Is the 'smart' battery a help or deterrent?

Cadex Electronics Inc
By Isidor Buchmann*, President, Cadex Electronics Inc
Sunday, 05 January, 2003

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

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.

Typical applications for 'smart' batteries are notebook computers and video cameras. Increasingly, these batteries are also used in biomedical devices and defence 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 forum, these batteries cannot be called 'smart'.

What makes a battery 'smart'? Definitions still vary among organisations and manufacturers. The SBS forum states that a 'smart' battery must be able to provide state-of-charge indications. In 1990, Benchmarq was the first company to commercialise the concept by offering a fuel gauge.

Today, several manufacturers produce such chips. They range from the single wire system, to the two-wire system to the system management bus. Let's first look at the single wire system.

The single wire bus

The single wire system delivers the data communications through one wire. A battery equipped with the single wire system uses these wires: the positive and negative battery terminals and the data terminal, which also provides the clock function. For safety reasons, most battery manufacturers run a separate wire for temperature sensing.

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.

The single wire system stores the battery code and tracks battery readings, including temperature, voltage, current and state-of-charge. 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 standardised state-of-health measurements. This produces problems for a universal charger concept. The Benchmarq single wire, 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 state-of-health 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 standardise on one communications protocol and one set of data. The Duracell/Intel SBS, which is in use today, was standardised in 1993.

It is a two-wire interface system consisting of a separate line for the data and clock.

The SMBus is based on a two-wire system using a standardised communications protocol. This system lends itself to standardised state-of-charge and state-of-health measurements.

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 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.

The SMBus is divided into Level 1, 2 and 3. Level 1 has been eliminated because it does not provide chemistry independent charging. Level 2 is designed for in-circuit charging. A laptop that charges its battery within the unit is a typical example of Level 2.

Another Level 2 application is a battery that contains the charging circuit within the pack. Level 3 is reserved for full-featured external chargers.

External Level 3 chargers are complex and expensive. 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 short cut.

Safety is always a concern, but users buy them because of lower prices. Serious industrial battery users operating biomedical instruments, data collection devices and survey equipment use Level 3 chargers with a fully-fledged charge protocol.

Among the most popular SMBus batteries are the 35 and 202 form factors. 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.

Available in Ni-Cad, NiMH and Li-ion chemistries, these batteries are used for laptops, biomedical instruments and survey equipment. A non-SMBus ('dumb') version with same footprint is also available.

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. They argue that enduring performance can only be guaranteed if their own brand battery is used.

This makes common sense but the leading motive may be pricing. In the absence of competition, these batteries can be sold at a premium.

Negatives of the 'smart' battery

The 'smart' battery has some notable downsides, one of which is price. An SMBus battery costs about 25% more than the 'dumb' equivalent. In addition, the 'smart' battery was intended to simplify the charger but a fully-fledged Level 3 charger costs substantially more than a regular model.

A more serious drawback is the requirements for periodic calibration or capacity re-learning. The engineering manager of Moli Energy, a manufacturer of lithium-ion cell, said:

"With Lithium-ion we have eliminated the memory effect; but is the SMBus battery introducing digital memory?"

Why is calibration needed? The calibration corrects the tracking errors that occur between the battery and the digital sensing circuit while charging and discharging. The most ideal battery application, as far as fuel-gauge accuracy is concerned, would be a full charge followed by a full discharge at a constant current.

In such a case, the tracking error would be less than 1% per cycle. In real life, however, a battery may be discharged for only a few minutes and the load may vary widely. Long storage also contributes to errors because the circuit cannot accurately compensate for self-discharge.

Eventually, the true capacity of the battery no long synchronises with the fuel gauge and a full charge and discharge is needed to 're-learn' the battery.

How often is calibration needed? The answer lies in the battery application. For practical purposes, a calibration is recommended once every three months or after every 40 short cycles. Many batteries undergo periodic full discharges as part of regular use.

If the portable device allows a deep enough discharge to reset the battery and this is done regularly, no additional calibration is needed.

However, if no discharge reset has occurred for a few months, a deliberate full discharge is needed. This can be done on a charger with discharge function or a battery analyser.

What happens if the battery is not calibrated regularly? Can such a battery be used in confidence? Most 'smart' battery chargers obey the dictates of the chemical cells rather than the electronic circuit.

