I found this information in a e-book called "Batteries in a Portable
World" by Buchmann. In our ongoing watered golf cart vs. expensive
maintenance-free gauze battery discussion, there are some very revealing
facts the author points out that I'm sure the battery sales people
wouldn't want you to look at too closely...

Sorry for the wordwrapping nonsense I can't seem to stop....

It says:

"The Lead Acid Battery
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, which 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 diverse applications, two
designations of batteries emerged.
They are the sealed lead acid (SLA),
also known under the brand name of
Gelcell, and the valve regulated lead
acid (VRLA). Technically, both
batteries are the same. No scientific
definition exists as to when an SLA
becomes a VRLA. (Engineers may
argue that the word ‘sealed lead acid’ is a misnomer because no lead acid
battery can be totally sealed. In essence, all are valve regulated.)
The SLA has a typical capacity range of 0.2Ah to 30Ah and powers portable
and wheeled applications. Typical uses are personal UPS units for PC
backup,
small emergency lighting units, ventilators for health care patients and
wheelchairs. Because of low cost, dependable service and minimal
maintenance requirements, the SLA battery is the preferred choice for
biomedical and health care instruments in hospitals and retirement homes.
The VRLA battery is generally used for stationary applications. Their
capacities range from 30Ah to several thousand Ah and are found in larger
UPS systems for power backup. Typical uses are mobile phone repeaters,
cable distribution centers, Internet hubs and utilities, as well as power
backup
for banks, hospitals, airports and military installations.
Unlike the flooded lead acid battery, both the 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, the SLA and VRLA can never be charged
to their full potential.
Among modern rechargeable batteries, the lead acid battery family has the
lowest energy density. For the purpose of analysis, we use the term
‘sealed
lead acid’ to describe the lead acid batteries for portable use and
‘valve
regulated lead acid’ for stationary applications. Because of our focus on
portable batteries, we focus mainly on the SLA.
The SLA 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. Whereas the
NiCd self-discharges approximately
40 percent of its stored energy in three
months, the SLA self-discharges the same
amount in one year. The SLA is relatively inexpensive to purchase but the
operational costs can be more expensive than the NiCd if full cycles are
required on a repetitive basis.
The SLA does not lend itself to fast charging — typical charge times are
8 to 16 hours. The SLA 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 the NiCd, the SLA does not like deep cycling. A full discharge
causes
extra strain and each discharge/charge cycle robs the battery of a small
amount of capacity. This loss is very small while the battery is in good
operating condition, but becomes more acute once the performance drops
below 80 percent of its nominal 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 SLA
battery is
recommended.
Depending on the depth of discharge and operating temperature, the SLA
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. Applying
charge/discharge
cycles does not prevent or reverse the trend.
There are some methods that improve the performance and prolong the life
of
the SLA. The optimum operating temperature for a VRLA 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. VRLA that would last for 10 years at 25°C would
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 temperature of 42°C (107°F).
The SLA has a relatively low energy density compared with other
rechargeable batteries, making it unsuitable for handheld devices that
demand
compact size. In addition, performance at low temperatures is greatly
reduced.
The SLA is rated at a 5-hour discharge or 0.2C. Some batteries are even
rated
at a slow 20 hour discharge. Longer discharge times produce higher
capacity
readings. The SLA performs well on high pulse currents. During these
pulses,
discharge rates well in excess of 1C can be drawn.
In terms of disposal, the SLA is less harmful than the NiCd battery but
the
high lead content makes the SLA environmentally unfriendly. Ninety
percent
of lead acid-based batteries are being recycled."

Most interesting....
No wonder they last longer up north where it's colder than here in the
tropics....contrary to the idea of pulling them out of the boat and
putting them in a warm place all winter... Looks like they're better off
in the boat under the snow!