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AGM (Absorbed Glass Mat) Batteries 

The AGM construction allows the electrolyte to be suspended in close proximity with the plate’s active material. This enhances both the discharge and recharge efficiency.

Since there are no liquid inside, these batterie scan perform better than flooded batteries in applications where maintenance is difficult to perform, however they are sensitive to over or under charging affecting their life and performance. AGM batteries perform most reliably when their use is limited to the discharge of no more than 50% of battery capacity.

AGM batteries are usually the type of batteries selected in off-grid power systems.

Gel Cell Batteries

Gel cell batteries have a water-acid in gel form. The electrolyte in a gel cell battery has a silica additive that causes it to set up or stiffen.  The recharge voltages on this type of cell are lower than the other styles of lead acid battery and gel cells are probably the most sensitive cell in terms of adverse reactions to over-voltage charging.

Gel batteries are best used in very-deep cycle applications and may last a bit longer in hot weather. Unfortunately a total deep discharge will irreversibly destroy the battery. If the incorrect battery charger is used on a gel cell battery, poor performance and premature failure is certain. 

Note: It is very common for individuals to use the term gel cell when referring to sealed, maintenance-free batteries, much like one would use Kleenex when referring to facial tissue. Be very careful when specifying a charger. More often than not, what someone is referring to a gel cell they really mean sealed, maintenance-free VRLA or AGM-style battery. Gel cell batteries are not as common as AGM batteries, and would be hard to source in humanitarian contexts.

Absorption Voltage Range

Float Voltage Range

Flooded Batteries

14.4 to 14.9 volts

13.1 to 13.4 volts.

VRLA Batteries

14.2 to 14.5 volts

13.2 to 13.5 volts.

AGM Batteries 

14.4 to 15.0 volts

13.2 to 13.8 volts.

GEL Batteries

14.0 to 14.2 volts

13.1 to 13.3 volts.


Capacity is defined as the total amount of energy a battery can store and reproduce in the form of electricity. Battery capacity is usually described in multiples and orders of magnitude of Watt-hours (Wh) – 1 Wh to one 1 kWh (1,000 Watt-hours). A Watt-hour is defined as the electrical energy required to supply a Watt of electricity for one continuous hour. For example, a standard 60W incandescent bulb would require 60Wh of stored energy to function for one hour. It is easy to see why properly estimating consumption needs are important for designing battery back-up systems, especially for security or mission critical related items.