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.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 or lithium 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.
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.
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.
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 slave that must follow the dictates of the battery.Battery-controlled charging makes sense when considering that some packs share the same footprint but contain different chemistries,requiring alternative charge algorithms.With the SMBus,each primary battery receives the correct charge levels and terminates full-charge with proper detection methods.Future battery chemistries will be able to use the existing chargers.
An SMBus battery contains permanent and temporary data.The permanent data is programmed into the battery at the time of manufacturing and includes battery ID number,battery type,serial number,manufacturer’s name and date of manufacture.The temporary data is acquired during use and consists of cycle count,user pattern and maintenance requirements.Some of this information is renewed during the life of the battery.Some lower cost chargers have emerged that accommodate SMBus batteries but are not fully SBS compliant.Manufacturers of SMBus batteries do not fully endorse this shortcut.Safety is always a concern,but customers buy them because of low cost.
Among the most popular SMBus batteries are the 35 and 202 form-factors (Figure 3).Manufactured by Sony,Hitachi,GP Batteries,Moli Energy and others,these batteries work (should work) in all portable equipment designed for this system.Although the 35 has a smaller footprint than the 202,most chargers accommodate both sizes.A non-SMBus (‘dumb’) version with same footprint is also available.These batteries can only be charged with a regular charger,or one that accepts both types.In spite of the agreed standard and given form factors,many computer manufacturers have retained their proprietary batteries.Safety,performance and form factor are the reasons.In the absence of competition,these batteries can be sold for a premium price.