The 'smart' battery

The battery has the inherit problem of not being able to communicate with the user. Neither weight, color, nor size provides an indication of the battery's state-of-charge (SoC) and state-of-health (SoH). The user is at the mercy of the battery.

Help is at hand in breaking the code of silence. 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. Typical applications for 'smart' batteries are notebook computers and video cameras. Increasingly, these batteries are also used in biomedical devices and defense applications.

There are several types of 'smart' batteries, each offering different complexities and costs. The most basic 'smart' battery may contain nothing more than a chip that sets the charger to the correct charge algorithm. 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. In 1990, Benchmarq was the first company to commercialize the concept by offering fuel gauge technology. Today, several manufacturers produce such chips. They range from the single wire system, to the two-wire system to the System Management Bus (SMBus). Let's first look at the single wire system.

The Single Wire Bus
The single wire system delivers the data communications through one wire. This battery uses three wires: the common positive and negative battery terminals and one single data terminal, which also provides the clock information. For safety reasons, most battery manufacturers run a separate wire for temperature sensing. Figure 1 shows the layout of a single wire system.

Figure 1: Single wire system of a 'smart' battery.Only one wire is needed for data communications. For safety reasons, most battery manufacturers run a separate wire for temperature sensing.

The single wire system stores the battery code and tracks battery readings, including temperature, voltage, current and SoC. Because of relatively low hardware cost, the single wire system enjoys market acceptance for high-end two-way radios, camcorders and portable computing devices.

Most single wire systems do not provide 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 the 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 only feasible if the original battery is used. Any discrepancy in the battery will make the system unreliable or will provide false readings.

The SMBus

The SMBus is the most complete of all systems. It represents a large effort from the electronics industry to standardize on one communications protocol and one set of data. The Duracell/Intel SBS, which is in use today, was standardized in 1993. It is a two-wire interface system consisting of separate lines for the data and clock. Figure 2 shows the layout of the two-wire SMBus system.

An addition problem with the SMBus battery is non-compliance. Unlike other tightly regulated standards, the SMBus protocol allows some variations. This may cause problems with existing chargers and the SMBus battery should be checked for compatibility before use. The need to test and approve the marriage between a specific battery and charger is unfortunate, given the assurance that the SMBus battery is intended to be universal. Ironically, the more features offered on the SMBus charger and the battery, the higher the likelihood of incompatibilities.

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