Pocketplate Nicads in Home Power Service

Richard Perez

I have just recovered from a pain in the neck that lasted twenty years. It started with our first lead-acid battery's arrival in 1970. I've had this pain for so long it became normal and I hardly noticed it anymore. The energy portion of my life revolved around these lead-acid batteries. All the decisions and compromises in our system were to accommodate the cranky lead-acid cells. The list of Dos and Don'ts was seemingly endless. "Thou shalt not discharge more than 80% of the energy in the lead-acid cells." "Thou shalt perform regular equalizing charges." "Thou shalt recharge the cells as soon as possible." "Thou shalt keep all connections clean and bright." A never ending litany of limitations and chores all related to the lead-acid cell's delicate and cranky nature-- a real prima donna... Well, things have changed. Nicad cells offer vastly better power storage for home power systems. We like these nicads so well that we are replacing all our lead-acid systems with nicads as quickly as we can afford it. What better compliment can we at Home Power pay?

The NiCad Saga

During the last nine months we've been experimenting with over 100 wet, pocket plate, nickel-cadmium cells. These cells are made by a variety of manufacturers. None of them are new and all were supplied by Pacific West Supply in Oregon. Some were reconditioned, others were not. Some cells are high rate cells and others are medium rate cells. These experiments have been carried out in four different test systems. Two of the systems are PV/generator sourced, one is stand-alone PV with no generator at all, and one is grid connected.

This article is not a primer on NiCads (already done in HP#12, pg. 16), or a test report on a particular cell type (HP#13, pg. 17). This article discusses the different types of nicads now available to home power users and how to effectively apply these cells in our systems. The info here was learned by actual hands-on testing and experience. These tests were conducted on cells between 57 and 2 years old. All had been retired from service by their original purchasers. What we have here is a wide cross section of used nicad cells. As such, any deficiency due to aging should be apparent. We are very pleased with the life potential of these cells. For example, we tested a Gould XWR7 nicad cell that is over 57 years old and still delivering its rated capacity!

Different Brands of Nicads

The different brands we have tested are -- SAB NIFE, Edison, Alcad, and Americad. SAB NIFE is a Swedish company and the oldest maker of nicads. Edison Battery Co. made all types of cells for years starting around 1900 and is now owned by SAB NIFE. Americad is a company also now owned by SAB NIFE. Alcad is a British company that has also been around for years. The point here is that pocket plate nicads are made by a variety of companies. And to confuse matters further, each company makes several types and many sizes of cells. The happy news for us home power types is that all pocket plate nicads are light-years ahead of even the finest lead-acid cells.

Different Types of Nicads

Regardless of manufacturer, pocket plate nicads come in three basic types-- low rate cells, medium rate cells, and high rate cells. All the different types share the same pocket plate construction described and illustrated in HP#12. The major difference between the types is the number and thickness of the plates that make up the cell.

Low Rate Cells

The low rate cells use a fewer number of thicker plates. They are designed for very slow discharge rates (<C/10) and are the least common type to find used or reconditioned. The reason for this is that fewer are initially made and sold by the various manufacturers. They are so rare that we've not yet found ten cells for a working 12 VDC pack, and so we have no direct data.

Medium Rate Cells

Most (»65%) of the cells we tested (like the ED-160 in HP#13) are medium rate cells. They have a greater number of thinner plates than do the low rate cells. They are designed with average discharge rates of C/10 to C/5 in mind. These cells are very plentiful since railroads, hospitals and airports use them for uninterruptible power. The medium rate cells are the easiest type to find reconditioned and/or used.

High Rate Cells

The high rate cells have the largest number of plates and the thinnest plates of all the types. The are designed for rapid discharge at rates around C/1 to C/0.1. They are mainly used to start large engines like diesel locomotives and jet aircraft. For example, a 120 Ampere-hour cell will be asked to deliver thousands of Amperes for several seconds to a minute. They are plentiful reconditioned and/or used. Please note: if we were discussing lead-acid cells, then thin plate construction results in reduced cell longevity. In a pocket plate nicad cell, with its supporting steel electrode framework, this it not true. High rate cells have the same high longevity potential as other nicad cell types.

Applying Nicads in Home Power Systems

So what manufacturer, type, and size of cells are best for me? Well, as to manufacturer, all the brands we tested met their specified Ampere-hour capacities and voltage/current curves. Regardless of brand, they all performed as their makers said they would-- and these are used (and sometimes not even reconditioned) cells. As to type, both the medium rate and high rate cells are designed for far more demanding current drains than they will ever see in a home power system.

