Can laptop batteries be repaired?

Isidor Buchmann
Cadex Electronics Inc.
isidor.buchmann@cadex.com
www.buchmann.ca
December 2001

Laptop batteries differ from other types of batteries in that they provide a relatively short service life and are expensive to replace. In this article we look at the reasons why these batteries do not last and examine the high replacement cost. We also look into the possibilities of repairing them.
Most of today's laptop computers are powered with Lithium-ion batteries.

Under good conditions, Lithium?ion provides 300 to 500 discharge/charge cycles or 2 to 3 years of service from the time the battery leaves the production line. The capacity loss occurs through increased internal resistance caused by cell oxidation. Eventually the resistance reaches a point where the battery can no longer deliver the needed energy although the energy may still be present in the battery. There are no remedies to restore the capacity when worn out. Heating the battery will momentarily improve the performance.

Figure 1 illustrates the recoverable capacity at various storage temperatures and charge levels over one year. Nickel-based chemistries, a chemistry that is also used in laptops, is illustrated on the right column. The capacity loss progresses on a quasi linear scale for the second and third year.

Figure 1: Non-recoverable capacity loss on Lithium-ion and nickel-based batteries after one year of storage. High charge levels and elevated temperatures hasten the capacity loss. The capacity loss past one year progresses on a fairly linear scale.

During use, the battery compartment in many laptops rises to about 45°C (113°F). The combination of high charge level and elevated ambient temperature presents an unfavorable condition for the battery. This explains the rather short lifespan of laptop batteries.

Most laptops batteries are 'smart', meaning that some form of communications occurs between the battery and user. The definition of 'smart' varies among manufacturers and regulatory authorities. Some manufacturers call their batteries 'smart' by simply adding a chip that sets the charger to the correct charge algorithm. The Smart Battery System (SBS) forum states that a 'smart' battery must provide state-of-charge (SoC) indications.

There are two common architectures of 'smart' batteries. They consist of the single wire system found on high-end radio communications devices and video camera equipment, and the two-wire system, typically used on laptops. The two-wire system is usually configured as System Management Bus (SMBus). Because of its common use in laptops, we will focus on the SMBus system. Figure 2 shows the layout.

Figure 2: Two-wire SMBus system. The SMBus is based on a two-wire system using a standardized communications protocol. This system lends itself to standardized state-of-charge and state-of-health measurements.

The SMBus battery has five or more battery connections consisting of the positive and negative battery terminals, thermistor, clock and data. The connections are commonly not marked and attempting to test this type of battery appears complicated. Figure 3 describes the functions of a battery with 6 connections.

Figure 3: Connections of a typical laptop battery. The positive and negative terminals are usually placed on the outside; no norm exists on the arrangements of the contacts.

The positive and negative battery terminals are commonly located at the outer edges of the connector. The inner contacts accommodate the clock and data. (On a one-wire system, clock and date is combined.) For safety reasons, a separate thermistor wire is brought to the outside. This allows temperature protection if the digital communication is disabled.

Some batteries are equipped with a solid-state switch that is normally in the off position. In such as case, no voltage is present. Connecting the switch control terminal to ground will turn the battery on. If this does not work, a proprietary code may be needed to activate the battery.

How can I find the correct terminals? To begin with, use a voltmeter to find the positive and negative battery terminals. Establish the polarity. If no voltage is available, a solid-state switch may need to be activated. With the voltmeter connected on the outer terminals, take a 100-Ohm resistor (other values may also work). Connect one end of the resistor to ground, and with the other end touch each terminal while observing the voltmeter. If no voltage appears, the battery may be dead or the pack requires a digital code to activate. The resistor protects the battery against a possible electrical short.

Once the connection to the battery terminals is established, charging should be possible. If the charge current stops after 30 seconds, a digital code may be required. Some battery manufacturers go as far as to add a defined end-of-life switch. If a preset age, cycle count or capacity is surpassed, the battery stops functioning. When asking why such codes are added, the manufacturers explain that enduring safety can only be guaranteed if the battery is tamper-free and well performing. This makes common sense but the leading motive may be pricing. In the absence of competition, replacement batteries can be sold at a premium price. Newer batteries are generally more service friendly than older ones.

It is recommended to utilize the thermistor during charge and discharge to protect the battery against over heating. The thermistor can be measured with the Ohmmeter. The most common thermistors are the 10 Kilo Ohm NTC type, which read 10kOhm at 20°C (68°F). NTC stands for negative temperature coefficient, meaning that the resister decreases with rising temperature. A positive temperature coefficient (PCT) will increase the resistance. Warming the battery with your hand may be sufficient to detect a change in resistor value.

An SMBus battery contains permanent and temporary data. The permanent data is programmed into the battery at 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.

