Sealed Ni-Cd cells may be used individually, or assembled into battery packs containing two or more cells. Small cells are used for portable electronics and toys, often using cells manufactured in the same sizes as primary cells. When Ni-Cd batteries are substituted for primary cells, the lower terminal voltage and smaller ampere-hour capacity may reduce performance as compared to primary cells. Miniature button cells are sometimes used in photographic equipment, hand-held lamps (flashlight or torch), computer-memory standby, toys, and novelties.
Specialty Ni-Cd batteries are used in cordless and wireless telephones, emergency lighting, and other applications. With a relatively low internal resistance, they can supply high surge currents. This makes them a favourable choice for remote-controlled electric model airplanes, boats, and cars, as well as cordless power tools and camera flash units. Larger flooded cells are used for aircraft starting batteries, electric vehicles, and standby power.
Ni-Cd cells have a nominal cell potential of 1.2 volts (V). This is lower than the 1.5 V of alkaline and zinc-carbon primary cells, and consequently they are not appropriate as a replacement in all applications. However, the 1.5 V of a primary alkaline cell refers to its initial, rather than average, voltage. Unlike alkaline and zinc-carbon primary cells, a Ni-Cd cell's terminal voltage only changes a little as it discharges. Because many electronic devices are designed to work with primary cells that may discharge to as low as 0.90 to 1.0 V per cell, the relatively steady 1.2 V of a Ni-Cd cell is enough to allow operation. Some would consider the near-constant voltage a drawback as it makes it difficult to detect when the battery charge is low.
Ni-Cd batteries used to replace 9 V batteries usually only have six cells, for a terminal voltage of 7.2 volts. While most pocket radios will operate satisfactorily at this voltage, some manufacturers such as Varta made 8.4 volt batteries with seven cells for more critical applications.
12 V Ni-Cd batteries are made up of 10 cells connected in series.
Comparison with other batteries:
Recently, nickel-metal hydride and lithium-ion batteries have become commercially available and cheaper, the former type now rivaling Ni-Cd batteries in cost. Where energy density is important, Ni-Cd batteries are now at a disadvantage compared with nickel-metal hydride and lithium-ion batteries. However, the Ni-Cd battery is still very useful in applications requiring very high discharge rates because it can endure such discharge with no damage or loss of capacity.
When compared to other forms of rechargeable battery, the Ni-Cd battery has a number of distinct advantages:
The batteries are more difficult to damage than other batteries, tolerating deep discharge for long periods. In fact, Ni-Cd batteries in long-term storage are typically stored fully discharged. This is in contrast, for example, to lithium ion batteries, which are less stable and will be permanently damaged if discharged below a minimum voltage.
Ni-Cd batteries typically last longer, in terms of number of charge/discharge cycles, than other rechargeable batteries such as lead/acid batteries.
Compared to lead-acid batteries, Ni-Cd batteries have a much higher energy density. A Ni-Cd battery is smaller and lighter than a comparable lead-acid battery. In cases where size and weight are important considerations (for example, aircraft), Ni-Cd batteries are preferred over the cheaper lead-acid batteries.
In consumer applications, Ni-Cd batteries compete directly with alkaline batteries. A Ni-Cd cell has a lower capacity than that of an equivalent alkaline cell, and costs more. However, since the alkaline battery's chemical reaction is not reversible, a reusable Ni-Cd battery has a significantly longer total lifetime. There have been attempts to create rechargeable alkaline batteries, or specialized battery chargers for charging single-use alkaline batteries, but none that has seen wide usage.
The terminal voltage of a Ni-Cd battery declines more slowly as it is discharged, compared with carbon-zinc batteries. Since an alkaline battery's voltage drops significantly as the charge drops, most consumer applications are well equipped to deal with the slightly lower Ni-Cd cell voltage with no noticeable loss of performance.
The capacity of a Ni-Cd battery is not significantly affected by very high discharge currents. Even with discharge rates as high as 50°C, a Ni-Cd battery will provide very nearly its rated capacity. By contrast, a lead acid battery will only provide approximately half its rated capacity when discharged at a relatively modest 1.5C.
Nickel-metal hydride (NiMH) batteries are the newest, and most similar, competitor to Ni-Cd batteries. Compared to Ni-Cd batteries, NiMH batteries have a higher capacity and are less toxic, and are now more cost effective. However, a Ni-Cd battery has a lower self-discharge rate (for example, 20% per month for a Ni-Cd battery, versus 30% per month for a traditional NiMH under identical conditions), although low self-discharge NiMH batteries are now available, which have substantially lower self-discharge than either Ni-Cd or traditional NiMH batteries. This results in a preference for Ni-Cd over NiMH batteries in applications where the current draw on the battery is lower than the battery's own self-discharge rate (for example, television remote controls). In both types of cell, the self-discharge rate is highest for a full charge state and drops off somewhat for lower charge states. Finally, a similarly sized Ni-Cd battery has a slightly lower internal resistance, and thus can achieve a higher maximum discharge rate (which can be important for applications such as power tools).