Sizing the Capacity of a Nicad Pack

Sizing nicads is not very different from sizing lead-acid storage. Watt-hours stored is Watt-hours stored. The battery should still be sized with at least four days of storage capacity. However, the nicads allow total cycling. This means that we can totally empty the cells, something we should never do to a lead-acid system if we want it to last. Since lead-acid systems require that 20% of their capacity never be used, we pick up a 20% reduction in the Ampere-hour capacity of the nicad pack. Since the nicads keep their voltage higher in relation to discharge rate, a smaller capacity pack will supply the high surge requirements of an inverter. In general, we've been sizing the nicad pack with about 30% less capacity than the lead-acid pack it replaces with no noticeable loss in system performance. With nicads, if there is not enough capacity then more can be added anytime.

In terms of charge efficiency and charge retention, the nicads offer about the same performance as brand-new lead-acid systems. The major difference here is that the lead-acid's efficiency drops radically as it ages (due mostly to increased self-discharge). The nicad's efficiency and low rate of self-discharge remain constant over its long lifetime. Mix and/or Match?

We've been experimenting with mixing different sizes and brands of nicad cells within the same battery pack. Here's what has been working and what hasn't.

• All nicad cells that make up a battery should be of the same cell type, either all high rate cells or all medium rate cells. Don't mix different rate nicad cells in the same pack, either as series or parallel elements.

• A series string of nicad cells (ten cells in series for a 12 volt system and twenty cells in series in a 24 volt system) must all be of the same size, type and brand.

• Parallel packs within the main pack may be of different brands and sizes. For example, a series string of ten ED-160s (160 A-h) may be placed in parallel with a series string of ten ED-80s (80 A-h). The resulting pack would contain 240 Ampere-hours at 12 VDC (note: all these cells are medium rate cells). The system we are now using at the Home Power office contains: a ten cell series string of Alcad 120 A-h cells in parallel with a ten cell series string of SAB NIFE 120 A-h cells. These are all high rate cells.

These configurations are experimental and they are working. Ideally, a nicad pack should be totally composed of identical cells. But considering that we are talking about reconditioned and recycled cells here, this isn't always possible, but always desirable.

Charging

We have been amazed at how well these nicads have functioned with power sources like PV modules designed with lead-acid charging characteristics in mind. The nicad cells are designed to be recharged rapidly, within a four to seven hour period. They are capable of accepting charge rates and voltages far beyond those usually found in our systems. A good analogy here is that a nicad battery in a home power system is like an NFL quarterback in a high school football game.

Voltage under Charge

If a nicad cell is fully charged and being recharged at rates as low as C/40, then the cell's voltage can rise as high as 1.65 VDC. This means that a single PV panel can push a nicad pack of ten ED-160 as high as 16.5 VDC. While this is not harmful to either the nicad cells or the PV panels, it can cause some 12 VDC appliances to overheat (the old fry&die syndrome). See charge curves printed in HP#13.

The nicads have an overall higher charge voltage profile than lead-acid systems. When any battery is under charge its voltage is elevated. The degree of elevation depends on several factors: cell electrochemistry, cell state of charge, recharging current, and cell temperature. Charge Regulation

I recommend that regulation be used in nicad systems even though the battery doesn't need it. Regulation is used to protect the many low voltage appliances on line. Number One appliance is the inverter. Most quality inverters will operate at »15.5 VDC (12 Volt models) and »31.0 VDC (24 Volt models). Thus, 15 to 15.5 VDC makes a good voltage regulation point in 12 Volt systems. And 30 to 31 VDC in 24 Volt systems.

Now, these voltage limits mean that the recharging current is reduced to the nicad pack before it is actually full. This makes the total refilling of the pack slower, but it still happens. And all the appliances on line are protected. What we really need is for inverter manufacturers, and all other low voltage DC appliance manufacturers, to widen the operating voltage range of their products. Consider that 12 VDC appliances should operate between 11 VDC and 18 VDC, and 24 Volt appliances should operate between 22 VDC and 36 VDC. If this were the case, then no charge voltage regulation would be required by nicad based systems. Let me be clear on this, the problem here is in the appliances, not the nicads or their power sources. We have been using the Heliotrope CC20 and CC60 PWM regulators on the PV arrays feeding the nicad cells. These regulators provide an adjustable voltage limit that is very effective. Heliotrope has also just introduced the CC60B, a 60 Ampere (either 12 or 24 VDC system) PV charge controller specifically configured for nicad storage systems.

Current

In terms of recharging current, home power systems are lightweights. These nicad cells are designed to be rapidly recharged at very high rates (»C/4 to C/7). The current input from our PV arrays, microHydros and wind machines is easily handled by the nicad. In fact, by recharging the cells at lower than design rates, we realize increases in cell operating efficiency. It is nice to know, however, that if we have to fire the engine/generator to recharge the nicad cells, then we can do the job quickly.

