Find out the challenges and limitations of repairing “smart” batteries
Most laptop batteries are smart. This means that the pack consists of two parts: the “chemical battery” and the “digital battery.” Batteries for medical and other industrial devices are also made smart, and the most common protocol is the System Management Bus, better known as SMBus technology. (See BU-601: Inner Working of a Smart Battery)
The typical SMBus battery has five or more battery connections consisting of positive and negative battery terminals, thermistor, clock and data. The connections are often unmarked; however, the positive and negative are commonly located at the outer edges of the connector and the inner contacts accommodate the clock and data. (The one-wire system combines clock and data.) For safety reasons, a separate thermistor wire is brought to the outside. Figure 1 illustrates a battery with six connections.
Figure 1: Terminal connection of a typical laptop battery
The positive and negative terminals are usually placed on the outside; no norm exists on the arrangement of the other contacts.
Courtesy of Cadex
Some batteries are equipped with a solid-state switch that is normally in the “off” position and no voltage is present at the battery terminals. Connecting the switch terminal to ground or pulling it up often turns the battery on. If this does not work, the pack may need a proprietary code for activation. Battery manufacturers keep these codes a well-guarded secret to which even service personnel have no access.
How can you find the correct terminals? Use a voltmeter to locate the positive and negative battery terminals and establish the polarity. If no voltage is available, a solid-state switch in the “off” position may need activating. Connecting the voltmeter to the outer terminals, take a 100-Ohm resistor (other values may also work), tie one end to ground, and with the other end touch each terminal while observing the voltmeter. Also try lifting the terminal up to voltage. If not responsive, the battery may be dead or the pack is locked by a security code. The 100-Ohm resistor is low enough to engage a digital circuit and high enough to protect the battery against a possible electrical short.
Establishing the connection to the battery terminals should now enable charging. If the charge current stops after 30 seconds, an activation code may be required, and this is often difficult if not impossible to obtain.
Some battery manufacturers add an end-of-battery-life switch that turns the battery off when reaching a certain age or cycle count. Manufacturers argue that customer satisfaction and safety can only be guaranteed by regularly replacing the battery. Such a policy tends to satisfy the manufacturer more than the user. Consumer and industrial users vote against such practices.
If at all possible, connect the thermistor during charging and discharging to protect the battery against possible overheating. Use an ohmmeter to locate the internal thermistor. The most common thermistors are 10 Kilo Ohm NTC, which reads 10kΩ at 20°C (68°F). NTC stands for negative temperature coefficient, meaning that the resistance decreases with rising temperature. In comparison, a positive temperature coefficient (PTC) causes the resistance to increase. Warming the battery with your hand may be sufficient to detect a small change in resistor value when looking for the correct terminal on the battery.
When the chemical battery is restored, the fuel gauge might not work, is inaccurate, or will provide wrong information. After repackaging, the battery may need some sort of initialization/calibration process. Simply fully charging and discharging the pack to reset the flags might do the trick. A “flag” is a measuring point to mark and record an event. (See BU-603: Battery Calibration)
The circuits of some smart batteries must be kept “alive” during cell replacement of the cells. Disconnecting the voltage for only a fraction of a second can erase vital data in the memory. The lost data could contain the resistor value of the digitized shunt that is responsible for the coulomb counter and other data. Some integrated circuits (IC) responsible for fuel gauge function have wires going to each cell, and the sequence of assembly must to be done in the correct order.
To assure continued operation when changing the cells, supply a secondary voltage of same voltage level through a 100-Ohm resistor to the circuit before disconnection. Remove the outside supply only after the circuit receives voltage again from the new cells. An analogy of cell replacement in a smart battery is open-heart surgery where doctors must also keep all organs of the patient alive.
Anyone repairing an SMBus battery needs to be aware of compliance issues. Unlike other tightly regulated standards, the SMBus allows some variations, and this can cause problems when matching battery packs with existing chargers. The repaired SMBus battery should be checked for compatibility before use. More information on SMBus is available on www.sbs-forum.org and www.acpi.info. Batteries for critical uses, such as heathcare, are typically replaced and not repaired.
Simple Guidelines when Repairing Battery Packs
Last updated 2015-06-15
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