I swear by the Big Crank ETX15L Battery:
Read about the AGM battery technology here: http://www.vonwentzel.net/Battery/01.Type/index.html
A newer type of sealed battery uses "Absorbed Glass Mats", or AGM
between the plates. This is a very fine fiber Boron-Silicate glass
mat. These type of batteries have all the advantages of gelled, but
can take much more abuse. We sell the Concorde (and Lifeline, made by
Concorde) AGM batteries. These are also called "starved electrolyte",
as the mat is about 95% saturated rather than fully soaked. That also
means that they will not leak acid even if broken.
AGM batteries have several advantages over both gelled and flooded, at
about the same cost as gelled:
Since all the electrolyte (acid) is contained in the glass mats, they
cannot spill, even if broken. This also means that since they are
non-hazardous, the shipping costs are lower. In addition, since there
is no liquid to freeze and expand, they are practically immune from
Nearly all AGM batteries are "recombinant" - what that means is that
the Oxygen and Hydrogen recombine INSIDE the battery. These use gas
phase transfer of oxygen to the negative plates to recombine them back
into water while charging and prevent the loss of water through
electrolysis. The recombining is typically 99+% efficient, so almost
no water is lost.
The charging voltages are the same as for any standard battery - no
need for any special adjustments or problems with incompatible
chargers or charge controls. And, since the internal resistance is
extremely low, there is almost no heating of the battery even under
heavy charge and discharge currents. The Concorde (and most AGM)
batteries have no charge or discharge current limits.
AGM's have a very low self-discharge - from 1% to 3% per month is
usual. This means that they can sit in storage for much longer periods
without charging than standard batteries. The Concorde batteries can
be almost fully recharged (95% or better) even after 30 days of being
AGM's do not have any liquid to spill, and even under severe
overcharge conditions hydrogen emission is far below the 4% max
specified for aircraft and enclosed spaces. The plates in AGM's are
tightly packed and rigidly mounted, and will withstand shock and
vibration better than any standard battery.
Even with all the advantages listed above, there is still a place for
the standard flooded deep cycle battery. AGM's will cost 2 to 3 times
as much as flooded batteries of the same capacity. In many
installations, where the batteries are set in an area where you don't
have to worry about fumes or leakage, a standard or industrial deep
cycle is a better economic choice. AGM batteries main advantages are
no maintenance, completely sealed against fumes, Hydrogen, or leakage,
non-spilling even if they are broken, and can survive most freezes.
Not everyone needs these features.