AGM & EFB BAT­TER­IES

Car Mechanics (UK) - - Vehicle 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.

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