|
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 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.
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.
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.
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.
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.
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.
Advantages
· 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.
Limitations
· 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. Thermal runaway can occur with
improper charging
About
the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver
BC.
Mr. Buchmann has a background in radio communications and has studied
the behavior of rechargeable batteries in practical, everyday applications
for two decades. Award winning author of many articles and books on batteries,
Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery
analyzers and PC software. For product information please visit www.cadex.com.
|