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Can the lead-acid battery compete in modern times?
(BU6)
The answer is YES. Lead-acid is the oldest rechargeable battery in
existence. Invented by the French physician Gaston Planté in
1859, lead-acid was the first rechargeable battery for commercial
use. 150 years later, we still have no cost-effective alternatives
for cars, wheelchairs, scooters, golf carts and UPS systems. The lead-acid
battery has retained a market share in applications where newer battery
chemistries would either be too expensive.
Lead-acid does not lend itself to fast charging. Typical charge time
is 8 to 16 hours. A periodic fully saturated charge is essential to
prevent sulfation and the battery must always be stored in a charged
state. Leaving the battery in a discharged condition causes sulfation
and a recharge may not be possible.
Finding the ideal charge voltage limit is critical. A high voltage
(above 2.40V/cell) produces good battery performance but shortens
the service life due to grid corrosion on the positive plate. A low
voltage limit is subject to sulfation on the negative plate. Leaving
the battery on float charge for a prolonged time does not cause damage.
Lead-acid does not like deep cycling. A full discharge causes extra
strain and each cycle robs the battery of some service life. 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. Lead-acid
is inexpensive but the operational costs can be higher than a nickel-based
system if repetitive full cycles are required.
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 lead-acid battery has one of the lowest energy densities, making
it unsuitable for portable devices. In addition, the performance at
low temperatures is marginal. 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. The high lead content
makes the lead-acid environmentally unfriendly.
Plate
thickness
The service life of a lead-acid battery can, in part, be measured
by the thickness of the positive plates. The thicker the plates,
the longer the life will be. During charging and discharging, the
lead on the plates gets gradually eaten away and the sediment falls
to the bottom. The weight of a battery is a good indication of the
lead content and the life expectancy.
The plates of automotive starter batteries are about 0.040"
(1mm) thick, while the typical golf cart battery will have plates
that are between 0.07-0.11" (1.8- 2.8mm) thick. Forklift batteries
may have plates that exceed 0.250" (6mm). Most industrial flooded
deep-cycle batteries use lead-antimony plates. This improves the
plate life but increases gassing and water loss.
Sealed
lead-acid
During the mid 1970s, researchers developed a maintenance-free lead-acid
battery that can operate in any position. The liquid electrolyte
is gelled into moistened separators and the enclosure is sealed.
Safety valves allow venting during charge, discharge and atmospheric
pressure changes.
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 larger Valve-regulated-lead-acid (VRLA). Both
batteries are similar. 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 because
excess charging would cause gassing and water depletion. Consequently,
these batteries can never be charged to their full potential.
To reduce dry-out, sealed lead-acid batteries use lead-calcium
instead of the lead-antimony.
The optimum operating temperature for the lead-acid battery
is 25*C (77*F). Elevated temperature reduces longevity. As a
guideline, every 8?C (15*F) rise in temperature will cut the
battery life in half. A 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 last a little
more than one year at a desert temperature of 42*C (107*F). |
Figure 1: Sealed lead-acid battery |
The
sealed lead-acid battery is rated at a 5-hour (0.2) and 20-hour
(0.05C) discharge. Longer discharge times produce higher capacity
readings because of lower losses. The lead-acid performs well on
high load currents.
Absorbed Glass Mat Batteries (AGM)
The AGM is a newer type sealed lead-acid that uses absorbed glass
mats between the plates. It is sealed, maintenance-free and the
plates are rigidly mounted to withstand extensive shock and vibration.
Nearly all AGM batteries are recombinant, meaning they can recombine
99% of the oxygen and hydrogen. There is almost no water is loss.
The charging voltages are the same as for other lead-acid batteries.
Even under severe overcharge conditions, hydrogen emission is below
the 4% specified for aircraft and enclosed spaces. The low self-discharge
of 1-3% per month allows long storage before recharging. The AGM
costs twice that of the flooded version of the same capacity. Because
of durability, German high performance cars use AGM batteries in
favor of the flooded type.
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.
- 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 if improperly charged.
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Created: April 2003, Last edited: January 2008
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.
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©
Copyright 2003 - 2005 Isidor Buchmann
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