A speaker 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 displaying the amount of reserve energy it holds. Neither weight, color, nor size provide 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 (Sony Vaio VGN-FZ battery).

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 (Sony VGP-BPS8 battery).

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’ (Sony VGP-BPL9 battery).

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. In 1990, Benchmarq was the first company to commercialize the concept of the battery fuel gauge technology. Today, several manufacturers produce chips to make the battery ‘smart’ (Sony VGP-BPS9 battery).

During the early nineties, numerous ‘smart’ battery architectures 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. Let’s look at the single wire system and the SMBus (Sony VGP-BPL11 battery).

The single wire system is the simpler of the two and delivers 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 1 shows the layout of a single wire system (Sony VGP-BPL15 battery).

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 (Sony VGN-FZ460E battery).

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 (Sony VGP-BPS11 battery).

On a further drawback, 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 (Sony Vaio VGN-FZ21M battery ).

The SMBus

The SMBus is the most complete of all systems. It represents a large effort from the portable electronic industry to standardize to one communications protocol and one set of data. The SMBus is a two-wire interface system; one wire handles the data; the second is the clock. It uses the I²C defined by Philips as its backbone (Dell INSPIRON 1420 Battery).

The Duracell/Intel SBS, which is in use today, was standardized in 1993. In previous years, computer manufacturers developed their own proprietary ‘smart’ batteries. With the new SBS specification, a broader interface standard is made possible. This reduces the hurdles of interfering with patents and intellectual properties. Figure 2 shows the layout of the two-wire SMBus system (Dell Inspiron 1501 battery).

In spite of the agreed standard, many large computer manufacturers, such as IBM, Compaq and Toshiba, have retained their proprietary batteries. The reason for going their own way is partly due to safety, performance and form factor. Manufacturers claim that they cannot guarantee safe and enduring performance if a non-brand battery is used. To make the equipment as compact as possible, the manufacturers explain that the common form factor battery does not optimally fit their available space. Perhaps the leading motive for using their proprietary batteries is pricing. In the absence of competition, these batteries can be sold for a premium price (ASUS A3000 Battery).

The objective behind the SMBus battery is to remove the charge control from the charger and assign it to the battery. With a true SMBus system, the battery becomes the master and the charger serves as a slave that must follow the dictates of the battery. This is based on concerns over charger quality, compatibility with new and old battery chemistries, administration of the correct amount of charge currents and accurate full-charge detection. Controlled charging makes sense when considering that some battery packs share the same footprint but contain radically different chemistries (ASUS Eee PC 900 Battery).

The SMBus system allows new battery chemistries to be introduced without the charger becoming obsolete. Because the battery controls the charger, the battery manages the voltage and current levels, as well as cut-off thresholds. The user does not need to know which battery chemistry is being used (ASUS Eee PC 1000HE Battery).



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