Pin-compatible ICs Type MAX712 and MAX 713 from Maxim are intended for fast charging of nickel-metal-hydride (NiMH) batteries: the MAX713 may also be used in nickel-cadmium (NiCd) battery chargers (see Table 1). In this table, C is the nominal capacity of the battery in Ah. Note that charging at currents >2 C should be undertaken only if the relevant batteries are suitable (see manufacturer’s data sheet).
The ICs fast-charge a NiMH or NiCd battery by forcing a constant current into it (dV/dt). Their output current is always in one of two states: high (fast charge) or low (trickle charge). Once a full charge is detected. the current is reduced to trickle charge. The ICs monitor three parameters to determine when the battery reaches full charge: voltage slope, battery temperature and charging time.
The charger described here is powered by a mains adaptor with an output voltage of 9 V d.c. and an output current rating of at least 1 A. This allows penlight NiMH batteries to be charged at a rate of 0.5 C, and NiCd batteries at a rate of 1 C. The maximum charging voltage is limited to 1.65 V per battery. Fast charging is discontinued when either the programmed charging time has elapsed, or the battery voltage drops (d V/d t method). The value of R5 is calculated from
R5 = 0.25/ /FAST,
where IFAST is the desired charging current. Examples to charge a 1.2 Ah NiMH battery at a rate of 0.5 C (two hours), a charge current of 1.2×0.5 = 0.6 A is required. This means that R5 becomes 0.25/0.6 = 0.42 O. Roughly the same resistance is needed when a 500 mph NiCd battery is to be charged at a rate of 1 C (one hour). In practice, the 0.47 c/ resistor shown in position R5 will work fine for both applications, since the charging current need not be **exact, and, in any case, the ICs monitor the individual cells.
The ICs have a built-in timer that can be set to a wide range of charging times. The theoretical charging rate of 1 C is approximated by the ’66 minutes’ setting. Similarly, 0.5 C corresponds to 132 minutes. When the programmed fast charging period has elapsed, D2 goes out, and the ICs automatically switch to trickle charging. The trickle charging current depends on the programmed charging time (see Table 2). As a rule of thumb, batteries should be given a 14-hour trickle charge once every five ‘fast’ charging cycles.
Construction of the charger is straightforward on the printed circuit board (Fig. 2). The number of cells and the charging time are set with the aid of wire jumpers, whose position may be taken from Tables 2 and 3. The power transistor, T1, is mounted on a heat-sink. Power resistor R5 is mounted a few millimeters above the board to assist in its cooling.
R1, R3 = 470 Ω
R2 = 820 Ω
R4= 150 Ω
R5 =0.47 Ω 3 W
C1, C3 = 10 uF 25 V
C1= 10uF 16V
C4= 10 nF
D1 = LED, red
D2 = LED, green
D3 = 1N4001 T1 = BD242C
IC1 = MAX713 or MAX712*
K1. K2 = 2-way PCB terminal block,
pitch 5 mm
Enclosure, e.g., Pactec
Heat-sink Type SK59
PCB REF. 934098
• Maxim Integrated Products (UK) Ltd., 20A Horseshoe Park, Pangbourne, Reading RG8 7JW. Telephone (0734) 845255, fax (0734) 843863
** OK Industries UK Ltd., Chickenhall Lane, Unit 1, Deacon Trading Estate, Eastleigh, Hants S05 5RR. Telephone: (0703) 619841, fax: (0703) 643279.
* Dau (UK) Ltd. 70-75 Barnham Road, Barnham, West Sussex P022 OES, Telephone (0243) 553 031.