In this case, the battery will fully charge regardless of the fuel gauge setting and function normally but the digital readout will become inaccurate. If not corrected, the fuel gauge simply becomes a nuisance.

An additional 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 charge 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.

The state-of-charge indicator

Most SMBus batteries are equipped with a charge level indicator. When pressing the 'Test' button on a fully charged battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery, all lights remain dark.

Although the state-of-charge is displayed, the state-of-health and its predicted runtime are unknown.

While state-of-charge information displayed on a battery or computer screen is helpful, the fuel gauge resets to 100% each time the battery is recharged, regardless of the battery's state-of-health.

A serious miscount occurs if an aged battery shows 100% after a full-charge, when in fact the charge acceptance has dropped to say 50% or less. The question remains: "100% of what?" A user unfamiliar with this battery has little information about the runtime of the pack.

The reserve capacity can only be established knowing the state-of-health.

Three imaginary sections of a battery consisting of available energy, empty zone and rock content. With usage and age, the rock content grows.

How can the three levels of a battery be measured and made visible to the user? While the state-of-charge is relatively simple to produce, measuring the state-of-health is more complex. Here is how it works:

At the time of manufacture, each SMBus battery is given its specified state-of-health status, which is 100% by default. This information is permanently programmed into the pack and does not change. With each charge, the battery resets to the full-charge status. During discharge, the energy units (coulombs) are counted and compared against the 100% setting.

A perfect battery would indicate 100% on a calibrated fuel gauge. As the battery ages and the charge acceptance drops, the state-of-health decreases.

The discrepancy between the factory-set 100% and the delivered coulombs on a fully discharged battery indicates the state-of-health.

Knowing the state-of-charge and state-of-health, a simple linear display can be made. The state-of-charge is indicated with green LEDs; the empty part remains dark; and the unusable part is shown with red LEDs. As an alternative, a numeric display indicating state-of-health and state-of-charge can be used. The practical location for the tri-state-fuel gauge is on the charger.

The Battery Health Gauge reads the 'learned' battery information available on the SMBus and displays it on a multi coloured LED bar. The illustration shows a partially discharged battery of 50% SoC with a 20% empty portion and an unusable portion of 30%.

The target capacity selector

For users that simply need a go/no go answer, chargers are available that feature a target capacity selector. Adjustable to 60, 70 or 80%, the target capacity, the selector acts as a performance check and flags batteries that do not meet the set requirements.

If a battery falls below target, the charge triggers the condition light. The user is prompted to press the condition button to calibrate and condition the battery by applying a charge/discharge/charge cycle.

The green 'ready' light at the end of the service reveals full charge and assures that the battery meets the required performance level. If the battery does not recover, a fail light indicates that the battery should be replaced.

This Level 3 charger serves as charger, conditioner and quality control system. It reads the battery's true state-of-health and flags those that fall below the set target capacity. Each bay operates independently and charges Ni-Cad, NiMH and Li-ion chemistries in approximately three hours. 'Dumb' batteries can also be charged but no SoH information is available.

By allowing the user to set the desired battery performance level, the question is raised as to what level to select. The answer is governed by the application, reliability and cost.

The nominal target capacity setting is 80%. Decreasing the threshold to 70% will lower the performance standard but pass more batteries. A direct cost saving will result. The 60% level may suit those users who run a low budget operation, have ready access to replacement batteries and can live with shorter, less predictable runtimes.

It should be noted that the batteries are always charged to 100%, regardless of the target setting. The target capacity simply reveals the energy, which a fully charged battery can deliver.

'Smart' batteries are predominantly used for high-end industrial applications. In spite of improvements made over the last 10 years, the 'smart' battery, the SMBus in particular, has not received the anticipated acceptance. Some engineers go so far as to suggest that the SMBus battery is a 'misguided principle'.

In the early 1990s, when the SMBus battery was conceived, price was not as critical as perhaps today. Now, buyers want economical products that are scaled down and perform the functions intended. But regardless of cost, the 'smart' battery will continue to serve a critical market. There are simply no alternatives for users to whom unexpected downtime is no option.

* Isidor Buchmann is the founder and CEO of Cadex Electronics Inc, in Vancouver BC. Mr Buchmann has a background in radio communications and has studied the behaviour of rechargeable batteries in practical, everyday applications for two decades.

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