This internal discharge (self-discharge) is a characteristic shared by all lead acid batteries, but the rate varies with battery type. The wet, or flooded lead acid types are constructed of a lead-antimony alloy, to provide structural integrity to the battery. This lead-antimony alloy construction leads to internal discharge rates between 8% and 40% per month in a flooded lead acid battery. The sealed lead acid battery types (SLA), AGM (Absorbed Glass Mat) and Gel, use a lead-calcium alloy, which does not have the same mechanical strength as the lead-antimony, since the construction (mats between plates in the AGM, gelled electrolyte in the gel cell) stabilizes the battery structure. The lead-calcium batteries have a lower internal discharge rate which is between 2% and 10% a month. This internal discharge leads to suphation on the battery plates, which can harden over time, either reducing battery capacity, or destroying a lead acid battery's ability to be recharged. A fairly high percentage of premature lead acid battery failure is from sulphation. Proper battery charging and use of floating chargers reduces or eliminates the sulphation problem.
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