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The grid structure in both pasted and
tubular plate batteries is made from a lead alloy. A pure
lead grid structure is not strong enough by itself to stand
vertically while supporting the active material. Other
metals in small quantities are alloyed with lead for added
strength and improved electrical properties. The most
commonly alloyed metals are antimony, calcium, tin, and
selenium.
The two most common alloys used today to
harden the grid are antimony and calcium. Batteries with
these types of grids are sometimes called
"lead-antimony" and "lead-calcium"
batteries. Tin is added to lead-calcium grids to improve
cyclability. The major differences between batteries with
lead-antimony and lead-calcium grids are as follows:
1. Lead-antimony batteries can be deep
cycled more times than lead-calcium batteries.
2. Flooded lead-antimony batteries require
more frequent maintenance as they near end-of-life since
they use an increasing amount of water and require periodic
equalization charges.
3. Lead-calcium batteries have lower
self-discharge rates as shown in the illustration below and
therefore, will draw less current while on float charge than
lead-antimony batteries.
4. Lead-calcium positive plates may grow in
length and width because of grid oxidation at the grain
boundaries. This oxidation is usually caused by long-term
overcharging, which is common to UPS and other batteries on
constant-float changing. Grids may grow in size sufficiently
to cause buckling or rupture of their containers.
Another type of grid alloy is lead-selenium.
In reality, this battery is actually a low lead-antimony
grid with a slight amount of selenium. Lead-selenium has
characteristics that fall somewhere between lead-calcium and
lead-antimony.
When pure lead is mixed with an alloy there
may be undesirable characteristics introduced in the
performance of the battery. Modern day battery manufacturers
try to reduce the amount of antimony and calcium by
introducing doping agents such as selenium, cadmium, tin,
and arsenic. When batteries containing arsenic and antimony
are charged (especially overcharged) the poisonous gases
arsine (AsH3) and stibine (SbH3) may
be released.
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