Challenges in Designing Fast NiCd Chargers
Developing fast NiCd charger poses an ongoing challenge, primarily in establishing the precise moment to cease charging and indicating when the battery has reached full capacity. The charger outlined in this description incorporates the latest advancements reported by various manufacturers. Nevertheless, the certainty of consistently delivering satisfactory results remains uncertain. The charging process involves applying a current, measured in milliamps (mA), that is ten times the nominal capacity of the battery, expressed in milliampere-hours (mAh). This translates to a substantial charging current of 5 A for an HP7 (AA; RG) type battery, a value 100 times greater than standard charging practices.
Controlled Charging with a Type 555 Timer IC
The charging process is under the control of a Type 555 timer IC configured as an astable. Charging occurs when the IC’s output is high, but there is a predetermined period during which no charging occurs (=R6C3). Once charging halts, an electronic switch (ICS) connects C1′ to the battery. The terminal voltage of C1′ is then compared to the maximum battery voltage set by PI using ICIa. The comparator’s output undergoes integration through R3 and C2, determining the stable period. If the maximum battery voltage is not attained, charging occurs for approximately 90% of that time. Conversely, if the maximum battery voltage is reached, charging is limited to 1% of the time (trickle charging). It is advisable not to keep the battery unnecessarily connected to the charger; the LED illumination indicates a fully charged battery.
Unreliable Battery e.m.f. as a Charge Indicator
Due to various resistances in the supply leads and connections during charging, the battery’s electromotive force (e.m.f.) becomes an unreliable measure for determining the state of charge. Consequently, the e.m.f. is measured immediately after a burst charge, ensuring precise voltage measurement. The critical question revolves around setting P1 to the correct e.m.f.; in essence, determining the accurate battery e.m.f. While opinions may vary, the prototype yielded favorable results with a value of 1.42 V at room temperature (21 °C). Drawing a mere 10–15 mA, the circuit achieves this current range with a 7805 regulator.
Charging Time and Current Regulation
The proposed charger aims to charge a single 1.5 V NiCd battery within 8–10 minutes. The charging current for a 500 mAh battery is approximately 5 A unregulated, as it is limited by R1. The value of R1 adheres to Ohm’s law. For instance, if the charging current is drawn from an 8 V source with a 2 V drop across the battery and T1, the voltage across the resistor is 6 V. Accordingly, its value should be 6/5 = 1.2 Ω. It is important to note that the power dissipated is 30 W, necessitating the connection of multiple resistors in parallel.
Considerations for Multiple Batteries in Series
For those intending to charge multiple batteries in series, adjust the level set by P1 accordingly (at 1.42 V per battery). However, it is crucial to use batteries pre-sorted by the manufacturer for equal capacity. Additionally, the charger circuit’s supply voltage must consistently be 2 V higher than the level set with P1. Lastly, after every five fast charges, administer a ‘normal’ charge (1/10 mAh over 14 hours) to the battery.