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Smart Battery Data Specification
SBS Implementers Forum -Page 5- Revision 1.1
4.2. Smart Battery Software Definition
The software interface is separated into three parts: SMBus Host-to-battery, charger-to-battery and battery-
to-charger or SMBus Host. Additionally, a discussion about error signaling and handling is included.
4.2.1. SMBus Host to Smart Battery
The SMBus Host to Smart Battery communication is used to get data that is either presented to a user or to
the SMBus Host's power management system. The user can get two types of data from the battery: factual
data and predictive data. Factual data can be measured, such as temperature, pack voltage or
charge/discharge current, or it can be a battery characteristic, such as the battery's chemistry. Predictive
data is calculated, based on the battery's present state and the battery's characteristics, such as the battery's
remaining life at the present rate of drain. Predictive data can also be calculated based on specified
parameters to predict battery performance at charge or discharge currents presently not measured by the
battery. (See the AtRate functions.) Additionally, since the battery has a clock, information can be
presented as a rolling average over a fixed interval.
The power management system may query a device driver to determine if an action will cause harm to the
system's integrity. For example, spinning up a disk drive at the end of the battery's charge might cause its
output voltage to drop below acceptable limits thus causing a system failure. In order to prevent this, the
device driver needs information from the battery that will cause it to do the right thing. If the driver queries
the battery and discovers that not enough power is available, it can request that the power management
system turn off a non-critical power use such as the LCD screen back-light and then try again.
SMBus Host to Smart Battery communications are performed:
· To allow the user to know the Smart Battery's remaining life
· To tell the user how long it will take to charge the Smart Battery
· To allow Smart Batteries to provide accurate information to their user
· To determine the SMBus Host's real-time power requirements
· To enable power management based on "real" information supplied by the battery
· To enable battery manufacturers to collect information about a Smart Battery's usage
· To allow battery manufacturers to electronically "stamp" batteries at time of manufacture
.
4.2.2. Smart Battery Charger to Smart Battery or Smart Battery to Smart Battery Charger
An internal or external battery charger must understand the characteristics of the battery it is charging.
Today's laptops, using either Ni- or Li-based batteries, must apply an appropriate current and voltage to the
battery for charging. End-of-charge is determined by various methods depending on the specific chemistry
and environmental conditions. The difficulty is determining the charge voltage and current for a particular
battery and chemistry. Even though the battery pack voltage and perhaps chemistry may be the same, the
charging characteristics may not. (For example, not all LiION chemistries require the same charging
voltages, even though their pack voltages are the same.)
A better method is to have the battery tell the charger when charging is complete and how to adjust the
charging voltage and current so they best match the battery's present state. Chargers that cooperate with the
battery have two distinct advantages: First, they provide the battery with all the power it can handle (that is,
maximum safe charge) without overcharging, and second, they will recognize and correctly charge batteries
with different chemistries and voltages, or same chemistry with different voltages.
To improve reliability and safety between the Smart Battery and the Smart Battery Charger, the ‘Safety
Signal’ (a secondary signaling mechanism) must also be supported by both devices. This mechanism
allows an independent communication path to be used to enhance the safety of the charging circuit. (See
section 4.4.4. ‘Safety Signal Hardware Requirements’ and the Smart Battery Charger Specification for
additional information.)