Q? Can the Lead-acid battery compete in modern times?
The answer is YES. Lead-acid is the oldest rechargeable battery in existence. It has retained a market share in applications where newer battery chemistries would either be too expensive or the upkeep would be too demanding. There are simply no cost-effective alternatives for such applications as wheelchairs, scooters, golf carts, people movers and UPS systems.
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 is used 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. They are 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 the Sealed-lead-acid and the Valve-regulated-lead-acid batteries 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.
Finding the ideal charge Voltage limit is critical. Any Voltage level is a compromise. The author of a paper in a battery seminar explained that: “Charging a sealed Lead-acid battery using the traditional float charge techniques is like dancing on the head of a pin.” A high Voltage limit produces good battery performance but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent. A low Voltage protects the battery and allows charging under a higher temperature but is subject to sulfation on the negative plate.
The Lead-acid is not subject to memory. Leaving the battery on float charge for a prolonged time does not cause damage. The battery’s charge retention is best among rechargeable batteries. The self-discharge is about 40 percent per year. 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.
Lead-acid does not lend itself to fast charging — typical charge times are 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.
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 rule of thumb, every 8°C (15°F) rise in temperature will cut the battery life in half. A Valve-regulated-lead-acid, 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). 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.
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 pulse currents. During these pulses, discharge rates well in excess of 1C can be drawn.
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.
· Inexpensive and simple to manufacture.
· Mature, reliable and well-understood technology — when used correctly, Lead-acid is durable and provides dependable service.
· The self-discharge is among the lowest of rechargeable battery systems.
· Low maintenance requirements — no memory; no electrolyte to fill on sealed version.
· Capable of high discharge rates.
· Low energy density — poor weight-to-energy ratio limits use to stationary and wheeled applications.
· Cannot be stored in a discharged condition — the cell Voltage should never drop below 2.10V.
· Allows only a limited number of full discharge cycles — well suited for standby applications that require only occasional deep discharges.
· Lead content and electrolyte make the battery environmentally unfriendly.
· Transportation restrictions on flooded Lead-acid — there are environmental concerns regarding spillage in case of an accident. Thermal runaway can occur with improper charging.
Isidor Buchmann, founder and CEO,