AGM & EFB BATTERIES
The demands of stop-start and regenerative braking systems are at odds with achieving a long life from flooded lead-acid batteries. Not only is this battery technology intolerant of prolonged, heavy discharges, but also the plates can buckle and electrolyte levels can fall as a direct result of overheating, caused by exposure to high power surges that originate from either a smart alternator, or regenerative braking systems.
Therefore, another type of sealed VRLA battery technology, advanced glass mat (AGM), was developed. As this type takes more than twice as long to manufacture, compared to a flooded battery, comprises additional materials and requires more complex manufacturing methods, AGMS are considerably more expensive. However, do not be tempted by false economy and substitute an alternative battery type. AGM batteries are not only more efficient than flooded types, they also have a longer lifespan and allow the stop-start system to operate correctly. Additionally they can tolerate a deeper discharge – down to 50%.
Instead of flooding the plates, the electrolyte is absorbed within sponge-like mats that are bound tightly against the plate surfaces. No acid leakage will occur, should the outer casing be punctured, and the low internal resistance speeds-up the chemical reactions between plate and electrolyte, so the incoming high current from the regenerative braking system and smart alternator is more likely to be converted into chemical energy, making the battery more efficient. A further advantage is that AGM batteries do not require a fluid electrolyte to be maintained at a level above that of the plates, so taller grids can be introduced, which can increase the battery’s capacity by around 20%. AGM technology is also less prone to sulphation.
As their name implies, enhanced flooded batteries (EFB) are based on the standard flooded lead-acid design but with many improvements. Being a halfway house between a standard flooded battery and an AGM, they offer more cost-effective routes for car-makers to offer emission-saving technology on less expensive vehicles, especially those where Eu-set Type Approval emissions limits are lower. The advantage to the cost-conscious motorist is that replacement EFB batteries are less expensive than AGMS. According to GS Yuasa, EFBS not only offer more efficient charge acceptance, but also do not suffer significant harm when operated in a reduced state of charge, offering a longer lifespan compared to standard flooded batteries.
The main internal differences include an increased number of cell plates, different manufacturing techniques for the grids to enhance durability and the addition of lithium to the electrolyte to boost charge acceptance. Carbon additives are incorporated within the porous lead materials of the negative plates, while the positive plates have polyester covering sheets, which reduce damage resulting from deeper discharge cycles, plus additives in the lead dioxide increase longevity.