Equalizing Charges

The nice thing about nicads and equalizing charges is that they aren't necessary. No cell equalization is required in nicad packs, while it is mandatory in lead-acid systems. Equalization is the controlled overcharge of a battery that is already full. Equalization is required by lead-acid batteries to keep all the individual cells at the same state of charge. Equalizing charges, by definition, represent energy produced and NOT stored. A basic waste. And in most of our systems, we use an engine/generator for equalization because it provides the constant current necessary for the seven hour controlled overcharge. None of this wasted energy is required by nicads.

In nicad cells wired into batteries, the individual cell voltages tend to converge as the cells function as battery. In lead-acid batteries, the individual cell voltages tend to grow apart, while in nicad cells they tend to come together. That's what I call a happy chemical reaction! Here is a sample of the data. We installed ten Americad HED-120 cells in a stand alone PV system (see Wizard's system this issue). The cells differed in individual voltages by 0.15 VDC, and that's alot! After six weeks of stand-alone PV service, the difference in voltage between the highest and lowest cell was 0.005 VDC. Bottom line is that the wasted energy and expense of equalizing charges is history in nicad systems.

Discharge

Discharging the nicads is much the same as lead-acid types, except that the voltage of the battery stays higher. This results in better appliance and inverter performance. All medium or high rate nicad cells we tested are capable of handling the surge currents demanded by large inverters. For example, our microwave consumes, via the inverter, over 500 Amperes for about 0.1 seconds as it starts. Even a small nicad pack of 120 Ampere-hours is capable of delivering stored power rapidly enough to satisfy the inverter's surge requirements. A well designed home power system uses at least four days of battery storage. This means that average discharge rates are low (»C/100). These cells will deliver at rates around C/7. They have no problem delivering the current. Nicad cells will take total discharge. This is to say that if the cells are occasionally fully discharged they will not lose any capacity. With lead-acid systems, any total discharge results in permanent loss of capacity and premature failure. As yet we haven't enough data to accurately discuss the relationship of depth of discharge to cell life. However, there is evidence that, while the nicads survive total discharge, it certainly doesn't do them any good. Early indications are that constant and regular deep cycling may reduce cell life. More on this as the data becomes hard.

Temperature

You can put your nicads outside. No longer do you have to shelter battery electrochemical reactions under your roof. Lead-acid batteries had to be kept warm in the Winter. Not only could they freeze (which ruins them forever), but they lost capacity and efficiency whenever they got below 50°F. Nicads will operate at -13°F (-25°C.) with only minimal loss in capacity. With special low temperature electrolytes (KOH up to 1.30 gr./ml.), nicads will operate at -58°F. (-50°C.). Eventhough nicads will not operate when frozen, they will not be damaged and will work as soon as they thaw out. At the average discharge rates encountered in home power systems (>C/10), nicads will deliver greater than 90% of their rated capacity at cell temperatures greater than -13°F.

Routine Maintenance

Nicads require only simple maintenance. They do, however, demand that the user perform this maintenance. How well the user performs this maintenance primarily determines the nicads lifetime. If the capacity of the pack is sized properly, then the quality of user maintenance is the most important factor affecting how long the cells will last. Cell Water Level

Electrolyte level should be checked at least monthly and DISTILLED WATER added if necessary. Use only distilled water. Do not use tap water, rain water, well water, spring water, or soda water. Electrolyte water loss is directly related to overcharging the cells. Moderate overcharging doesn't damage the nicad cells, but it does run up the distilled water bill. In no case should a nicad cell be operated with the electrolyte level below the tops of the plates. This can result in arcing within the cell and possibly explosion. In the cells we tested, the minimum and maximum levels for the electrolyte are marked on the transparent cell cases. It's easy to see at a glance where the level of the electrolyte is, and thus we have no excuse for letting the cells get low on water.

Cell Oil Layer

Check the thickness of the mineral oil layer floating on top of the electrolyte. This oil layer is there to prevent carbon dioxide in the air from reacting with the potassium hydroxide electrolyte. For technical data about this phenomena, see George Patterson's article in this issue. From a user's maintenance standpoint, there should be a layer of mineral oil between 1/8 and 3/16 of an inch thick floating on the surface of the electrolyte. If you need to add more oil then use Chevron "Utility Oil 22". Don't use motor oil, mineral oil from the drugstore, cooking oil, or anything else. If you fail to maintain the oil layer, then the cell's electrolyte will gradually become polluted with carbonates and will require replacement. If the oil layer foams when a fully charged cell is under charge, then this is a good indicator that the oil layer is too thin. So add more oil to foaming cells. Physical Maintenance

This is simple. Keep the cell tops free of moisture, oil, dust and sundry funk. The cells we've been using seal much tighter than lead acid types. This means that what is inside the cell stays inside the cell. Apart form dusting the cell tops with a damp paper towel occasionally, we've done no physical maintenance. Compared to a lead-acid system, corrosion of battery cables is nonexistent in nicads. Don't let this lull you into thinking that the chemical contents of the nicad are benign. Nicads contain a powerful base (caustic) electrolyte (like a solution of lye). The electrolyte will burn the skin, particularly eyes. Flush electrolyte from the body with copious quantities of fresh water. And be careful!

