All About the Gel cell battery. Sealed batteries have been steadily evolving for some time, and the gel electrolyte battery was the first innovative alternative to flooded cells.
These batteries are a recombinant battery, in that the oxygen generated from the positive plate recombines with the hydrogen given off by the negative plate to form water, subsequently no electrolyte replenishment is required.
They are also known as Sealed Valve Regulated (SVR) batteries as the oxygen is retained in the cell by sealing vents, which maintain positive internal pressure and this is essential to the recombination process.
The valve also has a safety function to vent any excess pressure that arises during the charging process, otherwise serious damage would occur.
Unlike normal lead acid flooded cells with liquid acid electrolytes the gel cell has a solidified thixotropic gel, which is locked into each group of plates, and one feature of thixotropic gels is that they possess a reduced viscosity under stress.
Gel cell battery electrolytes have a high viscosity, and during cycles of charge and discharge voids and cracks can develop in the gel. The effect is a resistance to charging and a loss in capacity.
Liquification of the gel occurs (thixotropic action) during charging due to this gas shearing effect, and it can take more than an hour to solidify again after charging ceases.
The gel cell battery is manufactured from a mixture comprising sulfuric acid, fumed silica, pure water and phosphoric acid.
The construction of a gel battery is different, as the plates are reinforced with calcium, and not with antimony, which results in a reduction in battery self-discharge rates, typically around only 1% per month. The newer battery types use phosphoric acid to assist in retarding the plate sulfation hardening rates, and grid designs are also undergoing change and improvement using copper calcium lead alloys. The plates are relatively thin, which is to facilitate the gel diffusion into them, and this results in higher charge acceptance rates than flooded cells, and so a more rapid charge rate is achievable.
The optimum life and performance requires constant potential, voltage regulated charging in the range 13.8 volts to a maximum of 14.1 volts at 68°F. As the open circuit voltage of a fully charged battery is 12.8 volts the charge voltage must exceed 13.8 volts to overcome internal resistance.
Fast charge regulators that go over this corrected for ambient temperature cannot be used. The relatively thick separators that are used also increase the distance between the plates, which reduces the high current transfer rates. Gel electrolytes also have lower densities, which also reduces the charging voltages with the low temperature performance is also better than flooded cells.
There are downsides, and while self discharge rates are very low, the charge acceptance rates high and no maintenance requirements are all positive advantages, unlike the flooded cell battery the gel cell is intolerant to high charging voltages such as in regulator failures, and this includes the application of equalising charges, and either will seriously damage the battery. In addition the cycling capability compared to quality deep cycles is less, along with considerably higher capital costs.
The expected lifespan of the gel cell battery in comparison to a quality deep cycle flooded cell lead-acid-battery is in the range of 800-1000 cycles of charge and discharge to 50%, where a quality deep cycle battery has a life of up to approximately 2500 cycles.
Gel cells however do have a much greater cycling capability than normal starting batteries, and in many applications they are ideal. If the safety valves malfunction the cells are also easily damaged by oxygen contamination, although in a quality gel cell battery this is uncommon. All About the Gel cell battery