Nickel-metal-hydride (NiMH) batteries are made by many manufacturers and have different charge ratings. Also, with repeated use, the maximum charge that NiMH batteries will hold diminishes at different rates. Therefore, it's difficult to gauge the useful life and charge capacity of these batteries. To measure NiMH battery performance, a multichannel battery discharge unit was designed. The unit includes a serial interface through Windows Hyperterminal and is powered by the interface itself, so it needs no external power.
A PIC18F2320 microcontroller runs the tester. The MCU has an 8-MHz internal oscillator with a 1% tolerance over its operating range. This allows the UART baud rate to be within error ranges without the need for an external crystal. The device also offers 10 10-bit analog-to-digital channels, eight of which are used in this application. Eight 2.2- Ω, 1-W resistors are used to discharge the batteries under test at approximately 500 mA, a rate that takes about two to four hours to fully discharge a battery (Fig. 1). The tester is powered through the RTS pin of the serial port, with a 5.1-V zener diode ensuring a constant reference voltage level. RTS and CTS are shorted, as are DTR and DSR.
The code that runs the tester was compiled using the Microchip MCC18 C compiler. The code can be found here.
When a battery is inserted into any of the tester's channels, a timer is initialized for that channel. As long as any channel's input voltage is above 0.9 V, the timers for all channels will be updated in the Hyperterminal display every three seconds. When the channel's input voltage drops below the 0.9-V threshold, the timer for that channel will stop incrementing. Thus, once all of the batteries fall below the threshold, the timer values will stop displaying. If every channel has a voltage below 0.05 V (that is, the tester has no batteries), the timer for each one will be reset. The format for the timer display is: Hours: Minutes: Seconds.
The tester outputs data using a serial interface set at 9600 baud, 8 bits, 1 stop bit, and no parity bit. There's no flow control. Windows Hyperterminal should be set up for this configuration. Also, the option, "append line feeds on incoming line ends," must be enabled under Properties => Settings => ASCII Setup. Figure 2 is a screenshot of sample test results in Hyperterminal. It took about one to three hours to discharge eight batteries down to 0.9 V. Not all of the batteries were fully charged.
