Lead Acid Battery Sealed Gel-Cell Rejuvenated Extended Life

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 Cells - Picture 1. Gel cells are specially constructed lead acid batteries. There are two major types of sealed lead acid cells. In true Gel Cells, 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.

Lead Acid Battery Sealed Gel-Cell Rejuvenated Extended Life Part 2

OddMix.com - PC Technical Note - PCN0701 - Karl Nagy

Picture 1. Cell vent assembly

Always wear protective glasses when working with batteries. Insert the tool into the gap between the cover disc and the battery housing at an angle and pry the top up. One by one remove all discs followed by all of the rubberlike caps Picture 1. It is not absolutely required, but it is good practice to keep them together, to help with the reassembly. Once all cells are open, use a thick pointless needle and fill a large syringe with distilled water. The syringe and a pointless needle combination is available for the refill of ink-jet printer cartridges. A thin plastic tubing can also be used instead of a metallic needle. For a seven Ampere hour battery, load the syringe with ten cubic centimeters (10 cc) of water and slowly and carefully inject 2 cc into each cell. Wait a little while and inspect each filler hole.

It is best to push the needle down at an angle and not too deep, as a metal needle can short and damage the plates, and any type of needle can create conductive bridges in it's track. Repeat the injections to all cells until they are properly rehydrated Picture 2. After each injection the water should promptly diffuse between the plates. Repeat this procedure until each cell received 10-12 cc of the destilled water. Do not overfill. It is best to underfill slightly. Pay attention, not to obstruct the inlet tube and allow the inside air to escape. Otherwise, the water will flush out some residual acid onto the top, and may cause burns on contact with the skin. If this happens wash the affected area immediately with plenty of water.

Picture 2. Adding water to cells.

If the battery was unmaintained for too long and became a "lost cause" before the filling procedure, then it may require more then the 10 cc water per cells. A completely dried up seven AHr battery can take up to twenty cc of water. This volume is entirely dependent of the battery's physical construction. Once all water is successfully inside the cells, it is time to start the slow charging process.

Reassemble everything, put the caps on, and apply a tiny droplet of acetone onto the underside of the covers, before push them into place. Make sure there are gaps for the air to escape during charging. Once charging begins, the internal charge carriers will produce a thorough mixing of the electrolyte. As the water mixes with the residue of old electrolyte it begins to attack and dissolve lead sulphite from the plates. Refilled batteries can function almost like new for long times. How well they function would depend on how much nonreversible capacity loss they suffered due to dry up and sulphate damage. Under constant float charge conditions, sulphates which quickly kills discharged batteries, grows very slowly.

The best indicator of battery health is its internal resistance. To determine the battery's internal resistance, connect a lamp with a 60 Watt light bulb to the UPS, turn on the lamp, then disconnect the power cord from the wall outlet. Carefully note the length of time when the light goes out. Record the lamps wattage and the time the UPS was able to power it and repeat the test monthly or at least once every three months. When the uptime shortens to about half of it's original value, it's time to replenish the water in the cells. Some UPS has a load test button, use it often to avoid surprises in a power out. A lot better test would be to measure and record, the actual battery voltage under load, every minute for five minutes, across the battery terminals.

For battery capacitance reference - a typical home computer system can be powered up 10 - 15 minutes (the exact time is dependent on monitor type and size and the computer processor) by a fresh and fully functional 7 AHr battery. A 60 Watt light should burn about twice as long. The electrical power in a fully charged 7AHr battery is 7 A x 12 V = 74 Watt Hours not allowing for any losses. Even with losses, a 60 Watt light should be powered for a full hour, before the battery will be completely exhausted. The longer the UPS is able to supply power to the light the less capacity the battery has lost. It is best to conduct these emergency capacity experiments with a desktop lamp and NOT with the computer.

Lead acid batteries should never be allowed to sit idle, disconnected from the float charger. It is most important to recharge them as soon as possible after discharge. Always discharge the battery as little as possible, and restore full charge quickly, and as soon as practical. Once the sulphatization progresses, more and more of the active plate material becomes covered by sulphate crystals. These crystals are not soluble in water and they are electrically nonconductors. As more of the active plate area is lost to crystallization the battery's internal resistance becomes higher and higher. Although it is possible to rejuvenate badly destroyed batteries, it takes a lot of effort and it is a difficult process even with regular wet cells. Therefore it is most important and beneficial to prevent battery degradation.

http://www.oddmix.com/tech/pc_rejuvenate_gelcell_2.html