Most laptop batteries are smart. This means that
the pack consists of two parts: the chemical cells
and the digital circuit. If the cells are weak, cell
replacement makes economic sense. While nickel-
based cells are readily available, lithium-ion cells
are commonly not sold over the counter, and
most manufacturers only offer them to authorized
pack assemblers. This precaution is
understandable given liability issues. Read also
about Safety Concerns and Protection Circuits .
Always use the same chemistry; the mAh rating
can vary if all cells are replaced.
A laptop battery may have only one weak cell,
and the success rate of replacing the affected cell
depends on the matching with the others. All cells
in a pack must have a similar capacity because
an imbalance shortens the life of the pack. Read
more about Can Batteries Be Restored? .
Furthermore, the state-of-charge of all cells being
charged for the first time should have a similar
charge level, and the open-circuit voltages should
be within 10 percent of each other. Welding the
cells is the only reliable way to get dependable
connection. Limit the heat transfer to the cells
during welding to prevent excess heat buildup.
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 on the battery terminals;
connecting the switch terminal to ground often
turns the battery on. If this does not work, the
pack may need a proprietary code for activation,
and battery manufacturers keep these codes a
well-guarded secret.
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. If no
voltage appears, the battery may be dead or the
pack will require 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, and
newer batteries do not include this feature.
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.
In some cases the chemical battery can be
restored, but 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
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.
The circuits of some smart batteries must be
kept alive during cell replacement. 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.
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 through a 100-
Ohm resistor to the circuit before disconnection
and remove the supply only after the circuit
receives voltage again from the new cells. Cell
replacement of a smart battery has a parallel with
open-heart surgery, where doctors must 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 .
Simple Guidelines when Repairing Battery Packs
Only connect cells that are matched and have the
identical state-of-charge. Do not connect cells of
different chemistry, age or capacity.
Never charge or discharge Li-ion batteries without
a working protection circuit unattended. Each cell
must be monitored individually.
Include a temperature sensor that disrupts the
current on high heat.
Apply a slow charge only if the cells have
different state-of-charge.
Pay special attention when using an unknown
brand of cells. Some may not contain a high level
of intrinsic safety features.
Li-ion is sensitive to reverse polarization. Observe
correct polarity.
Do not charge a Li-ion battery that exhibits
physical damage or has dwelled at a voltage of
less than 1.5V/cell.
When repairing Li-ion, assure that each cell is
connected to a protection circuit.
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