NiCd Nickel Cadmium Battery Charging

- details of the optimum charging & trickle charging techniques and circuits used with nickel cadmium or NiCd batteries to ensure the optimum performance.

Like any other form of rechargeable battery or cell, NiCds or nickel cadmium cells need to be charged in the correct manner.

Incorrect charging can result in shortened life or in some cases where charging is particularly in appropriate, it can result in safety issues.

Fortunately nickel cadmium or NiCd charging techniques are relatively straightforward and a there have been many suitable chargers on the market for these batteries and cells.

Prime charging NiCd batteries

NiCd battery manufacturers do not fully format their batteries before shipment so that they do not degrade as much in storage. As a result it is best to give new batteries a slow charge before use. This will typically take between 15 and 24 hours. This will ensure that each cell has the same level of charge as they have have self-discharged at different rates while in transit.

Additionally it is found that performance of new cells only reaches the optimum after a number of charge / discharge cycles. Typically cells should reach their specified level of performance after five to ten charge discharge cycles.

Beyond this, the peak capacity may be reached after around 100 or more charge discharge cycles after which the performance will start to drop.

This assumes that the NiCd batteries are charged and discharged in the required manner and they are not subject to abuse.

NiCd charging basics

Unlike the lead acid cells, NiCads are charged using a constant current source. Their internal resistance is such that if a constant voltage was used, they would draw excessively large currents which would damage the cells.

Normally cells are charged at a rate of around C/10. In other words if their capacity is 1 amp hour then they would be charged at a rate of 100mA. The charge time is usually longer than ten hours because not all the energy entering the cell is converted into stored electrical energy.

It is found that during the first stage of charging, up to about 70% of full charge, the charging process is nearly 100% efficient. After this it falls.

Fast charging

Sometimes equipment using nickel cadmium cells requires the use of fast charging techniques.

Typically charging takes place at rates of around C. However it is necessary to ensure that the NiCd charging is operated correctly and charging is terminated immediately on charge being completed.

As the charging efficiency is nearly 100% up until about 70% of full charge, the full speed charge is maintained up until this point, whereupon the charge rate is reduced as the temperature increases as the charge efficiency reduces.

It is found that a fast charge for NiCd cells also improves charge efficiency. At a 1C charge rate, the overall charge efficiency of a standard NiCd is about 90%, and the charge time is just over an hour.

Detecting end of charge

Whether slow or fast charging is used, it is necessary to ensure that any NiCd cells are not overcharged. It is therefore necessary to be able to detect end of charge. There are a number of methods of achieving this.

  • Basic charger:   Some of the very basic NiCd chargers that can be bought simply apply a charge of around C/10. They do not include a timer and assume that the user will remove the charging when the cell is charged. This mode is not at all satisfactory as cells will be overcharged if the user forgets and will suffer damage as a result. Also there is no way of knowing the exact charge state before charging begins.
  • Elapsed time / timer:   Some of the most basic chargers assume that cells will require a full charge and knowing their capacity, they can be given a charge for a given amount of time. This is a simple and straightforward method of charging NiCd cells and batteries. One of the main drawbacks of this form of end of charge termination is that it assumes that the batteries are all fully discharged before they are recharged. To ensure that batteries are suitable discharged, the charger may place item through a discharge cycle.

    This is not a particularly accurate method of recharging batteries and cells because the amount of charge they can hold changes over their useful lifetime. However it is better than no form of charge termination.
  • Temperature rise:   The technique used to detect when the fast charge should finish is that of temperature sensing. The problem with this is that this is inaccurate because the core of the cell will be at a much higher temperature than the periphery. For normal charge rates the temperature rise rate may be insufficient to accurately detect.

    Typically a temperature of 50°C is used as the cut-off temperature. Although a short period over the a temperature of 45°C can be acceptable if the temperature is able to fall quickly, any prolonged period at or above this causes the cell to deteriorate.

    More comprehensive chargers using more advanced techniques have been made available for fast chargers. Based around microprocessor technology they are able to detect the rate of change of temperature. Typically charge termination occurs when a rate of temperature increase of 1°C per minute is reached or an ultimate pre-determined temperature (often between 50°C and 60°C) is reached.

    Detection of the temperature increase rate is important because it determines when the cell is fully charged and energy entering the cell is not being converted into stored energy by lost as heat.

    One of the drawbacks of this method, is that NiCd cells or batteries reinserted into a temperature sensing charger, which is likely to be a fast charger can apply a harmful overcharge if the battery is reinserted without being fully discharged as in the case of someone wanting to make sure the battery has been charged.
  • Voltage signature:   Voltage signature NiCd chargers use the voltage signature of the nickel cadmium cell to determine where it is within its charge cycle.

    It is found that when a NiCd battery is fully charged, there is a small drop in the terminal voltage. Microprocessor based chargers are able to monitor the voltage and detect the full charge point when they will terminate the charging process.

    This form of NiCd charge termination is often called negative delta voltage, NDV. It provides the best performance with fast charging because the negative delta voltage point is more obvious when used with fast charging.

NiCd trickle charge

Often it is necessary to hold NiCd cells and batteries at full charge and overcome any self-discharge of the cell over time that would render them not immediately usable.

Once fully charged, it is possible to keep the NiCd at its full charge state by applying a trickle charge. This trickle charge can be achieved safely by applying a small current to the cell or cells at a level between about 0.05 C and 0.1 C. This needs to be achieved using a current source as the actual voltage of the cells may vary according to the temperature.

Often a much higher trickle charge may be applied to a cell or cells and this can result in overheating and some damage.

even though there is often a requirement to keep cells or batteries on a trickle charge to ensure they are ready for operation, if battery life is a consideration, it is not ideal to leave NiCd cells on trickle charge for more than a few days at a time. It is much better to remove them and recharge before use.

By Ian Poole


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