Some Questions and Answers About Nicad Batteries

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SOME QUESTIONS AND ANSWERS ABOUT NICAD BATTERIES

Roy Bourke

MAAC 204L

Toronto

The nicad battery pack is at the heart of every radio control system. It is probably the most important part of the system and is also the part the most prone to failure. And yet it seems to be the part least understood by modelers, judging from the concerns and the questions that arise as modelers suffer mishaps or struggle to make their radio systems more reliable.

Certainly questions have cropped up in my own mind over the past few years, and it is hard to find a single document or book that deals comprehensively with the operation and management of nicad battery packs. Here are some of the questions that I have struggled with and for which I have tried to get authoritative answers. There are many conflicting views and explanations in the literature about many of these questions, so what I present here are the explanations that came from sources with impressive qualifications, and which seemed the most logical to me.

Q. How does a Ni-Cd cell work?

A. A typical nicad consists of plates wound into a compact roll, isolated from each other by a porous plastic separator, and immersed in a caustic electrolyte solution (potassium hydroxide). Nickel hydroxide (NiOOH) is the active material in the positive plate (anode) which, during discharge, reduces to Ni(OH)2 by accepting electrons from the external circuit. Cadmium (Cd) is the active material in the negative plate (cathode) and during discharge it is oxidized to cadmium hydroxide Cd(OH)2 and releases electrons to the external circuit. (Electrons moving to and from the cell mean electric current in the circuit!) The reactions are reversed during charge. The reactions can be expressed as follows:

Cd + 2H2O + 2NiOOH <----> 2Ni(OH)2 + Cd(OH)2

When the cell becomes fully charged, the charging current is converted to heat and the cell becomes warm. A Ni-Cd has a negative temperature voltage coefficient so the warming of the cell results in a depression of the voltage (the "delta peak"). Peak chargers rely on this delta peak to determine when to stop the charge.

Q. What happens if a cell is overcharged?

A. When both plates are completely charged and charging current is continued, hydrogen is generated at the cathode and oxygen at the anode (by electrolysis of water), and gas pressure (and heat) builds up in the cell. A safety vent will release gas pressure if it gets too high, but the pressure is kept partly under control by making the cathode larger than the anode. The anode becomes fully charged first which inhibits the production of hydrogen at the cathode, and allows the cell to be overcharged at a slow rate safely for long periods of time without noticeable loss in performance. Overcharging at fast rates, however, will cause both heat and pressure to build very quickly, possibly more than the cell can handle, so it may self-destruct.

Q. What do the letters and numbers mean?

A. Cell designations usually start with letters;

"KR" means a "standard" cell

"N" means a cell for general use

"RC" means a cell developed for radio control applications

The number that follows refers, of course, to the capacity of the cell, expressed in milliamp-hours (mAh) for the cells used most frequently in Radio Control. (The capacity of a cell will be designated as "C" when we get to talking about charge rates)

The letters following the capacity refer to the physical size of the cell, or more specifically to the diameter. All "AAA" cells are the same diameter, but not necessarily the same length. Similarly all "AA" cells are the same diameter, as are all "A" cells, etc. "SC" refers to sub-C size cells. There are also C, D, etc. sized nicads, but these are not used much in radio control.

The final letters refer to the type (or construction) of cell;

No designator means a standard cell

"R" represents rapid charge/discharge type

"E" represents an extended capacity cell

"U" represents foamed substrate (further extended capacity)

"C" represents ultra polypropylene (also further capacity)

For example, an N-500AR Sanyo cell is a general-use, 500mAh cell the same diameter as other

A-cells, and is designed for quick charging.

Q. What is the difference between a rapid-charge and a large capacity cell?

A. The E-type cells get their extended capacity partly by using thinner plates with larger surface areas to get more capacity for their weight. These cells also have higher internal resistance (e.g.. 11 to 20 milliohms in the SCE type cells). The R-series cells have thicker, shorter plates resulting in lower surface area and lower capacity but also much lower internal resistance (4-5 milliohms in the SCR type), and the capability of much higher charge and discharge rates. (Supposedly the R-type cells have higher self-discharge rates too, but personally I have not found this to be the case.)

Q. What is the "memory" effect?

A. Here some controversy exists. One author (Wilson, Model Aviation) claims that the "memory" syndrome is a myth, and that most cases of a battery losing its capacity can be related to the weakening or failure of a single cell in the pack causing a voltage depression in the pack as a whole. This can be verified by monitoring the voltage of each cell in a pack during a discharge. However packs should be cycled through a deep discharge/recharge regularly to equalize the cells and to burn off any interplate shorts that may be developing.

