Periodic Charging – a Superior Method to Float Charging for Maintenance of VRLA Standby Batteries

Inhalt:
Because of their critical applications standby batteries must always be maintained at full charge. The universally accepted way of doing this is by constantly charging them, a process known as float charging. This is done by attaching the batteries to a constant voltage rectifier which is set at a voltage high enough so that sufficient current is flowing through the batteries to overcome parasitic self-discharge arising from the inherent thermodynamic instability of lead-acid batteries, the effects of impurities that enhance gassing and cell-to-cell chemical or manufacturing variations. Base transceiver stations (BTS), or cell towers, are almost universally equipped with VRLA batteries which do not allow water addition. There has been a tremendous increase over the last ten years of batteries deployed in uncontrolled outdoor installations and a high percentage of these are in tropical areas where high ambient temperatures are the norm. Although VRLA batteries have been very successful in reducing maintenance costs they have shorter life than the flooded type. They incorporate an exothermic oxygen recombination mechanism which generates heat inside them. Since there is minimal venting of gas this heat is trapped inside and can only be removed by inefficient transmission through the container walls. If they are closely packed in an installation very little heat can be removed. Therefore they operate at a higher temperature than flooded batteries which accelerates failure. In this paper we will discuss the mechanisms by which VRLA batteries fail and how these can be significantly reduced by a unique new maintenance system involving periodic charging instead of continuous float.