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Getting the Most Out of Nickel-Cadium Batteries

Many radio amateurs own handheld transceivers; for some hams, a hand-held is their only radio. Almost all hand-held users depend on battery power much of the time---- battery power from Nickel-Cadmium (NiCd) batteries. We value NiCds as a power source because they stuff considerable energy storage into small packages and because their output voltage stays relatively constant until just before they run down. Most importantly, though, we value NiCds because they're RECHARGEABLE. Instead of throwing them away, we can reuse them many times.

Hamdom reverberates with fact, opion, myth and legend about NiCd batteries and how to use them. We heat about memory affect, self-discharge, cell-reversal, slow charge, fast charge, trickle charge, overcharge and undercharge. We're told to discharge NiCds fully before recharging them; we're told that we can top them off anytime without hurting their performance. What's fact and what isn't?

One thing seems true: You want your NiCd batteries to last as long as possible. In this article, I'll discuss NiCd facts, myths, and legends, and some of the things you can do to extend your NiCD pack's life.

What Drives NiCd Evolution and know-how

Most of what we know about building better NiCds and using them effectively comes from satellite development. Of all battery applications, none demands more of battery life than satellites. In fact, battery life is the determining factor in predicting the useful life of a communications satellite, in which battery charge-discharge occurs daily for--hopefully--many years. With millions of dollars riding on the reliability of these electrochemical power plants, satellite designers choose only the best available batteries for their birds. Today, as at the dawn of the satellite era, the NiCd wins hands down.
Spacecraft power specialists thoroughly investigate anything that promises to lengthen satellite battery system life. What they discover either leads to improved space hardware or is discarded as useless. Because the results of such research--especially negative results-- trickle down to consumers slowly (if at all), false information about NiCd performance persists among consumers. Let's examine the biggest NiCd myth first: MEMORY

Memory

How many of us haven't heard these variations on a theme? "Memory killed my battery pack, so I have to buy a new one." "You can have it for nothing--memory ruined that pack and it won't hold a charge." "Always fully discharge a NiCd battery before recharging, or memory will result."

According to popular belief, NiCd memory works something like this: Using a battery for the same amount of time each day eventually trains the battery to that time and/or discharge limit. Beyond that point--zero capacity-- and the pack can't be retrained.

NiCd memory can occur, but only under very specific conditions, and I doubt that any ham has actually seen memory in a hand-held or power tool. The caracteristics of true NiCd memory are not what many people consider them to be.

Memory is not a condition where a cell "drops dead" after a short period of discharge. What does happen is that the cell potential drops several tenths of a volt below notmal and remains there for the rest of the discharge. The cell's total ampere-hour capacity is not significantly affected. Memory usually disappears if the cell is almost fully discharged and then recharged a time or two. Memory can occur during cyclic discharging to a definite fixed level, and subsequent recharging. Even then, memory rarely occurs--so rarely that battery manufacturers have considerable difficulty in forcing it to occur so they can study it!

Some satellite power systems, it's true, use NiCds in ways that require careful management to avoid memory. When a geosynchronous satellite passes within the Earth's shadow--once a day--it operates solely on battery power. The batteries recharge when the satellite emerges from Earth's shadow and it's solar cells once again provide power. A continious charge/recharge cycle this regular can promote memory occurrence. Memory rarely occurs in our hand-held transceiver battery packs because we rarely discharge them to the same level each time we use out hand-helds.

Even in geosynchronous satellites, memory is uncommon enough that batteries are reconditioned only once every year or so by taking a given battery off line, almost fully discharging and recharging it a time or two, and putting it back in service. This simple preventative maintenance process eliminates any memory condition which might have occurred and restores affected cells to full capacity.

Note that I said that satellite battery reconditioning involves almost fully discharging batteries. NiCds in communications birds are NEVER fully discharged! Doing so would shorten their life. Because such batteries are expected to last for seven or more years without failure, "running them down to zero" is definetly not one of the procedures!

What Actually Kills NiCds?

To examine the mechanisms for NiCd cell failure, let's look first at the NiCd cell itself. The eloctrolyte is potassium hydroxide, the uncharged positive plate is nickel hydroxide, and the uncharged negative plate is cadmium hydroxide. As the cell charges, the positive plate changes to nickel hydroxide, and the negative plate becomes metallic cadmium. Once the charging process has taken this conversion as far as it can go, the battery is fully charged, Further charging energy looks for, and finds, other chemical work to do: It breaks down electrolyte water--into oxygen gas at the positive plate, and hydrogen at the negative.