On the other hand, Robert Boucher of Astro Flight describes two types of memory, both related to the fact that when charging, the plate material nearest the terminals of each cell charges first, and the material deep in the interior of the cell charges last.

A "short-term" memory effect occurs if a battery is partially discharged (say during a flying session), then topped up at a fast charge rate for a short period of time. After top up, the battery may show full voltage even though only about 10% or 20% of the plate material (the material nearest the terminals) is fully charged. If the battery is put back into service immediately, the pack does not operate to full capacity and goes flat very quickly.

"Long-term" memory is an extension of this short-term effect. If a battery is only partially discharged and then topped up repeatedly, the outer plate material is continually "exercised" (i.e.. cycled through the oxidation/reduction process), but the material in the interior of the cell is not. A significant proportion of the plates become stagnant and subject to surface "poisoning" through a deposition of impurities from the electrolyte. These surface deposits accumulate and cause a high-resistance barrier to form on the inner plate

material. The battery begins to behave as if the coated portions of the plates did not exist, so the overall capacity of the battery is reduced. This can be prevented by periodically cycling the battery through several full charge/full discharge cycles to exercise the inner portions of the plates.

Q. What is the effect of heat and cold on cell capacity?

A. Both hot and cold environments will reduce the capacity of a Ni-Cd pack. Anyone who has tried to fly on a cold day in winter knows that you must be careful about your flight battery in cold weather. A Ni-Cd battery loses about 20% of its capacity at 20o F (-7o C).

What many fliers do not realize is that the battery loses even more capacity at higher temperatures. A Ni-Cd pack can lose 40% of its capacity at 120o F (49o C) and on a hot summer day, it doesn't take long for a battery wrapped in foam in the interior of an airplane to reach 120o F. Furthermore, a hot battery does not exhibit much of a delta peak, so if you try to top up a hot battery with a peak charger, the charger may not sense the peak and may overcharge. Allowing a hot battery pack to cool before trying to charge it would go a long way to prevent this.

Q. What charge rates should I use for my nicad packs?

A. For battery packs with standard or extended capacity cells, the safest charge rate is the "overnight" rate of C/10 (e.g.. a 60mA charge rate for a 600mAh battery). A full charge from a discharged state would take 14-16 hours, and even if left on charge for considerably longer, the battery will not heat up enough to be damaged. Also, if the cells of a pack start out at different states of charge (as is often the case if the pack has been stored for a period of time), this charge rate will bring all cells up to a common state of full charge without damaging the cells which become charged first.

However, delta-peak fast chargers have become very popular lately and standard nicad cells can be safely fast-charged at rates up to 2.5C, e.g. 1.5A for a 600mAh pack. (Nicads with extended capacities are less tolerant of fast charging). Some flyers use even higher rates and claim no problems. But a pack could easily be damaged if one cell that started out at a higher state of charge than the others became fully charged first. A high charge rate might cause that cell to overheat as the other cells approach full charge. If you are using fast charge rates, it is a good practice to use the overnight C/10 rate about every third or fourth charge to ensure that the cells do not become mismatched. The first charges after long-term storage should always be at the slow rate.

Rapid charge cells (R-type) have lower internal resistances and can handle much higher charge rates. Electric flyers charge them at rates of 5C or more without noticeable deterioration (at a 5C rate, a pack can be fully charged in 15 minutes). But even with R-type cells, it is a good idea to charge them at the overnight rate for the first time after long storage.

Q. How do I know what state of charge my batteries are at?

A. There is no real way of measuring the state of partial charge of a pack without completely discharging it. Voltage measurements can provide a rough indication, but can also result in big errors and lead to a false sense of security.

One approach to battery management is to experiment with your system at the beginning of the flying season, and determine how much flying time you can get from fully charged packs. Then keep track of how long you fly at a flying session (approximately) and estimate the amount of full charge you have used up. Then recharge the proportionate amount before the next flying session. If in doubt, fully cycle the packs.

Q. What about trickle charging.

A. Yes, you can keep your transmitter and flight packs charged between flying sessions by trickle charging. The best method is by pulse charging, which helps to prevent the formation of calcium bridges across the plates of the cells. You can set up a pulse charger at very low cost by using the wall-module charger that comes with your radio system. Simply buy a cheap appliance timer and connect the wall-module charger to it.

Set the trigger pins on the timer so that it is on for 1 hour per day. If you have a full day of flying, then when you get home set the timer so that the on/off triggers come up in about 14 hours and hook up your batteries so that the timer is on. In about 15 hours the timer will shut off the charger, and thereafter it will turn on for 1ÿhour/day to keep the battery charged and ready for your next flying session. If you fly only a couple of flights, then set the timer so you get only a few hours of charge before the timer goes into the 1ÿhour/day mode, according to the amount of flight time you estimate you have used up. For example, you have determined that fully charged batteries will give you 1-1/2 hours of flight, and you have flown for about 30 minutes, then set the timer to give you about 1/3 of a full charge at the C/10 rate, or about 5 hours.

