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NiMH Battery Monitor Touts Accuracy
Algorithms are key to wringing out longer useful life
By Design News Staff -- Design News, May 5, 2003
The latest application of Microchip Technology's PowerSmart® technology, the PS402, integrates all rechargeable battery monitoring functions for nickel metal hydride (NiMH) cells onto a single chip. Included on the chip are a precision time base (eliminating need for an external crystal clock), a regulator, and temperature sensor. The battery being monitored powers the PS402, thus an external regulator is not needed. If a user chooses, an external thermistor can be used to monitor battery temperature.
Smaller size and reduced parts count are always cost-effective developments. But the company says the key advantage of the PS402 is the ability to determine, within ±1% accuracy, the remaining battery capacity. This allows maximum safe lifetime of the battery to be achieved before recharge is needed.
Engineer Doug Phillips notes the need for maximum device life. "The power consumption in portable electronics has grown. Meanwhile, the increase in battery capacity has lagged," he says. So it is a case of trying to squeeze the maximum energy out of the cell technology available.
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Capacity Key: The PS402 Smart Battery System is designed to monitor NiMH battery capacity for stacks of six to twelve cells. For removable stacks, the device is onboard the battery pack and for embedded applications it can be part of the systems board. |
To do this, Microchip uses its Accuron™ technology consisting of battery models, customized tables, and patented algorithms that are stored in the on-chip programmable memory. The remaining capacity of the battery is computed. The capacity is a dynamic number based on temperature, usage rate, and the specific chemistry of the battery. Phillips says conventional battery fuel gauges usually use only one parameter to try and assess remaining lifetime.
Phillips cites one algorithm in particular used to predict shutdown capacity, which is having enough energy left to close the device while safely protecting any information that must be saved. "It tells the device to shut down at a certain capacity, not a voltage or temperature. It takes into account the discharge curve differences between a fully charged and nearly empty battery," he notes.
Finally, Phillips says battery management is not just silicon and firmware. Implementation can be problematic so the company has a set of development tools for configuring, calibrating, and testing. "These wizard-driven implementations install like Windows with features like auto-configuration after a user inputs design parameters." Microchip Technology Inc.www.microchip.com Enter 688
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