The lead-acid battery is a commonly used chemistry. The flooded version is found in automobiles and large UPS battery banks. Most smaller, portable systems use the sealed version, also referred to as gel-cell or SLA.
The lead-acid chemistry is commonly used when high power is required, weight is not critical, and cost must be kept low. The typical current range of a medium-sized SLA device is 2 Ah to 50 Ah. Because of its minimal maintenance requirements and predictable storage characteristics, the SLA has found wide acceptance in the UPS industry, especially for point-of-application systems.
The SLA is not subject to memory. No harm is done by leaving the battery on float charge for a prolonged time. On the negative side, the SLA does not lend itself well to fast charging. Typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state because a discharged SLA will sulphate. If left discharged, a recharge may be difficult or even impossible.
Unlike the common NiCd, the SLA prefers a shallow discharge. A full discharge reduces the number of times the battery can be recharged, similar to a mechanical device that wears down when placed under stress. In fact, each discharge-charge cycle reduces (slightly) the storage capacity of the battery. This wear down characteristic also applies to other chemistries, including the NiMH.
The charge algorithm of the SLA differs from that of other batteries in that a voltage-limit rather than current-limit is used. Typically, a multistage charger applies three charge stages consisting of a constant- current charge, topping-charge, and float-charge. (Reffer Figure) During the constant-current stage, the battery charges to 70% in about 5 hours; the remaining 30% is completed by the topping charge. The slow topping-charge, lasting another 5 hours, is essential for the performance of the battery. If not provided, the SLA eventually loses the ability to accept a full charge, and the storage capacity of the battery is reduced. The third stage is the float-charge that compensates for self-discharge after the battery has been fully charged.
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Figure: The charge states of an SLA battery.
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The proper setting of the cell voltage limit is critical and is related to the conditions under which the battery is charged. A typical voltage limit range is from 2.30 to 2.45 V. If a slow charge is acceptable, or if the room temperature can exceed 30°C (86°F), the recommended voltage limit is 2.35 V/cell. If a faster charge is required and the room temperature remains below 30°C, 2.40 or 2.45 V/cell can be used. Table compares the advantages and disadvantages of the different voltage settings.
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| Table: Recommended Charge Voltage Limit for the SLA Battery |