It doesn't hurt to crank in a few extra hours of charge at the C/10 rate because the cells won't be damaged by overcharging at that rate.

Q. How should I store my packs over long periods (winter, etc.)

A. Normal or extended capacity packs can be stored either charged or discharged. Over a period of time they are going to self-discharge anyway, but it is preferable to discharge them to a level of 1.1 volts/cell before storage. That way, the chances of cells becoming mismatched is reduced. Where possible, the batteries should be removed from the aircraft (or the transmitter) and stored in the refrigerator to help slow down natural chemical deterioration. As well, they can be more easily examined for problems such as corrosion, damaged connections or black-wire syndrome, when they are out of the aircraft.

R-type cells should also be stored in a discharged state (1.1 volts/cell). The R-type cells used in electric flight applications seem to thrive on fast charges, heavy discharges, and short or long term storage in a discharged condition.

At the end of the storage period, the voltage of the discharged pack should still show over 1 volt per cell. If it doesn't, check the voltage of each cell. Chances are that one or more cells have developed a short due to cadmium migration across the separator in the cell. Often such shorts can be "zapped" by charging and cycling the cell or the pack, but don't do it. A bad cell like that can be a time bomb just ticking away waiting to "zap" your airplane. Replace the whole pack, not just the bad cell.

Q. What causes the failure of a nicad?

A. Nicad cells can be damaged by heat, physical damage, and physical or chemical ageing. There are three main failure modes:

- Open circuit failure. This may be a mechanical break in the internal connections. Or a loss of electrolyte through the vent in the cell (evidenced by a white powder that accumulates around the positive terminal) may not leave enough electrolyte within the cell to permit current to flow from one plate to the other.

- Short circuit, caused either by failure of the separator to prevent the plates from touching one another or, most commonly and as mentioned earlier, cadmium migration forming a crystal or dendrite across the separator.

- "Worn out" cell! A cell becomes leaky, won't fully charge, loses capacity, self discharges rapidly, often develops a grayish powder around its terminal.

Q. Assuming no visible damage or corrosion, how can I tell if my packs are still good?

A. The best way is to invest in a good cycling system, and keep records of your packs' performance. Capacity measurements should be made after running the battery through several cycles. Keep records of each pack's measured capacity when it was new, and after long-term storage, and watch out for any degradation in performance. A degradation of more than 20% of rated capacity should be suspect.

Any observed increase in the self-discharge rate of a pack should also trigger a warning. A good test is to cycle the battery several times and record the capacity after the last discharge cycle. Then store the fully charged battery for 3 or 4 days and discharge again. A drop in capacity of over 10% should be suspect.

Batteries should be fully cycled periodically throughout the flying season, at least every three months (every month would be better), and the capacity monitored and recorded at these times as well.

Although the measurement of pack voltage can lead to incorrect conclusions, the flyers who do regularly check their batteries at the flying field with an expanded scale volt meter seem to crash less due to battery failure, so it is certainly a good idea to do so.

Q. To increase the capacity, can packs be connected in parallel?

A. Yes they can. Further they don't even have to be packs of the same rated capacity. They must be of the same voltage, however (i.e.. same number of cells). And they should be charged separately, not while still connected in parallel.

Q. What is the "black-wire syndrome"?

A. If you examine an older pack, particularly one that may have a damaged cell, you may find that the wire connected to the negative terminal of the battery is corroded, possibly black in colour, brittle, and impossible to solder. It is usually corroded for its entire length, from the negative terminal right up to the plug or switch it is connected to. And it is only the negative wire that suffers from this problem.

The cause of black-wire corrosion has been the subject of much conjecture and theory for many years, but the most plausible explanation is the escape of material from the inside of the cell, probably the potassium hydroxide electrolyte, through the bottom of the cell casing. However, the cause isn't important! Just examine your batteries occasionally to make sure they are not suffering from this malady.

The subject of nicad batteries is complex. Recommended practices have changed over the years, as have the design and characteristics of the cells themselves. But nicad cells are really tough little guys, and can tolerate quite a wide range of uses and abuses without significant deterioration in reliability. As modelers, we are quite lucky to have such tolerant power sources for our radio systems.

References - a partial list:

The Quiet Revolution...... …Robert J. Boucher

Electric Motor Handbook...... Robert J. Boucher

Seminars at Toledo...... ……Dr. Robert Suding

Model Aviation, May '98...... ….George Wilson

Web Site

Web Site http:/gnv.fdt.net/~redscho/storage.htm