The primary trade-off with Ni-Cd batteries is their higher cost and the use of cadmium. This heavy metal is an environmental hazard, and is highly toxic to all higher forms of life. They are also more costly than lead-acid batteries because nickel and cadmium cost more.
One of the biggest disadvantages is that the battery exhibits a very marked negative temperature coefficient. This means that as the cell temperature rises, the internal resistance falls. This can pose considerable charging problems, particularly with the relatively simple charging systems employed for lead-acid type batteries. Whilst lead-acid batteries can be charged by simply connecting a dynamo to them, with a simple electromagnetic cut-out system for when the dynamo is stationary or an over-current occurs, the Ni-Cd battery under a similar charging scheme would exhibit thermal runaway, where the charging current would continue to rise until the over-current cut-out operated or the battery destroyed itself. This is the principal factor that prevents its use as engine-starting batteries. Today with alternator-based charging systems with solid-state regulators, the construction of a suitable charging system would be relatively simple, but the car manufacturers are reluctant to abandon tried-and-tested technology.
Ni-Cd cells are available in the same sizes as alkaline batteries, from AAA through D, as well as several multi-cell sizes, including the equivalent of a 9 volt battery. A fully charged single Ni-Cd cell, under no load, carries a potential difference of between 1.25 and 1.35 volts, which stays relatively constant as the battery is discharged. Since an alkaline battery near fully discharged may see its voltage drop to as low as 0.9 volts, Ni-Cd cells and alkaline cells are typically interchangeable for most applications.
In addition to single cells, batteries exist that contain up to 300 cells (nominally 360 volts, actual voltage under no load between 380 and 420 volts). This many cells are mostly used in automotive and heavy-duty industrial applications. For portable applications, the number of cells is normally below 18 cells (24V). Industrial-sized flooded batteries are available with capacities ranging from 12.5Ah up to several hundred Ah.
Ni-Cd batteries can be charged at several different rates, depending on how the cell was manufactured. The charge rate is measured based on the percentage of the amp-hour capacity the battery is fed as a steady current over the duration of the charge. Regardless of the charge speed, more energy must be supplied to the battery than its actual capacity, to account for energy loss during charging, with faster charges being more efficient. For example, an "overnight" charge, might consist of supplying a current equals to one tenth the amperehour rating (C/10) for 14-16 hours; that is, a 100 mAh battery takes 10mA for 14 hours, for a total of 140 mAh to charge at this rate. At the rapid-charge rate, done at 100% of the rated capacity of the battery in 1 hour (1C), the battery holds roughly 80% of the charge, so a 100 mAh battery takes 120 mAh to charge (that is, approximately 1 hour and fifteen minutes). Some specialized batteries can be charged in as little as 10�15 minutes at a 4C or 6C charge rate, but this is very uncommon. It also exponentially increases the risk of the cells overheating and venting due to an internal overpressure condition: the cell's rate of temperature rise is governed by its internal resistance and the square of the charging rate. At a 4C rate, the amount of heat generated in the cell is sixteen times higher than the heat at the 1C rate. The downside to faster charging is the higher risk of overcharging, which can damage the batteryand the increased temperatures the cell has to endure (which potentially shortens its life).
The safe temperature range when in use is between −20°C and 45°C. During charging, the battery temperature typically stays low, around 0°C (the charging reaction absorbs heat), but as the battery nears full charge the temperature will rise to 45° 50°C. Some battery chargers detect this temperature increase to cut off charging and prevent over-charging.
When not under load or charge, a Ni-Cd battery will self-discharge approximately 10% per month at 20°C, ranging up to 20% per month at higher temperatures. It is possible to perform a trickle charge at current levels just high enough to offset this discharge rate; to keep a battery fully charged. However, if the battery is going to be stored unused for a long period of time, it should be discharged down to at most 40% of capacity (some manufacturers recommend fully discharging and even short-circuiting once fully discharged), and stored in a cool, dry environment.
A Ni-Cd battery requires a charger with a slightly different voltage than for a lead-acid battery, especially if the battery has 11 or 12 cells. Also a charge termination method is needed if a fast charger is used. Often battery packs have a thermal cut-off inside that feeds back to the charger telling it to stop the charging once the battery has heated up and/or a voltage peaking sensing circuit. At room temperature during normal charge conditions the cell voltage increases from an initial 1.2 V to an end-point of about 1.45 V. The rate of rise increases markedly as the cell approaches full charge. The end-point voltage decreases slightly with increasing temperature.