The continuing difficulty in designing fast NiCd chargers is determining when the battery is charged, that is when to stop charging.
The charger presented here is based on the latest developments as reported by several manufacturers. It is not at all certain whether, and under what conditions, the circuit will always give satisfactory results. The battery is charged with a current (in mA) that is ten times Its nominal capacity (in mAh). That means that, for instance, an HP7 (AA; RG) type battery is charged at 5 A, a current 100 times larger than used in standard charging.
The charging is controlled by a Type 555 timer IC, here connected as an astable. If the IC’s output is high, charging takes place. There is, however, a fixed period of time (=R6C3) during which no charging takes place. As soon as charging stops. C1′ is connected to the battery by electronic switch ICS. Its terminal voltage is then compared by ICIa with the maximum battery voltage set by PI: The output of the comparator Is integrated by R3 and C2 and then used to determine the period of the stable. If the maximum battery voltage has not been reached, charging takes place for about 90% of that time. If the maximum battery voltage has been reached, charging takes place for 1% of the time (trickle charging). Do not leave the battery connected to the charger unnecessarily: when the LED lights, the battery is fully charged.
During charging, owing to a variety of resistances, primarily in the supply leads and connections the battery e.m.f. is an unreli-able yardstick for determining the state of charge of the battery. Therefore, the e.m.f. is taken immediately after a burst charge, because then the voltage can be measured exactly. The important question is, of course, to what e.m.f. P1 should be set: in other words, what is the correct battery e.m.f.? Opinions vary, but with the proto-type good results were obtained with a value of 1.42 V at room temperature (21 °C). The circuit draws a current of only 10-15 mA, which may be obtained with a 7805 regulator.
The proposed charger is intended for charging one 1.5 V NiCd battery in 8-10 min-100, utes. The charging current for a 500 mAh battery is about 5 A, which need not be regulated since it will be limited by R1. The value of R1 is given by Ohm’s law. If, for instance, the charging current is drawn from an 8-V source, and assuming that the drop across the battery and T1 is 2 V, the voltage across the resistor is 6 V. Its value should thus be 6/5=1.2 O. Bear in mind that the power dissipated in it is 6×5=30 W: you will, therefore, have to connect a number of resistors in parallel.
If you want to charge a number of batteries in series, raise the level set by P1 accordingly (at 1.42 V per battery). Note, however, that you should use only batteries that have already been sorted for equal capacity by the manufacturer. Also, the supply voltage of the charger circuit must always be 2 V higher than the level set with P1. Finally, after every five fast charges, give the battery a ‘normal’ (1/10 mAh over 14 hours) charge.
