696 Chapter 19
just been taken off charge, but in all instances it should
adjust to about 2.12 V after a period of time.
Gel/Cells comes as a type A or type B. The type A
Gel/Cell is conservatively designed for 4–6 years of
continuous charging in standby power applications.
During this period over 100 normal discharge/charge
cycles can be expected. Even more are obtained if only
minor discharges are experienced. The end of life is
actually determined by when the equipment will no
longer perform its required function. Since the battery
may still have 40–60% of its initial capacity, the service
life may be much longer.
The type A Gel/Cell has near its full nominal
capacity upon shipment from the factory. Type A cells
are used for alarm systems, memory standby, etc. where
they are normally in a standby mode.
The type B Gel/Cell is designed to provide 3–5 years
of service in standby power applications or 300–500
normal discharge-charge cycles in portable power appli-
cations.
As the battery is discharged, the terminal voltage
will slowly decrease. For instance, when the rated
capacity of the battery is removed over a 20 h period,
the terminal voltage would decrease to 1.75 V per cell.
These batteries are rated at a 20 h current rate and room
temperature. This means a 2.6 Ah battery would put out
0.13 A for 20 h. This does not mean, however, that it
will put out 2.6 A for 1 h (it would put out about 1.7 A
for 1 h).
Lead-dioxide batteries can be charged by the
constant-current or constant-voltage method. The
constant-current method is used when charger cost is
the primary consideration. The battery is forced to
receive a constant amount of current regardless of its
needs. While charger component economy is achieved,
it is sometimes done at the expense of recharge time or
service life if the current is not properly set. Trickle
charging current ranges from 0.5–2.0 mA per rated Ah
capacity of the battery.
When charging with the constant-voltage method, a
voltage of 2.25–2.30 V per cell should be used. To
maintain the battery at 100°F (38°C), a voltage of
2.2 V/cell is required, while at 30°F (0°C), 2.4 V/cell is
required.
19.10.10 Absorbed Glass Mat Batteries
Absorbed glass mat batteries (AGM) are sealed batter-
ies that can be operated in any position. AGM was
developed to provide increased safety, efficiency, and
durability. In AGM batteries the acid is absorbed into a
very fine glass mat that is not free to slosh around. The
plates are kept only moist with electrolyte, so gas
recombination is more efficient (99%). The AGM mate-
rial has an extremely low electrical resistance so the bat-
tery delivers high power and efficiency. AGM batteries
offer exceptional life cycles.
The plates in an AGM battery may be flat like wet
cell lead-acid batteries, or they may be wound in a tight
spiral. Their construction also allows for the lead in
their plates to be purer as they no longer need to support
their own weight. AGM batteries have a pressure relief
valve that activates when the battery is recharged at
voltage greater than 2.30 V/cell. In cylindrical AGM
batteries, the plates are thin and wound into spirals so
they are sometimes referred to as spiral wound.
AGM batteries have several advantages over both
gelled and flooded, at about the same cost as gelled:- All the electrolyte (acid) is contained in the glass
mats so they cannot spill or leak, even if broken.
Since there is no liquid to freeze and expand, they are
practically immune from freezing damage. - Most AGM batteries are recombinant—i.e., the
oxygen and hydrogen recombine inside the battery.
Using the gas phase transfer of oxygen to the nega-
tive plates to recombine them back into water while
charging prevents the loss of water through electrol-
ysis. The recombining is typically 99+% efficient. - AGM batteries have a self-discharge of 1–3% per
month. - AGM batteries 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 so they withstand shock and vibration.
19.10.10.1 A Comparison of the Three Types of Deep
Cycle BatteriesSafety. Batteries can be dangerous. They store a tremen-
dous amount of energy, create explosive gas during
charge and discharge, and contain dangerous chemicals.
Both gel and AGM batteries are sealed batteries that use
recombinant gas technology. AGM is more efficient in
the AGM process and completes its gas recombination
near the plates. Gel recombinant gas batteries should
incorporate automatic temperature-compensated voltage
regulators to prevent explosions associated with their
overcharging. Flooded batteries will spew acid, will defi-
nitely spill and leak if tipped over, and they generate
dangerous and noxious explosive gases. AGM batteries
are best at protecting both equipment and passengers.