Difference between GEL and AGM batteries. How do they work?

Both GEL and AGM batteries are Valve-regulated Lead-Acid (VRLA) batteries. Both are sealed, non-hazardous and nonspillable.

The electrolyte does not flow like a regular liquid in an AGM or GEL battery. In AGM batteries, the electrolyte is trapped in a sponge-like mat of woven glass fibre separator material; in a GEL battery, the electrolyte is mixed with a silicate additive which immobilizes (changes) the electrolyte into a GEL-like material consistency.

GEL and AGM batteries are considered to be of a starved electrolyte design. The “acid-starved” condition of these batteries protects the plates during heavy deep- discharges. The more acid-starved the batteries are, the more protection is given to the active materials. The sulfuric acid in the battery electrolyte is fundamental to the discharge process. When the acid is depleted, the discharge process cannot go on.

Due to the physical properties of the GEL electrolyte and the batteries' inherently higher internal resistance, GEL battery power declines faster than an AGM battery as the temperature drops below 32˚F (0˚C).

AGM batteries excel for high-current, high-power applications in extremely cold environments, while GEL batteries tend to live longer in hot climates. It is important to note that the difference between GEL and AGM batteries for higher-current, high-power applications is very small at room temperature.

AGM batteries are better replacements for flooded batteries than GEL when used in alternator-equipped applications. GEL batteries require lower (precisely controlled) charging voltages (mainly lower finishing voltages) than AGM or flooded types. AGM battery charge voltages are more similar to flooded charge voltages (14.4V – 14.7V) than GEL charge voltages (13.5V to 13.8V).

How do DRY CELL AGM or GEL CELL batteries work?

Discover DRY CELL AGM and GEL CELL batteries are designed using proven gas recombination technology which removes the need for regular water addition by controlling the evolution of hydrogen and oxygen during charging. This means that the oxygen normally produced on the positive plates of all lead-acid batteries is absorbed by the negative plate through a porous medium (see Figure 1) without being vented.

The woven fibreglass mat is the porous medium in a DRY CELL AGM battery.

The porous medium in a GEL DRY CELL battery is the cracks in the GEL electrolyte. This suppresses the production of hydrogen at the negative plate. Water (H20) is produced instead, retaining the moisture within the battery. It never needs watering and should never be opened as this would “poison” the battery with additional oxygen from the air.

The retained oxygen produces an overpressure within the cell. This is normal.

The battery’s high-quality sealing valves will not open at too low pressure because this would allow too much oxygen to escape and be irretrievably lost. If the highly controlled and defined opening pressure is achieved, the sealing valves will open for a short time to release overpressure caused by the accumulated gas. Under normal operating conditions, this gas consists mainly of hydrogen. Under unfavourable conditions (high charge voltages at high temperatures, for instance), oxygen would also escape. In quality DRY CELL AGM and GEL batteries, the quantity of electrolyte as a ratio to active material (plates and oxides) is controlled in such a way as to allow the battery to attain an expected performance and design life under normal operating conditions, taking into account:

• The rate of recombination
• The corrosion of the positive electrode (oxygen consumption)
• The losses by diffusion through the cell container