Normally, for any a NiCd cell it may take more than 15 hours to charge them fully, which is a very long time. The fast NiCd charger explained in this article will charge any modern NiCd cell within a period of just 1 hour.
The modern NiCd cells are usually built using sintered electrodes, which it makes it possible to charge them initially with a high charging current rate of around 500 mAh, so that they can charge rapidly at the start.
However, the challenge in doing so is to switch OFF the charging operation exactly at the full charge limit of the cell, which is usually 1.5 V.
In this article we make sure that the NiCd cell is initially fast charged with a 0.5 amp current and then switched OFF when the cell reaches about 80% of its full charge level. The switch OFF is done by an op amp which is configured like a compartaor.
If for some reason the op amp fails to cut off at the right time, we also have a backup protection in the form of a IC 4060 timer circuit which automatically cuts off the high current charging to the NiCd after about 54 minutes.
After this the fast charge is switch OFF and replaced with a float charge of 50 mA, which continues for some more time until the battery is removed.
Circuit Stages
The circuit is made up of 4 different stages as depicted in Fig. 3. The Fig. 3a indicates the input voltage section using rectifier, Br, and an IC1 7805, 5-V voltage regulator, along with a the IC 4060 timer, IC4.
It has to be taken into account that the NiCd batteries cannot be wired in series in a voltage-controlled charger. This is mainly because the batteries will never be completely charged simultaneously.
The main charger controller circuit includes a charging voltage monitor stage along with a switch, IC2, and a darlington transistor, T1, which is wired up to work like a current source for the fast charging of the NiCd battery.
The input voltage supply stage consists of an power 'on' indicator, D11. The input supply is received through a mains transformer with an 8-V, 1.5 A secondary. After rectification the voltage is filtered, and cleaned by the smoothing capacitor C3. The IC 7805 regulates this supply to 5 V output which is then applied as a reference voltage and a supply input for the charger circuit.
The IC 4060 is configured to work with a frequency of 2.5 Hz, which is set by the timing components R10 and C6. After 213 (8192) clock pulses (which is after approximately 54 mins) after the reset key S1 had been activated; output Q14 (pin 3) is turned logic high.
The regulated source from the 7805 IC is used as the reference voltage for IC2, through a potential divider made from R1-P1-R2. This reference voltage is set in accordance with the battery level at which the charging operation needs to be cut-off, and this value has to be undoubtedly a 1.5 V.
How the Circuit Works
The inverting input of IC2 get the reference voltage through the resistor R3 . The non-inverting input of the op amps is connected with the NiCd battery voltage. The IC2/IC4 work like bistables, which are both initiated through the switch S1 start set pulse during the start of the charging process. As soon as S1 is pushed ON momentarily, the inverting input of op amp temporarily gets linked with the +8 V input supply through D1.
This just becomes enough to turn the op amp output to 0V so that the transistor T1 switches ON and connects the NiCd cell with the charging supply. For a discharged battery, its voltage will be below that of the reference voltage, which will cause the comparator to latch ON in this condition and keep T1 switch ON while the battery charges.
In this situation a fast charging current of approximately 0.5 A begins passing through the battery keeping the LED D4 illuminated.
This LED does a twin job, it indicates the ongoing charging process, and also, along with D3 provide a voltage reference for the transistor T1. The level of the charging current is fixed by the resistor R8 value.
Now as the battery charges, due to the high current it quickly gets charged within an hour. When this happens, its terminal voltage increases above the reference voltage set by the preset P1 at the inverting input of the op amp.
Due to this the op amp now toggles, and its output changes state, and becomes high.
The high output from the op amp switches OFF the transistor T1, which immediately stops the charging process of the connected NiCd cell.
Once the charging process is terminated, a small float charge current continues to flow through the resistor connected across the emitter/collector of T1.
This float charge resistor must be selected in such a away that only a minimal amount of 50 mA is able to pass through it. Any value between 70 and 100 ohms should do the job for this resistor.
In case the op amp delays the switch OFF process due to some reason, the IC 4060 timer now kicks in, and after exactly 54 minutes it switches OFF the NiCd fast charging process of the circuit by supplying a high pulse to the non-inverting input of the op amp via the diode D5.
Therefore it seems that the IC 4060 stage is actually optional, and could possibly be eliminated, and the whole process may be left to the op amp comparator itself.
