe conditions the output from a healthy battery is well below the potential maximum, so a faulty battery is clearly evident. Assuming that the battery condition
is the source of the problem (after checking cabling, starter motor and switching arrangements), the following checks will confirm the diagnosis.
Visual inspection
The battery should be checked for obvious damage around the external casing (cracks etc.). A white powdery corrosion of metal parts around the battery indicates leakage of acid. This can be removed via washing with ammoniated water. Evidence of bulging of the container suggests that the plates have deformed internally and this results in a reduction of battery capacity. This is normally caused by overcharging. Fluid levels in the cells should be checked (if accessible). If they are not covered by electrolyte then it will be necessary
to top up with distilled water before proceeding further. The terminals (see Figure 3.15) are also important as corrosion can cause a high resistance. If the terminal is covered in white powder or green-white paste then the battery should be removed and washed down with ammoniated water. The cable end terminals must be cleaned in the same way, and if there is any doubt about their integrity they should be replaced. On reassembly, the terminals (battery and cable) should be coated with petroleum jelly.
Figure 3.15 Battery connectors
Specific gravity check
This check indicates the state of charge of the battery and is done using a battery hydrometer which consists of an acid lifter (glass tube with rubber suction ball) containing an aerometer (a float marked with a scale showing electrolyte relative density) (see Figure 3.16).
A fully serviceable battery should give a reading of 1.280 with a variation between cells of less than 0.050. This result is temperature dependent. Figure 3.17 shows the correction factor which must be applied if the ambient temperature deviates significantly from the standard condition of 15°C.
Typical results of a hydrometer test are shown in Table 3.1.
Figure 3.16 Hydrometer
Figure 3.17 Specific gravity correction
Most batteries are now sealed and do not allow access to the electrolyte, but certain types are fitted with built- in hydrometers as shown in Figure 3.18.
Load tests (high rate discharge)
This is a severe test which should only be performed on a charged battery showing at least 1.200 per cell during a specific gravity check. The test simulates the electrical load demanded from the battery under extreme starting conditions. For this reason the test should not be performed for an extended period of time (normally it should be performed for about 15 seconds).
Simple testers of this type consist of a low-resistance strip with a voltmeter connected across it. This is
Table 3.1 Results of hydrometer testing
Reading Variation Action
1.270 less than Battery in good condition;
0.050 confirm with drop test
1.190 less than Discharged battery; recharge
0.050 for 10 hours at the battery's
bench charge rate and retest
Some cells more than Battery should be scrapped
less than 0.100
1.200
84 Power storage Fundamentals of Motor Vehicle Technology: Book 3
connected to the battery, hence drawing a significant amount of power which is dissipated as heat at the resistor. The voltmeter shows the voltage whilst supplying this current. Generally, a healthy battery should maintain 9.6 V for 15 seconds during this test. A typical hand-held unit is shown in Figure 3.19.
More sophisticated versions of this tester are available and are used in preference to the traditional ‘drop’ type tester above. These incorporate a carbon pile loading unit that can be adjusted to suit the battery capacity. A load of half the cold-test current for the battery is typically specified. The cold-test current and other information can be found on the battery manufacturer’s label.
Note that the performance of the battery is temperature dependent and this must be considered when testing a battery. If the battery does not maintain the voltage for the specified time then it is classified as unserviceable. Removing the vent caps (if possible)
allows visual monitoring of the electrolyte, and if a cell is faulty often the electrolyte can be seen ‘boiling’ in that cell. This is a vigorous reaction which is due to the cell being faulty and should not be confused with ‘gassing’ which is a normal cell reaction when the battery approaches full charge.
Modern battery test technology incorporates sophisticated electronics and new test techniques to evaluate the state of the battery. This involves impedance and conductance test modes where the test method involves applying a small AC voltage of known frequency and amplitude across the cell and measuring the in-phase AC current that flows in response to it. The impedance is calculated by Ohm’s law and the conductance is similarly calculated as the reciprocal of the impedance.
Note that the impedance increases as the battery deteriorates while the conductance decreases. Thus conductance correlates directly with the battery’s ability
to produce current whereas impedance gives an inverse correlation. The conductance of the cell therefore provides an indirect approximation to the state of health of the cell. This measurement can be refined by taking other factors into account like temperature etc. In addition, impedance and conductance tests will obviously detect cell defects such as shorts and open circuits.
A typical hand-held unit is shown in Figure 3.20.
but under certain conditions external chargers are needed (i.e. the battery has become completely discharged, the vehicle has been out of use). These off-vehicle chargers can be subdivided as follows.
Bench chargers
Normally used in a workshop environment, these can be standalone units for charging single batteries or larger units that can charge a number of batteries at once in a series-parallel arrangement (see Figure 3.21).
The charger operates from mains voltage and incorporates a transformer (to reduce voltage/increase current) and a rectifier (which converts AC to DC current). A typical simple circuit for a bench battery charger is shown in Figure 3.22.
Figure 3.19 High-rate discharge tester
Figure 3.20 Hand-held impedance and conductance tester
Figure 3.21 Bench charging
Figure 3.22 Layout of a battery charger