Battery Charger

Fast Charging Of NiCd And NiMH Batteries Schematic Circuit Diagram

Maxim ICs MAX712 and MAX713 for Battery Charging

Designed to be pin-compatible, the Maxim ICs Type MAX712 and MAX713 are specifically crafted for the swift charging of nickel-metal-hydride (NiMH) batteries. The MAX713 can additionally be applied to nickel-cadmium (NiCd) battery chargers, as detailed in Table 1. In this table, ‘C’ denotes the nominal capacity of the battery in ampere-hours (Ah). It’s crucial to note that charging at currents exceeding 2C should only be undertaken if the respective batteries are deemed suitable, as indicated in the manufacturer’s data sheet.

Charging Process with MAX712 and MAX713 ICs

These ICs employ a constant current, indicated by dV/dt, to rapidly charge NiMH or NiCd batteries. The output current operates in one of two states: high (fast charge) or low (trickle charge). Upon detecting a full charge, the current is then reduced to a trickle charge. The ICs closely monitor three key parameters—voltage slope, battery temperature, and charging time—to accurately determine when the battery has reached full charge.

Fast charging of NiCd and NiMH batteries Schematic diagram

Design Description of the Charger:

This charger is powered by a mains adaptor featuring an output voltage of 9 V d.c. and an output current rating of at least 1 A. With this configuration, penlight NiMH batteries can be charged at a rate of 0.5 C, while NiCd batteries can be charged at a rate of 1 C. The maximum charging voltage is capped at 1.65 V per battery. Fast charging ceases either when the programmed charging time concludes or when the battery voltage drops (dV/dt method is employed). The value of resistor R5 is determined by R5 = 0.25/FAST, where IFAST represents the desired charging current.

Charging Parameters and Resistor Calculation:

To illustrate, consider the scenario of charging a 1.2 Ah NiMH battery at a rate of 0.5 C (two hours). This necessitates a charge current of 1.2 x 0.5 = 0.6 A. Consequently, R5 is computed as 0.25/0.6 = 0.42 O. Similarly, when charging a 500-mph NiCd battery at a rate of 1 C (one hour), a resistance approximately equivalent to 0.47 O, as denoted in position R5, proves effective for both applications. The precision of the charging current is not crucial in practice, as the ICs actively monitor the individual cells.

Built-in Timer Features of the ICs:

Equipped with a built-in timer, the ICs offer flexibility in setting a wide range of charging times. The ’66 minutes’ setting approximates a theoretical charging rate of 1 C, while 0.5 C corresponds to 132 minutes. Upon completion of the programmed fast charging period, D2 extinguishes, triggering an automatic switch to trickle charging. The trickle charging current is contingent on the programmed charging time, as outlined in Table 2. Following a general guideline, batteries should undergo a 14-hour trickle charge once every five ‘fast’ charging cycles.

Straightforward Construction of the Charger:

The construction of the charger is uncomplicated on the printed circuit board (refer to Fig. 2). Wire jumpers play a crucial role in setting the number of cells and the charging time, with positions conveniently outlined in Tables 2 and 3. The power transistor, T1, is affixed to a heat sink, ensuring efficient heat dissipation. Additionally, the power resistor R5 is positioned a few millimeters above the board to enhance its cooling capabilities.

Fast charging of NiCd and NiMH batteries Schematic diagram

Fast charging of NiCd and NiMH batteries Schematic diagram

Parts list

  • R1, R3 = 470 Ω
  • R2 = 820 Ω
  • R4= 150 Ω
  • R5 =0.47 Ω 3 W
  • C1, C3 = 10 uF, 25 V
  • C1= = 10 uF, 16 v
  • C4= 10 nF
  • D1 = LED, red
  • D2 = LED, green
  • D3 = 1N4001; T1 = BD242C
Integrated circuits:
  • IC1 = MAX713 or MAX712
  • K1. K2 = 2-way PCB terminal block,
  • pitch 5 mm
  • Enclosure, e.g., Pactec
  • Model HM,
  • Type 6600-902**
  • Heat-sink Type SK59
  • (Fischer)
  • PCB REF. 934098

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