Continuous float charging flooded SLI batteries accelerates the deterioration of polyethylene plate separators to a life shorter than that which battery designers intended. When subjected to continuous float charging, the dominant and earliest failure mode tends to be premature failure of the polyethylene separator. Separator failure typically results in short circuits that prevent the battery from functioning normally. And, because of the large amount of current available in a battery, short circuits are more likely to cause catastrophic (i.e. sudden release of large amounts of energy) battery failure than other types of failures. In contrast, in vehicle applications with intermittent charging, nearly all components in the battery tend to fail around the same time.

Battery separators keep apart the positive and negative electrodes of a galvanic cell to prevent short circuit. A separator has to serve over a long period of time to physically separate the electrodes without excessive hindrance to ionic current flows. In the hostile environment of a lead-acid battery, i.e. sulfuric acid and very strong oxidizers such as lead dioxide or nascent oxygen, only a few substances are stable over a long time. These substances include primarily silica, glass fibers, a few salts and a few synthetics. In contrast, all organic separator materials including polyethylene decompose and, via several intermediate steps, oxidize to carbon dioxide and water. In addition to sensitivity to oxidation, solubility and temperature, the amount of electric charge (in particular overcharge) accelerates oxidation.

Emergency generator and diesel fire pump regulations specify continuous float charging of lead-acid battery packs. Around-the clock charging greatly exceeds the limited daily charging time anticipated by the batteries’ designers. The mandated continuous charging accelerates oxidation of polyethylene separators such that the separator fails before other battery components. SLI batteries used in these applications thus fail sooner, and more catastrophically, than batteries used in the vehicle applications for which they were designed.

It is this mechanism of separator oxidation that appears to account for the relatively shorter life and more sudden failure of genset batteries than the same batteries employed in vehicles.

It must be noted that this polyethylene separator failure mode has yet to be validated by statistically significant laboratory analysis of failed genset batteries. There is no known “junk bin” study differentiating the failure mechanisms of batteries used in gensets from those used in vehicles.

There is another important fact to note. Genset starting batteries account for less than 5% of the total market for SLI batteries used for engine starting. This small market means that there is little incentive for suppliers of SLI batteries to manufacture special genset starting batteries designed to survive continuous float charging. The solution to this problem therefore lies elsewhere

****This article is collected from https://sens-usa.com/.

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