How to service laptop batteries (BU47)

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, consisting of the single wire system found on high-end cameras and radio communications devices, and the two-wire system typically used on laptops. The two-wire system is usually configured to the System Management Bus (SMBus). Because of its common use in laptops, we will focus on the SMBus system. Figure 1 shows the layout.

Figure 1: 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.

Battery connection

The SMBus battery has five or more battery connections consisting of positive and negative battery terminals, thermistor, clock and data. The connections are commonly unmarked and attempting to test this type of battery appears complicated. Figure 2 describes the functions of a battery with 6 connections.

Figure 2: Connections of a typical laptop battery. The positive and negative terminals are usually placed on the outside; no norm exists on the arrangement 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 are 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 a case, no voltage is present. Connecting the switch 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 locate 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, an activation code may be required. This code is often difficult, if not impossible to obtain.

Some battery manufacturers even add an end-of-battery-life switch. At a preset age, cycle count or capacity level, the battery stops functioning. Manufacturers explain that customer satisfaction and safety can only be guaranteed if the battery is regularly replaced. Such policy tends to satisfy the manufacturer more than the user. Newer batteries generally do not have this feature.

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 10 Kilo Ohm NTC or 10kOhm at 20°C (68°F). NTC stands for negative temperature coefficient, meaning that the resistance decreases with rising temperature. A positive temperature coefficient (PTC) will increase the resistance. Warming the battery with your hand may be sufficient to detect a small 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 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.

Repairing a 'smart' battery

Laptop batteries can be repaired but the work is often time consuming. 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 battery can be fully restored but the fuel gauge may be inaccurate or its data is corrupt.

Anyone attempting to repair SMBus battery must be aware of some non-compliance. Unlike other tightly regulated standards, the SMBus allows some variations. This may cause problems with existing chargers and the SMBus battery should be checked for compatibility before use. More information on SMBus is available on www.sbs-forum.org and www.acpi.info.
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. 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 the 'smart' battery may need to 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 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 to limiting the amount of heat transferred to the cells during welding. Excess heat can damage the cells.

During storage, each cell may have self-discharged to a different charge level. This is especially evident on nickel-based batteries. To assure proper charge of all cells without overcharging some, trickle charge the newly repaired pack for about 14 hours, then discharge and recharge normally. Such a cycle is also needed to reset the battery's fuel gauge circuit. Lithium-ion can accept a normal charge in about 3 hours. The service should also include calibrating the battery. (Refer to "Choosing the right battery for portable computing", Part Two).
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Created: May 2003, Last edited: July 2003