Nickel-cadmium batteries. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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Nickel-cadmium batteries, commonly referred to as nickel-cadmium cells, are somewhat different from most dry cells, such as the manganese-zinc battery commonly used in flashlights. As the battery discharges, it loses some of its voltage. This effect is manifested in the brightness of the flashlight bulb. As the battery drains, the glow becomes more and more dimmer until it stops altogether.

In contrast, nickel-cadmium cells hold the voltage quite stable during the discharge. This can be seen from the constancy of the glow up to a deep charge. After the element is discharged, the voltage on it quickly drops and the glow stops. On fig. 1 for comparison shows the dependence of the voltage on the degree of discharge of the elements of the two mentioned types.

As you can see, to determine the remaining life of a manganese-zinc cell, you simply need to measure the voltage across it. For a nickel-cadmium element, this is not so easy to do. An 80% discharged cell produces the same voltage as a freshly charged cell. Thus, when recharging a nickel-cadmium cell, some difficulty arises. Until the element is completely discharged, we cannot judge its condition. In addition, nickel-cadmium cells are very sensitive to overcharging, which can damage them. So a partially discharged cell poses a really tricky question: how much charge can it take on?

Recharging nickel-cadmium cells

To better understand the principle of operation of the charger, you must first familiarize yourself with the operation of the nickel-cadmium cell itself. You can start consideration with a completely discharged element. To charge it, you need to pass current through it.

Due to its design, the nickel-cadmium cell has a fairly high internal resistance, which is inversely proportional to the amount of charge accumulated in the cell: the lower the charge, the higher the resistance.

Due to the presence of internal resistance, part of the energy of the charging current is converted into heat. Therefore, it is necessary to start the charge with a small current, otherwise the energy dissipated in the internal resistance in the form of heat will lead to the failure of the element.

As the charge increases, the internal resistance of the cell decreases. The lower the resistance, the less heat is dissipated and the more efficiently the charge of the cell flows. In addition, more charging current can now be passed through the cell, which will further speed up the charging process. In practice, it is possible to complete the charge cycle at a current significantly higher than the initial current.

However, it is very difficult to regulate and maintain such a charge mode. For simplicity, manufacturers recommend the maximum safe current regardless of battery condition.

For disk nickel-cadmium cells, this current does not exceed 330 mA. Even a completely discharged cell with a high internal resistance can be charged with such a current without fear. However, the answer to the question has not yet been received: what amount of charge will not harm the element?

The charging current mentioned above can only be maintained until the battery is fully charged. This usually takes 4 hours. If you continue recharging, there is a danger of overcharging the cell, which can lead to a decrease in battery life or, worse, destruction of the cell. Thus, if the battery is only half discharged, it can be easily recharged without even knowing it.

This is why the manufacturer recommends slow recharging. For a disk element, the charging current should not exceed 100 mA. With slow charging, you can charge the cell without fear of overcharging for the recommended 14 hours required to charge a fully discharged cell. In fact, it is possible to constantly lightly charge the element without fear of its destruction: the charge rate is quite low and excess energy is easily dissipated by the element.

Author: Alexander Torres; Publication: N. Bolshakov, rf.atnn.ru

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