Smart cells gain power

Smart cells gain powerCreated some years ago the Smart Battery Standard took off slowly, but all that is changing. Steve Bush finds out what is behind it all
Out on assignment, the battery power indicator of the office laptop is the bane of my life.
In all the times I have used the machine, I have never fathomed the relationship between the indicator and the actual battery charge state. ‘Full’ sometimes means full, but Ihave it die two minutes later, ‘empty’ can mean anything and I once got a full two hours of use from a nominally flat battery.  
 
A few years ago, with this kind of random ‘gas-gauging’ in mind, Duracell, Intel and a few others got together and drew-up the Smart Battery Standard, or SBS.
Behind the SBS are two assumptions: that battery packs have some in-built intelligence, and that the packs are the best source of information about their own status.
By putting intelligence in a battery, information on charge state lives with the battery. Swapping packs never confuses the equipment being powered and a much-cycled pack is able to indicate its remaining capabilities.
SBS defines a two-wire physical interface (remarkably similar to Philips’ I2C bus) and a comprehensive set of standard information-passing messages. These include data on the charge state, chemistry, number of charge-recharge cycles and its age.
While the standard was welcomed, adoption has been somewhat slower than was hoped.
This is partially due to Duracell’s diminished interest. Its idea was to include standard physical dimensions for packs in the SBS, then manufacture packs to those dimensions. The company’s lethargy set in when it became clear that there was little industry interest in standard pack sizes and Duracell shed its smart battery interest a year ago into a company called PowerSmart which now designs SBS-compliant silicon and offers SBS consultancy. Who is interested in SBS?
Analog Devices, Benchmarq, Intel, Linear Technology, Maxim, Micrel, Microchip Mitsubishi, Motorola, National Semiconductor, Philips and Xicor are among the companies in the SBS forum. Most of these companies have SBS capable products or are planning to introduce them.
Linear Technology’s (LTC’s) SBS offerings include a chip to overcome one of the limitations of the SMBus standard. “SMBus has a relatively weak pull-up on the line driver,” said LTC spokesman Ian Scott, ” this means that rise-times are slow if there is a lot of capacitance on the lines.”
LTC’s answer to high capacitance or multiple loads is the LTC1694. This is a dual active pull-up in a five-pin SO-23 package. “It senses the line voltage starting to rise and pulls it up,”said Scott.
Over in the Maxim camp, the MAX1660 is a universal, digitally controlled, battery fuel gauge, monitor and protector, with an SMBus interface, a claimed accuracy of one percent between 60mA and 4A and 4 to 28V operation.
The Smart Battery System Implementers Forum has a conference in Tokyo in March this year  
 
Despite not causing a huge initial impact, SBS has steadily increased it penetration. “Taiwan shipped six million notebooks last year and 50 per cent were SBS compliant. In 1999 it look like it will be 60 or 70 per cent of 7.5 million,”said Lonnie Conklin, PowerSmart’s sales and marketing v-p.
Microsoft is now strongly supporting the SBS and has included it in its own ACPI (advanced configuration and power interface). Although the use of SBS communication between the pack and computer is not compulsory in the ACPI standard, any proprietary alternative will have to mimic SBS messages to communicated with the ACPI.
The physical interface and message structure of SBS are together defined in the System Management Bus (SMBus) standard. This comes from Intel’s early involvement and was conceived as a general purpose in-PC bus as well as a basis for battery pack communications.
“The SBS standard has done very well and we are beginning to see some decent adaptations and products,” said David Heacock, director of portable products at power chip maker Benchmarq. He sees the potential for SMBus products that will perform general thermal and power management tasks. These might include an SMBus thermometer, controlling and monitoring fans, and setting processor speed.
At system level, a number of point-to-point SMBus ‘branches’ will spread out from the SMBus control blocks within the keyboard controller part of a PC chipset to communicate with the SMBus peripherals.


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