At the time of writing, the latest AA NiMH (Nickel metal Hydride) batteries have a capacity of up to 2900 mAh. Using an original-type conventional battery charger (supplying 125 mA), the charging time will be extremely long.
The charger we propose here should accelerate the recharging process of NiMH batteries, which hare becoming more and more common (we must do our bit for the environment).
The design is based on the MAX712 made by Maxim (Integrated Products to be precise, which was bought by Dallas Semiconductor; quite a long story), operating in switched mode, it can supply a maximum fast charge current calculated as
/dims, = 250 mV / R1
or not less than 1 A if R1 = 0.25 ohms. Under these conditions, the battery will be charged in just over two hours.
The Maxim circuit is not only intelligent, but it also includes an ADC (analogue to digital converter), a system to detect charge completion, a timer, and a temperature monitoring module. The four configuration pins that are included allow users to set the parameters as they please. These pins are used to set the parameters for the number of cells to be charged, the maximum charging period, as well as the method to detect when it is fully charged (inflexion point or negative slope). You can refer to the datasheet to find out more. The MAX712 is intended for NiMH batteries, with charge completion at the inflexion point of the voltage curve (8wat = 0).
The maximum power supply voltage is 15 V. The power supply voltage must be at least 2 V above the maximum charging voltage in order to compensate for voltage fluctuations during charging. Therefore, for a maximum charging voltage of 1.6 V per cell, a 15-V power supply voltage is used to charge 8 series-connected batteries. A 12-V voltage level (supplied, for example, by a car battery) is used to recharge six cells. The power supply must be able to supply 1 A. It is important to be certain of its specification. If the requirement is not fulfilled, the integrated circuit will not operate correctly and may not correctly detect completion of the fast-charge (entailing a risk of damage that could affect the connected batteries).
Setting the circuit parameters
- The PRGMO/PRGM1 pins are used to regulate the number of cells to be charged. A note concerning the use of a battery cradle: during recharging: each contact canrepresent a 1-1″2 series resistance, which is seen as a 1-V potential difference at 1 A. The power supply voltage may not be adequate for this configuration — therefore, it is preferable to verify this detail before beginning the project.
- For security reasons, it is preferable to properly configure the maximum charging period with the PRGM2/PRGM3
- On this setup, the temperature control circuit for the batteries is deactivated. At the end of the fast-charge, the circuit will power the batteries with a maintenance charge (trickle). Let’s examine the circuit’s T1 is uses as a current source supplying the 8 mA necessary to power the MAX712. D3 ensures that the battery does not discharge into the circuit in case it is not powered.
The LED D1 lights up when the circuit is in fast-charge mode. T5 may be mountd
on heatsink, if necessary. The characteristics of coil L1 are not critical; a traditional 100 itH/5 A suppressor choke will work fine. The same holds true for diodes D2, D3 and the MOSFET transistor T5; they too are not critical in this application. You can use any Schottky diode that can withstand 3 amps and include any MOSFET with a lower drain resistance.
A compact PCB was designed for the circuit. Mounting the components should be all plain sailing, but do not forget the two wire links on the board. Inductor L1 is a toroid ‘suppressor choke’ with a good size. Connectors K1-K4 allow different charging parameters to be set up.
Since the calculation principle is the same as for the NiCd charger in the MAX713 in the other article, we refer you to the calculation example proposed there. Use the same tables to set the parameters of this circuit as the ones given in that article.