How to repair a 'smart' battery

Some basic rules must be followed in repairing a 'smart' battery. If the cells are weak, cell replacement makes economic sense. While Nickel-based cells are readily available, Lithium-ion cells are not sold on the open market. Most manufactures offer them only in a completed battery pack, together with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the cells are assembled in a careless way. Always replace the pack with the same chemistry cells.

During cell replacement, the circuit of many 'smart' batteries must be kept alive with a supply voltage. Disconnecting the circuit, if only for a fraction of a second, can erase vital data and render the circuit unusable. To assure continued operation when changing the cells, connect a secondary voltage through a 100 Ohm resistor before disconnecting the cells. Remove the secondary supply only after the circuit is fed with the needed operating voltage from the new cells.

The open terminal voltages of the replacement cells should be within 10% of each other. Welding the cells is the only reliable way to get dependable service. Attention must be paid in limiting the heat transferred to the cells during welding. Excess heat can damage the cells.

During storage, each cell has self-discharged to a different charge level. This is especially important on Nickel-metal-hydride. To assure proper charge of all cells without overcharging some, trickle charge the newly repaired pack for about 14 hours, then apply a charge, discharge and charge cycle. Such a cycle is also needed to reset the battery's fuel gauge circuit. Lithium-ion can accept a normal charge lasting about 3 hours. The Cadex C7000 Series battery analyzers feature a program that performs this priming function automatically.

How to calibrate the 'smart' battery

With use and time, a tracking error occurs between the chemical battery and the digital sensing circuit. This results in a loss of accuracy of the SoC readout.

The most ideal use of the 'smart' battery, as far as fuel-gauge accuracy is concerned, is 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 longer synchronizes with the fuel gauge and a full charge and discharge is needed to 're-learn' or calibrate the battery.

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.


If no full discharge occurs for a few months as part of normal operation, a deliberate full discharge is needed. This can be done on the equipment itself, on a charger with discharge function or with a battery analyzer. Cadex manufactures SMBus chargers and battery analyzers, both of which can be used to test and calibrate the battery. The Cadex SM2+ (Figure 4) is a level-3 SMBus charger featuring a target capacity selector that is adjustable to 60%, 70% or 80%. The target capacity selector checks performance and flags batteries that do not meet the set requirements. The charger works like this:

If a battery falls below target, the charger 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 illuminates if the capacity is met at full charge. If the battery does not recover, a fail light recommends replacement. The SM2+ charger accommodates batteries with the 5-prong knife connector by AMP. The charger services both SMBus and non-SMBus batteries. "Dumb' batteries do not provide state-of-health indications.

Figure 4: The Cadex SM2+ charger This level-3 charger serves as charger, conditioner and quality control system. The charger reads the battery's state-of-health and flags those that fall below the set target capacity. Each bay operates independently and charges Nickel-cadmium, Nickel-metal-hydride and Lithium?ion chemistries in approximately three hours. 'Dumb' batteries can also be charged but no SoH information is available.

For full battery service, a battery analyzer is recommended. The Cadex C7400 is a programmable battery analyzer capable of rapid testing, charging, priming and reconditioning a large variety of batteries. The battery packs connects by custom SnapLock battery adapters, programmable cables or the Cadex FlexArm™ adapter. The analyzer does not check the SMBus.

Figure 5: Cadex 7400 battery analyzer The programmable Cadex 7400 services lithium, nickel and lead-based batteries. SnapLock battery adapters simplify the interface with different battery types. A quick test program measures battery state-of-health in 3 minutes, independent of charge. Nickel-based batteries are automatically restored if the capacity falls below the user-defined target capacity.

The QuickTest™ program measures the battery state-of-health in three minutes by gathering data from six variables and combining them to derive the test results. Boost restores seemingly dead Lithium-ion batteries by re-activating the protection circuit that has been disabled through low discharge. Prime prepares and calibrates a new battery by repeatedly applying charge/discharge cycles until the peak capacity is reached. Auto reconditions nickel-based batteries if the user-set target capacity cannot be reached. Custom allows the setting of unique cycle sequences composed of charge, discharge, recondition, trickle charge or any combination, including rest periods and repeats. OhmTest™ measures internal battery resistance.

Summary
Laptop batteries can be repaired but such work only makes economical sense for smaller operators. The success rate varies with battery type. One must remember that the 'smart' battery consists of two parts, the chemical cells and the digital circuit. In some cases, the chemical part can be fully restored but the fuel gauge may become inaccurate or other data may get corrupted.
Anyone attempting to repair SMBus battery must be aware of some non-compliance in the SBS forum. 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 fact that the SMBus battery is being promoted as being universal. Ironically, by adding more features to the SMBus charger and the battery, the higher are the likelihood of incompatibilities. Tighter regulations are desirable. More information on SMBus is available on www.sbs-forum.org and www.acpi.info.

About the Author
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 behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com