Invented by the French physician Gaston Planté in 1859, the lead acid was the first rechargeable battery for commercial use. Today, the flooded lead acid battery is used in automobiles and other heavy duty applications. Most portable equipment utilize the sealed or valve regulated versions, one of which is sold under the trademark gelcell.

Unlike the regular lead acid battery, the SLA is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge, preventing water depletion. Consequently, the SLA has a good storage life but is never recharged to its full potential. Among rechargeable batteries, the SLA has the lowest energy density.

The SLA is commonly used when bulk power is required, weight is not critical and battery cost must be kept low. A typical capacity range for portable applications is 1Ah to 30Ah and applications include wheelchairs, UPS units and emergency lighting. Some transportable cellular phones and camcorders also use the SLA battery. Because of its low self-discharge and minimal maintenance requirements, the SLA is the preferred choice for biomedical instruments in hospitals. Larger SLA batteries for stationary installations range from 50 to 200Ah.

The SLA is not subject to memory. No harm is done by leaving the battery on float charge for a prolonged time. Its charge retention is the best among rechargeable batteries. Whereas the NiCd self-discharges approximately 40% of its stored energy in three months, the SLA self-discharges the same amount in one year. The SLA is inexpensive to purchase but can be more expensive than the NiCd if a lot of full cycles are required.

The SLA does not lend itself to fast charging. Typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state. Leaving it in a discharged condition causes sulfation, a condition that makes the SLA difficult, if not impossible, to recharge ( Charging the SLA Battery ).

Unlike the NiCd, the SLA does not like deep cycling. A full discharge causes extra strain similar to that of a mechanical device. In fact, each discharge/charge cycle robs the battery of a small amount of capacity. This loss is very small while the battery is in good operating condition, but becomes more acute once it drops below 80% of its nominal capacity. This wear-down characteristic also applies to other battery chemistries in varying degrees. To ease the strain of deep discharging, a slightly oversized SLA battery can be used.

Depending on the depth of discharge and the operating temperature, the SLA provides 200 to 500 discharge/charge cycles. The primary reason for its relatively short cycle life is an expansion of the positive plates which is a result of a chemical reaction within the cell. This growth phenomenon is most prevalent at higher temperatures. Applying charge/discharge cycles does not prevent the growth, neither can deep-cycling reverse the trend. However, there are methods to improve the performance of the SLA .

An additional difficulty with the SLA is the relatively low energy density compared with other rechargeables, making it unsuitable for devices demanding compact size. This deficiency becomes more critical with cold temperatures since its ability to deliver high load current below freezing is much reduced. Paradoxically, the SLA performs quite well with an intermittent high-rate pulse discharge. During these pulses, discharge rates in excess of 1C can be drawn (C-Rate)

Because of its high lead content, the SLA is not environmentally friendly but is less harmful than the NiCd if carelessly disposed.

Charging The Sealed Lead Acid (SLA) Battery
Restoring The Sealed Lead Acid (SLA) Battery

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