BMW are about to fit the new Varta AGM car battery as original equipment to their New Mini to meet the increased electrical demands of this vehicle. The AGM (absorbent glass mat) car battery is a relatively new type of sealed construction battery that uses saturated absorbent glass (boron silicate) mats between the plates instead of a gelled or liquid acid electrolyte. The AGM car battery is more expensive than the acid filled type but has distinct advantages.
The acid electrolyte in an AGM car battery is contained in a series of glass mats. This eliminates spillage, even if the battery casing is damaged. This type of car battery is therefore safer in use and of less potential damage to the environment. (A very positive feature as far as the etyres green policy is concerned.) Each AGM car battery is therefore less expensive to move from the factory to our mobile battery fitting branches. This somewhat offsets the much higher basic cost. Additionally, because an AGM car battery does not contain liquid there is no chance of problems resulting from freezing or liquid expansion.
AGM batteries are termed recombinant. This describes the design technology in which the oxygen and hydrogen recombine inside the car battery. Recombinant batteries use gas phase transfer of oxygen to the negative plates to enable recombination back into water while charging. This prevents water loss through electrolysis. This recombining has an efficiency of more than 99%..
The charging voltage of this type of car battery is the same as for a standard car battery. This eliminates the need for any special adjustments and problems with incompatible chargers or charging controls. The internal resistance of an AGM car battery is extremely low. This almost completely eliminates heating of the car battery even under heavy charge and discharge currents.
An AGM car battery has a very low standing discharge rate of approximately 1% to 3% per month. As a result storage for much longer periods is achieved without the need for the periodic re-charging that is required by a standard car battery.
The hydrogen emission of an AGM car battery, even under severe overcharge conditions, is far below the 4% maximum specified for aircraft and enclosed space situations. The plates in an AGM car battery are tightly packed and rigidly mounted, and will withstand shock and vibration better than a standard car battery.
The downside is that AGM technology costs up to three times as much as for a traditional, acid filled car battery, of the same capacity. It is assumed that as the AGM car battery becomes more widely fitted as standard equipment prices will reduce. We assume that this will be made possible by longer production runs and increased amortisation of development costs.
The AGM car battery provides a useful upgrade option for any vehicle that has retro-fitted electrical equipment. etyres is frequently asked to advise on increasing the specification for large 4x4s and MPVs where in-car entertainment for rear seat passengers has been added.
The BMW decision is based on the fact that the original car battery specification from about 1990 called for approximately 950 watts of power but this is has now increased to almost 3500 watts. The huge jump is because a car battery now has to cope with electronic engine management, ABS, air bags, a/c, electrically heated seats DVDs and satellite navigation. It is therefore essential that any vehicle that is fitted with an AGM car battery as standard should only have this type of car battery fitted as a replacement.