Can laptop batteries be repaired?
Cadex Electronics Inc.
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
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.
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.
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
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)
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
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
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
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
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
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.
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
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
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