Lead-acid does not lend itself to fast charging. Typical charge time is 8 to 16 hours. The battery must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation, a condition that makes the battery difficult, if not impossible, to recharge.
Unlike nickel-cadmium, the lead-acid does not like deep cycling. A full discharge causes extra strain and each cycle robs the battery of a small amount of capacity. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger battery is recommended.
Finding the ideal charge voltage limit is critical. Any voltage level is a compromise. A high voltage limit (above 2.40V/cell) produces good battery performance but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent. A low voltage (below 2.40V/cell) is safe if charged at a higher temperature but is subject to sulfation on the negative plate.
Lead-acid is not subject to memory. Leaving the battery on float charge for a prolonged time does not cause damage. The self-discharge is about 40% per year, one of the best on rechargeable batteries. In comparison, nickel-cadmium self-discharges this amount in three months. Lead-acid is relatively inexpensive to purchase but the operational costs can be more expensive than the nickel-cadmium if full cycles are required on a repetitive basis.
The sealed lead-acid battery is rated at a 5-hour discharge or 0.2C. Some batteries are rated at a slow 20-hour discharge. Longer discharge times produce higher capacity readings. The lead-acid performs well on high load currents. During these pulses, discharge rates well in excess of 1C can be drawn.
Depending on the depth of discharge and operating temperature, the sealed lead-acid provides 200 to 300 discharge/charge cycles. The primary reason for its relatively short cycle life is grid corrosion of the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Cycling does not prevent or reverse the trend.
The optimum operating temperature for the lead-acid battery is 25°C (77°F). As a guideline, every 8°C (15°F) rise in temperature will cut the battery life in half. VRLA, which would last for 10 years at 25°C (77°F), will only be good for 5 years if operated at 33°C (95°F). Theoretically the same battery would endure a little more than one year at a desert temperature of 42°C (107°F).
Among modern rechargeable batteries, the lead-acid battery family has the lowest energy density, making it unsuitable for handheld devices that demand compact size. In addition, performance at low temperatures is poor.
Invented by the French physician Gaston Planté in 1859, lead-acid was the first rechargeable battery for commercial use. Today, the flooded lead-acid battery holds a domineering position in automobiles, forklifts and large uninterruptible power supply (UPS) systems.
During the mid 1970s, researchers developed a maintenance-free lead-acid battery that could operate in any position. The liquid electrolyte was transformed into moistened separators and the enclosure was sealed. Safety valves were added to allow venting of gas during charge and discharge.
Driven by different market needs, two lead-acid systems emerged: the small sealed lead-acid (SLA), also known under the brand name of Gelcell, and the large valve-regulated-lead-acid (VRLA). Technically, both batteries are the same. (Engineers may argue that the word 'sealed lead acid' is a misnomer because no rechargeable battery can be totally sealed.)
Unlike the flooded lead acid battery, both SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential.
In terms of disposal, the lead-acid is less harmful than nickel-cadmium but the high lead content and the electrolyte make the lead-acid environmentally unfriendly.


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