Alkaline Cell Operating Tips Richard Perez All right, spring is here and part of spring cleaning is getting the battery back into shape after a hard winter's work. This article begins a series on alkaline cell operation and reconditioning. This is for hands-on alkaline cell users. If the info here reads like greek, then check out the battery articles in Home Power issues #27, #28, and #29, these articles are a quick course in battery basics. First examine your system Undercharging is the most common cause of alkaline cell problems. The cells making up a battery are never identical, especially reconditioned cells. After months of partially cycling the battery, some cells become more discharged than others. It only takes one weak cell in a series string to limit the capacity of the entire series string. Check your voltage regulator or charge controller. If the voltage cutoff or regulation point of the regulator is set too low, then the battery is not being fully recharged. Set the regulator for at least 1.6 VDC for each series connected cell in an alkaline system. This means 16 VDC for a ten series cell nicad or nickel-iron battery. This information applies PV, wind, and hydro systems using either series or shunt regulators. Charge and Discharge Test Recharge the battery until you are really sure it is totally full and measure how many Ampere-hours you can get out of it. Even if your sure that you have weak cells, the charge and discharge test is the best prelude to cell reconditioning. Get to know your cells on an individual basis. Give each cell a name or number. Get a clipboard and record all the measurements. If you have a battery Ampere-hour meter (like the Cruising Equip. models), then you already have an instrument that will be of great aid in diagnosing lost capacity. If not, then you will need an accurate voltmeter. How do we know when the battery or cell is fully recharged? Just look within the cell. Is it gassing furiously? Is the electrolyte alive with cloudy mass of bubbles that burst on its surface? Sustained, heavy gassing is a sure sign that the cell is as recharged as possible. Measure the voltage each cell. Write this data down next to the cell's number on the clipboard. While still undergoing recharging, the voltage difference between the highest and lowest cell should be no more than 0.1 VDC. If continual recharging does not bring the cells within this voltage spread, then the lowest cells have problems and should be singled out for further treatment. If you have an Ampere- hour meter, then overcharge the battery to between 130% to 160% of its rated capacity. If you are using a voltmeter, then continue recharging for five to seven hours after the cell reaches 1.6 VDC. Now discharge the battery or cell and measure number of Ampere-hours that it delivers. This is simple to determine if you have an Ampere-hour meter. Discharge cutoff voltage for an alkaline cell is 1.0 VDC. After discharging, read the number of Ampere-hours removed from the battery on the Ampere-hour meter. If you don't have an Ampere-hour meter, then discharge the battery or cell at a constant rate, measure this discharge rate with an ammeter, and multiply this rate by the number of hours it takes the battery (or cell) to reach the discharge cutoff voltage of 1.0 VDC. This results in the number of Ampere-hours removed from the battery - its capacity. More complete instructions for this charge / discharge regime are in may battery articles in HP#27 and #28. I am repeating some of this info here because most battery problems are really just chronic undercharging. This method is both tedious and sure. Finding weak cells the easy way While the recharge / discharge routine will find the weak cells, the process is both time and energy intensive. Here is a procedure that does not use the refill/empty routine and requires no record keeping. Wait until your alkaline battery voltage is low (below 11.5 VDC for ten series connected alkaline cells). Measure the voltage of each cell in the battery. The weak cells will have a low cell voltage at this point (between 0.5 and 0.8 VDC per cell). The good cells will have higher voltages (between 1.1 and 1.2 VDC). The voltage differences between the weak and good cells are very dramatic at this point. We recently did this to our 150 cell, reconditioned, nicad battery. We are a 12 Volt system. Our battery is configured as fifteen series strings of ten cells. Each ten cell string is wired in parallel with the next ten series string. Out of the 150 cells we found the ten cells with the lowest voltages. Oddly enough, no two of the ten lowest cells occupied the same series string. This meant that a single cell in was limiting the performance of the other nine cells in the string. Our battery had this problem in ten of its series strings. What should have been a 1,500 Ampere-hour battery was behaving like a 700 Ampere-hour battery. "I had to rearrange their faces and give them all another name..." We then rearranged our battery so that all of the ten weakest cells were in the same series string. This means that ten weakest cells are not holding back the performance of ninety good cells. The weak cells now have their own series string and limit only the performance of each other. The performance difference was immediate and well worth the small amount of effort. After the next sunny period the battery recharged and we went into another discharge cycle. This discharge cycle showed the battery behaving like it contained over 1,000 Ampere-hours. Successive cycling has produced even further capacity increases. If the weak cells are low in capacity because they have not been properly recharged, then this procedure will restore their lost capacity. If the weak cells do indeed have diminished capacity, then at least they are isolated where they have the least effect on the remaining good cells. If the battery's performance doesn't increase after this procedure, then something else is wrong with the cells. The dance of carbon chemistry The electrolyte used in alkaline cells is a 25% solution of potassium hydroxide (KOH) in water (about a 5N solution for chem techies). Carbon dioxide (CO2) will enter into chemical change with the KOH and form potassium carbonate (K2CO3). High concentrations of carbon compounds in the electrolyte manifests as