AGM batteries

Tests were also performed using VRLA batteries with gelled electrolyte [9]. The additives gave benefits similar to those obtained with AGM batteries. It was concluded that the porosity additives could have a positive effect on battery capacity. Future work with these additives will continue and involve full-scale batteries. 

The service-life of a conventional flooded battery under the harsh operating scheme in taxis, i.e., extended periods of standstill with lights and other equipment switched on, is only 6-12 months in Europe. In high-temperature climates, this may be even shorter. Taxi operation generates much more cycling of the battery than is found for the same type of car in private use, and this results in a tremendous reduction in battery life. Field tests with AGM batteries have shown an extension of taxi-duty life by a factor of 2 3 over flooded batteries. Therefore, a major European manufacturer has equipped all new taxis with AGM batteries since 2001. 

Automotive AGM batteries with a nominal voltage of 12 V (Fig. 12.9) have been on the market for several years [19-22]. While SLI batteries with free electrolyte are produced only with prismatic cells with a multiplicity of electrode plates, AGM... 

To complicate the situation even more, many of the properties of an AGM battery are already exhibited by modern, totally maintenance-free batteries with liquid electrolytes. This is particularly true for starter batteries as they are only expected to have a service-life of about 2000 h, which is considerably less than one-tenth of that which industrial battery manufacturers claim their products achieve when used in constant-voltage, float-charging applications. As a result, a VRLA battery will be used in vehicles with conventional electric power systems only if its advantages over a flooded battery are really needed. These include the following. 

These application scenarios exploit the high power density, the high cycling stability at moderate temperatures, and the advantageous behaviour in recovery from complete discharge that are provided by AGM technology. Such attributes justify the additional cost of AGM batteries. 

Key features of AGM batteries, which have also been achieved in modem, high-end, flooded starter batteries, inelude ... 

The main benefits of AGM batteries in present-day automotive applications, as compared with flooded battery designs with the same outer dimensions, are (i) higher power capability (ii) improved cycling capability, e.g., as in taxis ... 

This broad overview of start-idle-stop systems shows that the demarcation between the use of AGM batteries and the use of other electrochemical storage systems in future vehicle electrical systems with high cycling duty is not yet clear. There will be no either/or situation, but several battery systems have advantages and disadvantages and will exist side-by-side. What is unclear today is the portion of the overall automotive market each will cover. 

An example of an electrical system layout that is used in a European high-end sports car launched in 2001 is shown in Fig. 12.16. A cycling AGM battery, directly connected to the consumer harness and to the alternator, buffers electric energy, as... 

An accelerated life-time test, at a temperature of 55°C and a float-voltage of 2.27 V per cell on 12-V, 50-Ah, front-terminal AGM batteries, confirms the expected life of 12 years at 20°C. At intervals of about two months, capacity was checked at the 10-h discharge rate, and the test results are shown in Fig. 13.15. During more than 300 days the capacity remained at a high level and rather stable. Afterwards, the capacity decreased significantly, and the test was stopped after 380 days when the capacity had dropped to 70% of the nominal value. The float life at 55°C is, therefore, 1 year, and this equates to a life of 12 years for a float operation at 20°C. 

AGM batteries designed to provide very high power (180WP at the 15-min discharge rate) may offer a life of 10 years and are produced as 6 and 12-V units with capacities between 60 and 180 Ah. Such batteries are often used for applications... 

Fig. 13.14. 12-V, 100-Ah AGM battery (Marathon-type) with front terminals for telecommunications applications.

Fig. 13.16. Discharge power per cell of a 40-Ah, AGM battery (Sprinter-type) for discharge time periods between 3 and 60 min at different cut-off voltages between 1.60 and 1.90 V per cell.

All the AGM batteries diseussed here are designed to aeeommodate a short eireuit. This means, they ean survive a 30-day short eireuit and, after recharge, have virtually the same capaeity as before the test. This is also true for the gel battery-types described earlier. 

The charge behaviour shown in Fig. 13.17 was measured with AGM batteries with medium plate thickness. The trend is to reduce the plate thickness in order to achieve even higher power performance. This will further improve the charge acceptance of the battery. 

The temperature inside an AGM battery increases during fast charging [77,78]. Tests have shown a significant increase in the temperature although, normally, this... 

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