7. The ‘Smart’ Battery
Aspeaker at a battery seminar remarked that, “The battery
is a wild animal and artificial intelligence domesticates
it.” An ordinary or ‘dumb’ battery has the inherit problem
of not being able to display the amount of reserve energy
it holds. Neither weight, color, nor size provides any indication
of the battery’s state-of-charge (SoC) and state-of-health
(SoH). The user is at the mercy of the battery when pulling
a freshly charged battery from the charger.
Help is at hand. An increasing number of today’s rechargeable
batteries are made ‘smart’. Equipped with a microchip, these
batteries are able to communicate with the charger and user
alike to provide statistical information. Typical applications
for ‘smart’ batteries are notebook computers and video cameras.
Increasingly, these batteries are also used in advanced biomedical
devices and defense applications.
There are several types of ‘smart’ batteries,
each offering different complexities, performance and cost.
The most basic ‘smart’ battery may only contain a chip to
identify its chemistry and tell the charger which charge algorithm
to apply. Other batteries claim to be smart simply because
they provide protection from overcharging, under-discharging
and short-circuiting. In the eyes of the Smart Battery System
(SBS) forum, these batteries cannot be called ‘smart’.
What then makes a battery ‘smart’? Definitions
still vary among organizations and manufacturers. The SBS
forum states that a ‘smart’ battery must be able to provide
SoC indications. Benchmarq was the first company to commercialize
the concept of the battery fuel gauge technology. Early IC
chips date back to 1990. Several manufacturers followed suit
and produced ‘smart’ chips for batteries.
During the early nineties, numerous ‘smart’ battery
architectures with a SoC read-out have emerged. They range
from the single wire system, the two-wire system and the system
management bus (SMBus). Most two-wire systems are based on
the SMBus protocol. This book will address the single wire
system and the SMBus.
7.1 The Single Wire Bus
The single wire system is the simpler of the
two and does all the data communications through one wire.
A battery equipped with the single wire system uses only three
wires, the positive and negative battery terminals and the
data terminal. For safety reasons, most battery manufacturers
run a separate wire for temperature sensing. Figure 7-1
shows the layout of a single wire system.
The modern single wire system stores battery-specific
data and tracks battery parameters, including temperature,
voltage, current and remaining charge. Because of simplicity
and relatively low hardware cost, the single wire enjoys a
broad market acceptance for high-end mobile phones, two-way
radios and camcorders.
Most single wire systems do not have a common
form factor; neither do they lend themselves to standardized
SoH measurements. This produces problems for a universal charger
concept. The Benchmarq single wire solution, for example,
cannot measure current directly; it must be extracted from
a change in capacity over time.
In addition, the single wire bus allows battery
SoH measurement only when the host is ‘married’ to a designated
battery pack. Such a fixed host-battery relationship is feasible
with notebook computers, mobile phones or video cameras, provided
the appropriate OEM battery is used. Any discrepancy in the
battery type from the original will make the system unreliable
or will provide false readings.
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Figure 7-1:
Single wire system of a ‘smart’ battery.
Only one wire is needed for
data communications. Rather than supplying the clock
signal from the outside, the battery includes an embedded
clock generator. For safety reasons, most battery manufacturers
run a separate wire for temperature sensing.
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Batteries in a Portable World 2nd Ed.
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