NiCD Charging Types

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Basics

In many ways the nickel cadmium battery is the most satisfying battery to charge. The main reason for this is that the charging process is strongly endothermic - the battery cools during charge. This makes it possible to charge very quickly, as the joule heating and endothermic charging balance each other.

The coulometric charging efficiency of nickel cadmium is about 83% for a fast (C/1 to C/.24) charge, and 63% for a C/5 charge. This means that at C/1 you must put in 120 amp hours in for every 100 amp hours you get out. The slower you charge the worse this gets. At C/10 it is 55%, at C/20 it can get less than 50%. (These numbers are just to give you an idea, battery manufacturers differ).

When the charge is complete oxygen starts being generated at the nickel electrode. This oxygen diffuses through the separator and reacts with the cadmium electrode to form cadmium hydroxide. This causes a lowering of the cell voltage which can be used to detect the end of charge. This so-called minus delta V/ delta t bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very temperature dependent.

As the battery reaches end-of-charge oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured with a thermistor.. This is the safest way to detect end-of-charge during a fast charge.

Nickel cadmium battery chargers should cut the charge off when the temperature exceeds the maximum charging temperature, typically 45 degrees C for a controlled fast charge, and 50 degrees C for an overnight or fast charge.

Overnight Charging

The cheapest way to charge a nickel cadmium battery is to charge at C/10 (10% of the rated capacity per hour) for 16 hours. So a 100mAH battery would be charged at 10mA for 16 hours. This method does not require an end-of-charge sensor and ensures a full charge. Cells can be charged at this rate no matter what the initial state of charge is. The minimum voltage you need to get a full charge varies with temperature - at least 1.41 volts per cell at 20 degrees C. The best charging practice is to use a timer to prevent overcharging to continue past 16 hours. Our chargers use a microprocessor to report the state of charge via an LED as well as performing the timing function.

Faster Charging

Some nickel cadmium cells are designed to be "quick chargeable." This is just a timed charge at C/3 for 5 hours, or C/5 for 8 hours. This is risky because the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of venting the battery. One way to ensure this doesn't happen is to have the charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours. The advantage of this method is to eliminate any chance of battery memory. PowerSupplies online offer several such chargers. If you do not have such a charger you could modify the microprocessor board on some makes of charger to do the discharge. A power dissipating package would be needed in order to dissipate the energy from a partially charged battery in a reasonable amount of time.

Fastest Charging

If a temperature or voltage monitor is used NiCad batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in amp-hours for 1.5 hours).

The termination can be done with minus delta V, when the battery voltage drops -10 to -20 mV per cell. To terminate the charge on temperature requires a temperature slope measurement.

This board also has the ability to sense voltage and current for more sophisticated algorithms required for ultra-fast charging. These algorithms require constant monitoring of the voltage, temperature, and sometimes pressure, to actively determine the amount of current a battery can take without damage. This is sometimes called a smart charge, or a controlled fast charge.

A cheaper version of the fast charger can be made by just monitoring absolute temperature. The charge rate is set at C/2 until 45 degrees C is reached then switched over to a C/10 charge to complete the charge.

Trickle Charging

In standby mode you might want to keep a nickel cadmium battery topped up without damaging the battery. This can be done safely at a current of between 0.05 C and .06 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

Last Amendment: 30/11/2006 23:11:44