a loss in electrolyte density (specific gravity) and a loss in the cell's ability to store electricity (diminished capacity). If you really want to measure the exact amount of carbonate in the electrolyte, then perform a titration. The process is relatively simple desk top chemistry, but involves some lab equipment and reagents. See the side bar for specific titration instructions. If you are equipped to do titrations, then by all means do them before reconditioning any cells. If you are not equipped, then either hire the job out, or recondition the cells with titrating their electrolyte. While you may waste your time and resources trying to recondition unfit cells, you will not damage any reusable cells. BEGIN SIDEBAR TEXT Titration for K2CO3 in Alkaline Electrolyte Materials required Hydrochloric Acid, 1 N Buret, 25 ml. Buret stand, white porcelain base Pipette, 5 ml. Phenolphthalein pH indicator, 1% Methyl orange, 0.1% (w/v) Aqueous Erlenmeyer flask, 250 ml. Graduated cylinder, 100 ml. Distilled water Procedure 1. Using pipette remove 5 ml. of electrolyte from the cell and transfer it to the 250 ml. Erlenmeyer flask. 2. Add 50 ml. distilled water to the Erlenmeyer flask. 3. Add 2 drops phenolphthalein pH indicator to the Erlenmeyer flask. Note the pink color to the liquid. 4. Using the buret, titrate with 1N HCL until clear. Record the number of milliliters 1N HCL to clear as A= ml. 5. Add 4 drops methyl orange to Erlenmeyer flask. Note the yellow-orange color. 6. Continue the titration until the yellow-orange color changes to pink-orange. Record this final value of the titration as B= ml. (2(B-A)/B) 100 = % KOH in the electrolyte For alkaline cells up to 15% carbonate concentration is considered all right. If the K2CO3 exceeds 15%, replace the electrolyte. END SIDEBAR TEXT Causes of Electrolyte Carbonation There are several ways to rapidly carbonate an alkaline cell's electrolyte. The first and most common way is to neglect the mineral oil layer floating on top of the cell's electrolyte. Pocket plate nicads and nickel-iron cells use a 1/8 inch thick layer of pure mineral oil (Chevron Oil 22) floating on top of the electrolyte. Since this oil is less dense than the electrolyte, the oil floats. This thin oil layer isolates the electrolyte from contact with ambient air (which contains carbon dioxide). Another cause of carbonation is chronic overcharging. When the cell is gassing heavily, the gas bubbles stir up the oil layer and work air into the electrolyte. Daily sustained gassing of the cell can result in electrolyte carbonation within a few months. Distilled water, in a sealed but half filled jug, will dissolve carbon dioxide from the air and can transfer this CO2 to the cell Another cause of carbonation is not thoroughly cleaning the cells when they are reconditioned. Reconditioning alkaline cells essentially means cleaning out the cell and replacing its electrolyte. Graphite is used as an electrically conductive packing material in the cell's anodes. Over time (a shorter time if the cell is routinely overcharged) the graphite gradually works itself out of the pocket and into solution with the electrolyte. Graphite is mostly carbon and contributes to electrolyte carbonation Äa harbinger of cell failure. If you have ever wondered what that black scum floating on top the electrolyte was, well now you know Ä it's graphite. The graphite that is not buoyed up by the oil layer sinks and forms a thick black sludge on the bottom of the cell. This sludge must be removed during the reconditioning process or it will rapidly contaminate the replacement electrolyte. If you are buying "reconditioned" cells, reject any that have the black sludge! All carbonation reactions take place faster at higher temperatures (over 120øF or 40øC). A Question of Carbon You may ask yourself why does a maker of alkaline cells deliberately add a carbon containing material to a cell that is sensitive to carbon contamination. I know I have. Thomas A. Edison did. Graphite is added as a flexible, low resistance, path binding the powered nickel oxide. Finely powered graphite is mixed with nickel oxide to make the cells active anode material. This mixture is compressed into the anode's pockets during folding and pressing the plate. The graphite provides good electrical connection between the active anode material (nickel oxide) and the framework of the plate. The only cells of this type that do not use graphite are the new fiber nickel- cadmium cells made by Hoppecke. Carbon compounds in the cell's electrolyte eventually attack the cathode's active material Ä cadmium oxide (CdO). The cadmium forms cadmium carbonate (CdCO3) which is a poor conductor of electricity. Cadmium carbonate coats the negative plate and chokes out the more active cadmium oxide. Changing the cells electrolyte decomposes the cadmium carbonate and converts the active cathode back into cadmium oxide. And the dance of carbon still goes deeper. The graphite used in pocket plates is very pure; it contains very few impurities. High on the list of graphite impurities is any iron compound, specifically iron oxide Ä rust. If a cell is operated with high graphite concentrations in its electrolyte, then eventually the iron impurities (from rust molecules in the graphite) attack the cell's active anode material Ä nickel oxide. This terminal reaction is known as "iron poisoning of the anode" in classical electrochemical literature. Anodes can also be iron poisoned by adding water with dissolved iron compounds to the cell. A word on alkaline cell storage Alkaline cells are best stored with their electrolyte in place. If you must remove the electrolyte and are not ready to replace it with new electrolyte, then fill the cell with distilled water. Do not store unsealed cells in a dry condition for over 30 days. Atmospheric degradation begins immediately and can ruin a cell within a month. What next? If you've run these tests and examined your cells, then you have a good idea of candidates for reconditioning. Next issue's battery article will have specific details on the reconditioning process, including making your own electrolyte and proper disposal procedures for spent electrolyte. Till then get to know your cells in preparation. Access Author: Richard Perez, c/o Home Power, POB 520, Ashland, OR 97520 ù 916-475-3179