OddMix.com - PC Technical Note - PCN0701 - Karl Nagy
|Picture 1. Seven AmpereHour gel cell battery|
Sealed lead acid batteries
are often referred to as Gel Cell
s - Picture 1. Gel cell
s are specially constructed lead acid batteries. There are two major types of sealed lead acid cells. In true Gel Cell
s, the sulphuric acid electrolyte is thickened (gelled) to allow the battery to work in all positions. Unfortunately this electrolyte thickening decreases the mobility of the ions which results in increased internal resistance. The other more often used type of sealed cell uses glass fibers and regular liquid electrolyte. The capillary action keeps the electrolyte within the fiberglass between the plates. Both types of batteries are packaged in a sealed container. It is much easier to extend the life of the fiberglass type sealed battery.
|Picture 2. Battery warning label|
There are other differences resulting from gelling, the most significant detriment is that the electrolyte can not be removed. Washing out of the cells to help rejuvenate the battery in true gel cells are just not possible. Sealed batteries also cost much more then ordinary wet cell types. Presently the very common seven Ampere hours [AHrs] sealed cell battery cost more than twice as much as a new garden tractor battery wich has three times the AHr's capacity.
|Picture 3. Cell cover removal|
Almost all smaller (up to about 1,000 VA) Uninterruptible Power Supplies [UPS], used by many personal computer users are outfitted with these sealed type batteries. Smaller UPS uses one, larger ones use two or more. The UPS continually "float" charge its batteries. Typical float charge for a the seven AHrs battery is around 50±25 mA [milliAmps]. Most uninterruptible power supplies continuously charge the internal sealed cell battery, as long as the UPS unit is plugged into a working wall outlet. The float charge is applied even when the power switch is in the "off" position. As a result of this charging the internal battery will be fully ready for any forthcoming emergency. It is precisely what is required to keep the battery's cells from sulfation, a nonreversable serious degradation. Reconverting these lead sulphate crystals are not easy even in conventional lead acid batteries. It is easier to rejuvenate gel cells then to attempt to desulphate them.
|Picture 4. Cell valve removal|
When the battery reaches full charge, the continued flow of current disassociates the water in the electrolyte into its gaseous components - hydrogen and oxygen. In the course of a short few years, this nonstop charging action results in completely dehydrated, dried up cells. UPS manuals advise users to frequently test their units. Occasionally, on more expensive power supplies, a test button is provided for this purpose. As water loss and the battery's internal resistance gradually increases, it's Ampere Hour capacity steadily decreases. This loss is apparent in shorter and shorter backup times. Since backup batteries are "sealed" units, replace them may be the only apparent remedy. Then the same cycle repeats again. Usually the life time of a sealed battery in UPS service is between two to four years. Precisely, because of the high replacement cost, and to protect our overburdened fragile environment, it is most advantages to extend the life of those batteries as much as possible. A small time spent on maintenance pays good dividends, not only in cost avoidance, but improved emergency readiness as well. Revitalization of these sealed cells are best done, as preventive maintenance, before they are completely dry up.
Luckily for our purpose, these "sealed" batteries are not really completely sealed. Each of the cells have a tubular opening on top of the cell containers, which are capped by a small rubber like plastic stopper. This little cover Pict 4. functions as a check valve. When the electrolysis generated gas develops over pressure, this cap separates a little from the tube it covers, and allows the gas free passage to the outside. That is why the warning label on Pict. 2 cautions against charging without ventilation, and about sparks hazards. To make sure that the stopper can't fly off, a plastic disc is cemented above it. As visible on Pict. 4, the cover disc is cemented only at a few - four in our battery - spots, to allow passages to the outside for the escaping gases. All of this helps us greatly, since all that is required to get access to the inside of the cell's container is to pop off this disc as shown - Pict. 3. Use a sharp pointed tool, it makes removal much easier. Be extremely careful not to hurt yourself in the process. As shown a dental tool works great, but a sharpened small screw driver, or a scribe might be equally effective.