Electrolyte Replacement

After a period of years (about 5 to 20) all nicad cells require that their electrolyte be replaced due to atmospheric carbonate contamination. How long depends how well the user maintains the oil level of the cells. See George Patterson's article in this issue for the technical details of electrolyte replacement. The procedure can be accomplished by a careful and responsible user.

Access

From the number of calls and letters we've been getting recently at HP, the interest about nicads is very high. Home Power is read by many folks who have had the lead-acid experience and are looking for something better. We urge you to communicate your nicad data and experiences to the common fund. Do this so we may all share what works. I will chew the rag about nicads via phone: 916-475-3179 or write me C/O Home Power Magazine.

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Experiences with NICAD Cells from Pacific West Supply

George Patterson

NICAD cells were picked up from Pacific West Supply in Amity, OR on Saturday, Dec. 8, 1989. They consisted of twenty-three ED-80's, nine ED-160's, four HIP-8's and one XWR7 dating back to 1933. The XWR7 is a pocket plate NICAD cell with a rated capacity of 35 amp-hour at the 8 hour rate. The Jungner Nickel-Cadmium Pocket-Type (NIFE) HIP-8's are high rate cells with a capacity of 80 amp-hr. and a cell weight of 6.4 kg. The Edison ED-160 cells are rated at 160 amp-hr. for the 5 hr. rate. The ED-80's at 80 amp-hour for the 5 hr. rate. Edison ED series cells are medium rate cells. The design of the ED-80 and ED-160 cells are the same except that the ED-80s are 12 1/4" tall while the ED-160s are 18 1/4" high. Two ED-80's are otherwise equivalent to one ED-160 cell electrically.

Testing

Testing this myriad of cells started with adjusting the electrolyte of each cell to a specific gravity of 1.190 gr./ml.. This was accomplished using a high quality hydrometer. Use only a brand new hydrometer that has NEVER been used to test lead-acid cells. All cells that were not yet reconditioned were filled with distilled water to the maximum level mark on the cell's case. The specific gravity was then adjusted either by adding distilled water or more highly concentrated electrolyte. The concentrated electrolyte is a solution of KOH in water with a specific gravity of between 1.19 and 1.22 gr./ml.. The cell was then charged and gassed for about 15 minutes in order to completely mix the solution. After another 10 minutes of charge, the specific gravity was measured with a hydrometer. It took several such episodes to achieve the desired value of 1.190 gr./ml., approximately 40 minutes per cell on average. Excess electrolyte was then removed from each cell to bring the level to the maximum mark.

Titration for Potassium Carbonate

Potassium carbonate concentrations in the cells' electrolyte were measured by titration and recorded. All cells were then charged prior to testing their ampere-hour capacity using a computer controlled system that produced the discharge curve of each cell with capacities to 1.1, 1.0, and 0.9 volts. Most of the Edison cells obtained were of the "Low Temperature" variety. They all had specific gravities for the electrolyte of approximately 1.220 gr./ml. after being filled with distilled water to the maximum level. Although the higher specific gravity has a low freezing point, <-36 degrees Centigrade, the higher density has a somewhat detrimental influence on the cycle life of the cells. The positive electrodes tend to lose capacity on cycling more rapidly than when the usual electrolyte concentration is employed. As cycle life is my most important consideration, the value of 1.190 gr./ml. was chosen.

Foaming & Battery Oil

During charging, three of the ED-160 cells foamed up and out of the vent caps. Upon inspection of all of the ED-160 cells, battery oil Chevron "Utility Oil 22" was added to bring the oil level on top of the electrolyte to 1/8"-3/16". This immediately reduced the foaming and the charging proceeded at the C/10 rate until >140% of rated capacity was reached. The cells were then allowed to rest for at least 24 hours, then discharged during the capacity test.

Battery Cell Testing System

The cell testing system consists of a computer with printer and digital voltmeter controlled over a IEEE-488 instrument control bus. The computer is a standard IBM-PC (IBM and PC AT are registered trademarks and PC XT is a trademark of IBM corp.) clone with software written in the Turbo Pascal (Turbo Pascal is a trademark of

Borland International, Inc.) language. The software controlling the digital voltmeter functions over the IEEE-488 bus. All data is repeated every 30 seconds with the computer performing the necessary calculations and data storage. After all of the data is collected, a graph of the discharge curve is plotted on the color display and the printer provides a hardcopy of the discharge curve. Figure 1 shows a schematic of this computerized cell testing system.

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