As of the current date, the most recent AA NiMH (Nickel Metal Hydride) batteries boast a capacity of up to 2900 mAh. When charged with a conventional battery charger that provides 125 mA, the charging process can be exceptionally lengthy.
The charger proposed here aims to expedite the recharging of NiMH batteries, which are increasingly prevalent (our contribution to environmental conservation). The design centers around the MAX712 by Maxim Integrated Products, which, to be precise, was acquired by Dallas Semiconductor—a story spanning quite a length. Operating in switched mode, this charger can deliver a maximum fast charge current calculated as follows:
/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.
Maxim Integrated Circuit Features and Configurations
The Maxim circuit stands out due to its intelligence, integrating an ADC (analog to digital converter), a charge completion detection system, a timer, and a temperature monitoring module. Users have the flexibility to set parameters according to their needs, utilizing the four configuration pins. These pins facilitate customization for variables like the number of cells to be charged, the maximum charging duration, and the method to detect full charge, be it at the inflection point or through negative slope detection. Referencing the datasheet provides more in-depth information about these settings. The MAX712 is specifically designed for NiMH batteries, achieving charge completion at the voltage curve’s inflection point (8wat = 0).
Voltage Specifications and Power Supply Requirements
The circuit’s maximum power supply voltage stands at 15 V. To accommodate voltage fluctuations during charging, the power supply voltage should exceed the maximum charging voltage by at least 2 V. For example, when charging eight series-connected batteries with a maximum charging voltage of 1.6 V per cell, a 15 V power supply voltage is utilized. Alternatively, when recharging six cells, a 12 V voltage level sourced from a car battery is employed. The power supply must be capable of delivering 1 A. Ensuring the power supply meets these specifications is crucial. Failure to do so may lead to the integrated circuit malfunctioning and potentially failing to detect the completion of fast charging, risking damage to 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.
LED Indicator and Heat Dissipation
The LED indicator D1 illuminates when the circuit is operating in fast-charge mode. If necessary, T5 can be mounted on a heatsink for efficient heat dissipation. The specifications of coil L1 are not highly critical; a conventional 100 µH/5 A suppressor choke functions well. Similarly, diodes D2, D3, and the MOSFET transistor T5 are not stringent in this application. Any Schottky diode capable of handling 3 amps can be used, along with any MOSFET having a lower drain resistance.
Compact PCB Design and Component Mounting
A compact PCB layout has been devised for this circuit. Mounting the components should be straightforward, but it’s crucial not to overlook the two wire links on the board. Inductor L1 is a toroid “suppressor choke” of adequate size. Connectors K1-K4 enable the configuration of different charging parameters.
Setting Parameters Using Existing Calculation Principles
The calculation principle aligns with that of the NiCd charger in the MAX713 article. For detailed calculations, we recommend referring to the example provided there. Utilize the same tables to configure the parameters for this circuit as the ones outlined in the mentioned article.