NiCd-cell manufacturers deliberately make the positive plate smaller than the negative to allow the positive plate to reach full charge before the negative. Overcharging then generates oxygen first, which diffuses through the insulating separator material between the plates and reacts with the negative plate's cadmium metal to convert the cadmium back to it's uncharged (cadmium hydroxide) form. In practice, this means that keeping overcharge current to low levels allows the oxygen generated to combine with the negative plate and keep it from reaching full charge. Increasing overcharge current beyond the point at which the oxygen generated can combine with the negative plate causes cell outgassing-- oxygen and hydrogen if both plates reach full charge. Whatever gas develops builds up pressure in the cell. The oxygen will slowly combine with the negative plate and reduce cell pressure if overcharging ceases at this point; any hydrogen produced, however, will remain. If overcharging continues, pressure builds until the gas escapes through the cell's pressure-relief vent--a good thing, because oxygen supports combustion, and hydrogen can burn with explosive force.

Gas venting indicates water loss, which means electrolyte loss. Electrolyte loss reduces cell capacity. If you overcharge a NiCd battery vigorously enough to cause outgassing, you're losing electrolyte and throwing away battery capacity. How much you overcharge the battery--that is, how much overcharge current you push through the battery how fast, and for how long-- makes the difference between a destructive overcharge and an overcharge your battery can handle.

Chargers

Charging your NiCd batteries and cells goes a long way toward maximizing their life. Generally, you'll be okay if you use the charger that matches your hand-held's battery pack--use it according to the manufacturer's instructions, that is. With homemade and aftermarket chargers, you need to be more careful. Here's an overview of the charger types and charging techniques you're likely to encounter.

Wall chargers, plug-in transformer/rectifier units, come with many hand-helds. Typically, they're designed to supply a constant current of about 10% of the cells' ampere-hour (Ah) rating. (In battery jargon, this is referred to as "C-Over-10" rating--C standing for capacity, equal to 100%--and is written as C/10. A 5% charge rate would be C/20 or "C-Over-20." and so on.)

Wall chargers are safe and effective, can be left plugged in and charging for extended periods of time without damaging a battery. The price you pay for this convenience and safety is time: A wall charger typically takes 15 to 16 hours to charge a fully discharged battery. A second battery pack is the simplest way around this limitation; Use one pack while the other charges.

High-rate chargers, sometimes available as extra-cost options for hand-held transceivers, can recharge a battery in much less time than a wall charger, with some manufacturers chaiming recharge times as short as 1 to 2 hours. The charger and battery must be made for each other, because high-rate chargers can damage cells not rated to accept the charge currents they produce. Chargers capable of taper charging (reducing their current as the battery reaches full-charge terminal voltage) are generally safer for a battery. (Exception: If the battery contains one or more shorted cells, a taper charger will damage the pack's remaining good cells by attempting--and failing-- to charge them to the full-charge voltage appropriate for an undamaged pack.)

Pulse charging NiCds is somewhat controversial. Pulse charging, according to it's adherents, charges NiCds efficiently with little heating by means os strong, very short current pulses. It seems that one commercial organization, experimenting with pulse charging for spacecraft batteries, has achieved greater charging efficiency--not cell life--with pulse charging under specific conditions. The benefits were apparently insignificant, however, because my contacts in the space field have confirmed that pulse charging remains unused.

The pulse charger in this aerospace application produced very-high-current, low-duty-cycle pulses. In contrast, the pulse charges described in several popular magazine articles I've seen consist of little more than a rectifier and step-down transformer operating at 60 Hz. Ordinary wall chargers achieve essentially electrically identical "pulsing" because they contain almost no filtration. (A charging NiCd battery absorbs and smooths ripple far better than the largest filter capacitor installable in a tiny wall charger.) Overall, pulse charging has yet to prove its worth.

Dischargers

Entrepreneurship has recently brought another product to the NiCd-accessories field: Battery dischargers. Avoid these at all costs. The deeper you discharge a rechargeable battery, the more you stress it. Why deliberately wear out a battery by increasing its discharge? It's never done industrially, because industrial NiCd users know that memory is not an issue. Intentional battery discharge increases the likelihood of cell outgassing and polarity reversal--two effects that can kill a battery pack. Don't needlessly discharge your batteries-- put their stored energy to work.

Summary

I hope that the information presented here boosts your faith in NiCds. Although they certainly don't deserve the poor reputation they receive via the rumor mills, they can be abused. Applying proper NiCd-care know-how should net you better performance and longer life from these popular batteries.



Written by Ken Stuart, W3VVN
Courtesy: QST Magazine, Special Ed.pg.20