Simple Characterization Circuit for Rechargeable AA Cells

Posted on Feb 4, 2014

This characterization circuit, plus a PC and some software, accurately measures the full discharge cycle for a rechargeable AA cell. Cell capacity and output resistance can easily be determined from the curve resulting from these measurements. Rechargeable AA cells are very popular for a large variety of portable applications. The number

Simple Characterization Circuit for Rechargeable AA Cells

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of brands is also high and the cell quality can differ considerably. Cell capacity is sometimes different from that specified, and the capacity often deteriorates dramatically after a limited number of discharge cycles. The proposed characterization circuit, in combination with a PC and AA-Cell Characterizer software, can accurately measure a full discharge cycle. From the resulting curve you can easily determine the cell capacity and output resistance. The circuit is built around an IC intended for fuel-gauging applications in battery packs ( DS2756 ). The DS2756 provides the key components necessary to accurately estimate remaining battery-pack capacity by integrating low-power precision measurements of temperature, voltage, current, and current accumulation (integral of current). This hardware can also record the discharge cycle of a single AA cell (either NiCd or NiMH ) with accuracy. In Figure 1, the simple circuit is connected to the USB port of a PC. No external power source is required because power is extracted from the USB bus voltage. U1 ( DS9490R ) is a USB dongle that converts the USB protocol to the 1-Wire ® protocol, thereby allowing the PC to communicate with the key circuit component (U2). U2 measures the cell voltage on pin 1 (VIN) through R1, the cell current through R4, a 25m © sense resistor connected to pins 7 (SNS) and 2 (VSS). The programmable I/O at pin 3 (PIO) allows PC software to control the gate...

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